Dynamics III – 4.2 Friction

When a piece of cloth lies down on the floor or on something else, and a force – like gravity – is applied on it to drive it forward, then the contact of that cloth with the floor will work against it. Up till some limit the cloth won’t move at all, that limit is called the static friction. When that limit is exceeded it will move but will still apply a force against it. This latter is called dynamic or kinetic friction. Generally, dynamic friction is less than static. Friction is a surface quality only, it does not depend on cloth density nor on the cloth speed over the surface. It is said that rough surfaces have more friction, but that might be as well the other way around: cloth with more friction is experienced rougher. It is said that alike surfaces have more friction than surfaces which are far from similar, which is why insects can walk on vertical glass panels. It is said that surface structure from a geometrical point of view has not that much impact, friction is the result of (electrostatic) forces between the surface molecules. Take your pick. For Poser use, let’s stick to the roughness concept for the sake of it.

Total friction in green, Static portion in red, dynamic portion in blue.

An experiment the get a feel for friction is easy to do at home. Clean up a smooth long table, or a smooth plank, put a piece of cloth flat on one end and tilt the table. Until some tilt angle is reached, the cloth will stay put. That’s static friction. When the tilt becomes larger than that, the cloth starts to move smoothly. That’s dynamic friction working against the gravity pulling it down.

Poser Friction

In Poser, friction comes in various flavors:

Static and Dynamic between cloth element and collision object, from the collision object point of view.
It sort of states the roughness of the surface of the collision object. These parameter values can be set in panel 2, where I manage the collision objects. Each object from the list has its own settings.
Static and Dynamic, from the cloth element point of view, it’s about the roughness of the cloth surface. These values can be set in panel 4, where I manage the cloth behavioral details.
Self-friction between cloth elements themselves, which is always about cloth surfaces.

For the moment I’ll concentrate on Static and Dynamic and leave Self-friction alone. Just leave it at 0 in the parameters panel, no harm done.

In life, friction not only depends on surface structures but also on complex interactions. Silk might do smooth and fine over glass and female bodies, but can turn in disaster when moving over plastics due to electrostatics, or when moving over brushed aluminum because the fine surface structures seem to ‘fit’.
In Poser I can define frictions for the object as well as for the cloth, but I’ve got to tell Poser which ones to use in the calculations.

If I tick the Collision Friction box in panel 4, then this particular cloth element will experience the friction as defined for each collision objects it will be sliding over.
If I uncheck the box, then all collision elements will experience the friction as defined for this piece of cloth.

So, the friction values for a specific collision object hold for the interactions with all cloth elements that will slide over it – as long as these cloth elements have their Collision Friction ticked. And the friction values for a specific cloth element hold for the interactions with all collision objects that it will slide over, when its Collision friction is not ticked. Hence, there is no combination of values that takes object roughness, cloth roughness as well as some interaction into account.

For example:
the girl in my scene wears a silk blouse and a long leatherish skirt. The skirt collides to the ground as well, which is covered with a rough carpet. For the skirt, the friction with the girl and with the ground drastically differ and I want to show that, so for the skirt I tick the Collision Friction box and I assign the girl and the carpet appropriate values, the latter substantially larger than the former. For the blouse I can’t check the box too, as the friction of silk with the body differs too much from the friction of the skirts leather with the body. So I leave the box unticked and I give the blouse its own – rather low – silky friction values.

Friction is important for cloth room. While density (gravity) and air-resistance address the interaction between the cloth and the room itself, friction addresses the interaction between the cloth and the figure it collides with. Simply stated: when the cloth vertices hit the figure in a perpendicular way we’re talking collision, and when they interact in a parallel way, we’re talking friction. So friction might be as relevant as collision. Might not be so much for still images, but it certainly plays a relevant role in believable animations using dynamic clothes.

Finding real world values

With a flattened and stretched box and a square piece of cloth, I just rebuild the “tilted plank” experiment mentioned above. And after each run of the sim, I gathered positions for the various frames in the animations. Position changes over time make velocity, velocity changes over time make acceleration.

For this acceleration a, I know that a = G*sin(z) – D*cos(z) for tilt-angle z (with the horizon, as set in Z-Rotate), for dynamic friction effect D (under investigation) and for gravity acceleration G = 9.8 m/s² = 1.089 cm/f² when switching to the Poser Cloth Room units cm and frames, at 30 fps. That’s basic mechanics.

The question is: how does D in this theory relate to the setting in the Poser cloth parameters?

Well, it IS the setting !! The Poser dynamic friction is not a dimensionless ratio between forces as in the physics literature, it is the resisting acceleration from the cloth on to some surface (or the other way around, in the collision object properties), under Earthy gravity, expressed in Poser Cloth Room units: cm and frames. So, when you set D to the default 0.1, the cloth on the tilted box is accelerated with 1.089*sin(z) – 0.1*cos(z), which determines its speed and displacement. Now I can appreciate why Dynamic Friction has a max on 1.0, where the critical tilting angle is about 45*. More is quite meaningless, it would lock the cloth to the object in about all positions.
Poser Cloth Density had no effect on these findings, tested from 0.001 to 0.500. As in the theory. That’s something.

When G*sin(z) – D*cos(z) < 0 the net force on the cloth is negative, and the cloth eventually would stop moving, or vice versa: doesn’t even start. At least, that’s the idea. Not starting, in formula: S>G*tan(z) introduces S for Static Friction.

I observed that the formulas did not hold very well at low cloth speeds, where Dynamic and Static frictions both became a factor of influence. This was not the case at higher cloth speeds. I observed that no value for Dynamic Friction could bring the cloth to a stand still once moving, nor could prevent it the cloth from starting to move. Only Static friction could do that. The other way around, the Static Friction parameter has no effect in the “tilted table” experiment, except for low dynamic friction (<0.1), small tilt angles (< 20*), low speeds (I sometimes needed over 3200 frames in animation / simulation). From this and the above I infer that static friction is something extra, having noticeable effects at zero or low speeds only. I can expect a minor effect from stretch, as gravity stretches the cloth, and therefore it moves the center of mass. I then guess that the Static Friction is here to prevent the entire cloth from moving as well.

In other words, it’s Static Friction at zero speed, some mixture of Static and Dynamic at low speeds (1-10 cm/frame) and Dynamic only for higher speeds. The figure intends to give some idea of this (red: static, blue: dynamic, green; total).

In the mixture, Static became noticeable only at low Dynamic values (< 0.1) or at Static values close to the critical G*tan(z) one, where it prevents the cloth from moving at all and velocities are very low.

Two issues in here, on the mixture at low speeds:

Determining Static friction at low speeds at low Dynamic Friction is very hard. It either requires very small tilt angles or precise observation in the few frames after the start of the cloth movement. Not very accurate, that is, and therefore all conclusions get drowned in a sea of measurement errors.
unfortunately, the low speed 1-10 cm/frame = 30-300cm/sec range mentioned above is the one for clothes under normal moving conditions. Girl getting seated: 60cm/sec=2cm/frame. Default cloth falling due to gravity: 245cm/sec=8cm/frame. See Density & Air Damping (previous section) and
Cloth-the Sim Side on air damping and (animation) speed limits. That’s bad luck for simulating clothes. We need both the Frictions.
When I use or analyze python scripts that address the Cloth Room, I find a parameter VelocityCutOff associated with the Frictions, set to 30. Sounds like 30cm/sec = 1 cm/frame to me. Perhaps that’s the moment Dynamic Friction kicks in.

www.hypertextbook.com/facts and a few other places on the net present acceleration and static numbers for human skin and cloth-to-cloth like info. These vary between 0.65 (skin to metal), 0.70 (skin to paper, cloth to cloth) and 0.75 (skin to plastic). Other reasonable values for Poser use (cloth, skin) ranged from 0.3 to 0.6. The latter suggests that the Poser default for Static, 0.5, is reasonable. The former suggests that the Poser default 0.1 for dynamic friction is far too low. Unless we stick to the idea that de default stand for light silk. Silk is extremely smooth, and perhaps 0.1 is fine for silk over a polished wooden table or a lacquered car surface. It does not represent normal clothing over human skin though.
Next to that there are hardly relevant numbers available for our Poser use in the literature. Lots of industrial materials, and long / heavy duty applications. Like tire rubber on concrete, like steel on steel (bolts), and so on. In general, static values are a bit higher than dynamic but not too different. Values for industrial materials can vary wildly: from 0.04 (Teflon) to over 1 (iron to iron on railroad tracks).

From the S = G*tan(z) formula, setting the angle z for a tilted plank and altering Static Friction till the cloth just did/did-not started moving revealed values for Static Friction that did not resemble any physical meaning to me.

Cloth Friction

Then I had a peek into Cloth Friction. I clothified the plank itself, put all its vertices in a choreographed group, combined both the plank and the former piece of cloth in one simulation and – of course – I checked the cloth-cloth collision box.
The first results were a nightmare, as the cloth started to wrinkle and crumble, and fell through the clothified plank. This was repaired by raising the fold-resistance (from 5 to 100).

Since then, I have not found any effect of varying this friction parameter on the position or speed of the cloth at any moment. the results are different from Static or Dynamic, but the same for all values of the Cloth friction. Some literature suggest 0.3 as a decent cloth-to-cloth value.
On top of all this, the SM page http://my.smithmicro.com/tutorials/2313.html does not show any differences between values 0.001 and 0.9, and notices that the effects will mainly be visible in animation. Well, not in mine!

So my question to you all: has anyone seen any noticeable effects in animation or stills of changes in this parameter? because if not, no investigation can be done. And then there is no need to, as any value will do for anything.

Engineering stuff

The concept of Static and Dynamic friction is sort of understood by most people: Static holds the cloth in place until the ‘driving force’ gets too large, and Dynamic works against the driving force when the cloth is moving. Both are independent of cloth density, and Dynamic friction is independent of cloth speed (unlike for instance air damping).

Wikipedia has good info on the theory, in case you need some. Friction is a force, which works against the force that drives the cloth over a surface. The friction is proportional to the force which presses the cloth onto the surface. When the surface (e.g. a plank) is tilted, the driving force from gravity reads F = d * S * g * sin(z) for density d, cloth surface S, gravity acceleration g and angle z with the horizon (flat = 0°). And the friction force reads F = f * d * S * g * cos(z) for friction parameter f (as the rest equals the force down to the box).

At the angle where static friction just prevents the cloth from moving, both forces are equal, and all collapses to f = tan(z), having most values between 0.3 and 0.6 in real nature, as I found while wading those loads of physics tables on the net. Values for f larger than 1 are rare.

For dynamic friction, we’ve got Coulombs Law stating that the force is independent of the sliding velocity.
So, when the cloth moves, we should see a constant acceleration of the cloth with gravity force F = d * S * g * sin(z) minus friction F = f * d * S * g * cos(z), over the cloth mass d*S. Hence, the acceleration reads g * [sin(z) – f*cos(z)].

For a given angle z this is like free fall along the boxes surface, so we might expect

falling speed v = g * [sin(z) – f*cos(z)] * t (t for time) and
distance s = 1/2 * g * [sin(z) – f*cos(z)] * t².

Correcting for unit conversions, and noting that the friction parameter is a ratio between forces and therefor unit-less, we should be able to interpret the Poser measurement results. So I created an animation, 240 frames in total, and made the box rotate along the Z-axis in the first 120 frames, up till the angle of interest, say 30 degrees. I had to do this slowly, because otherwise the cloth would get launched. And it’s a good idea to zero out Air Damping to prevent hovering.

First I turned down Dynamic Friction, till its lowest 0.0001 value. Then I started playing with the Static friction, till I found the critical value that started / stopped the cloth from moving when the box was at its largest angle, and the cloth was on top. Just a fraction less and the cloth started moving. That ‘critical value’. Each angle of interest has its own critical Static Friction value, and vice versa. These were my results:

This means that at a tilt angle of 30 degrees the cloth started moving when the Static friction value came below 0.13, while physics theory says that at that angle the friction value is 0.58 (=tan(z)). So, the Static friction does not resemble anything in real life to my current knowledge. It prevents the cloth from moving indeed, not effected by Dynamic friction, but there is no relationship (discovered yet) between measured values and the literature ones. The concept fits but the model does not. So those who want to match cloth behavior to life, need a re-direction.

A similar experiment could be done for Dynamic Friction. Same setup, I took a Static Friction value a bit below the critical one, so the cloth would move but not before it had reached the top at frame 120. I varied the Dynamic Friction value and noted the frames (the time) that the cloth passed halfway and the end of the box. Higher friction values made lower speeds and therefore larger pass-by frame numbers.

After measuring the size of the box I could infer the speed, meters per frame or per second, as a result from the dynamic fraction, at that angle limit of the box. This is a shipload of details, so I’m not posting them. In the meantime, I noticed a few effects while playing with the parameters.

Density has no effect on friction, at the larger angles. This is physically correct. But it does have effect at the smaller angles.
The stiffness parameters (fold/shear/stretch) do not have effect (which is correct), until the stiffness passes values like 400.
In some cases with large parameter values, the cloth started rotating while coming down.

I have no physical interpretation for any of these. It might be something in the simulation algorithm. But most important, while I got neat looking results of cloth displacement over time, I could not make any physical sense out of it.

To summarize, my Poser Cloth Room experimental measurements do not fit physical theory. That’s it, plain and simple. The static friction angle vs critical value list does not follow the simple f = tan(z) or anything alike. The dynamical friction values do lead to neat distance vs time relationships, but not the one from Coulombs Law.

After a (long) while of puzzling, I finally got the message. Dynamic Friction in Poser is independent of speed and density (Poser matches theory), but it’s not a ratio between forces but a material-dependent acceleration by itself. So 1 stands for: 1 cm/frame² which is comparable to the 1.089 of the gravity constant in Cloth Room. It won’t vary when I alter the tilting angle of the plank in the experiment. This matched my findings for higher cloth speeds (> 10 cm/frame) only. At lower speeds the effect of Dynamic Friction decreases and the results are effected by Static Friction as well. Which might be a good idea from a simulation point of view, but it’s not according to the books, and it does not help my determining of values and Poser behavior.

So, for Dynamic Friction we’ve got an acceleration D instead of the f*g*cos(z) on the plank, and units are cm and frames. Now let’s face it: when a sleeve of a shirt moves over my arm, and my arm is held under a small angle (say < 15°), the sleeve is not going to move in a way that Dynamic Friction becomes important. And when I hold my arm under a large angle, say >30°, the sleeve comes loose a bit which reduces the effect of Dynamic Friction too. This means that in practical cases where Dynamic Friction can be relevant, g*cos(z) has a value between 0.95 and 1.05 (note that g=1.089 in cm and frames). And when we allow for a range 0.90 – 1.10, we’re considering all tilt angles from 0 to 35°.

The conclusion from that is, that within a 10% accuracy, I can use the literature values (for friction constant f) for the Poser dial value D. That’s a breakthrough. And according to www.hypertextbook.com/facts and some other places on the net life values vary between 0.65 (skin to metal), 0.70 (skin to paper, cloth to cloth) and 0.75 (skin to plastic). If you find reasonable values somewhere else, you can just plug them in (and please tell me about them). Industrial values are plenty (teflon 0.04, iron-on-iron 1.0 or more, good for railroad tracks), but normal cloth over normal skin is scarce in the literature.

From this we can infer that the Poser default value 0.1 is not too bad for silk over a lacquered wooden table, but is far too low for cloth over cloth or skin. Good for Cloth Room, not too well for Clothes Room :). We might guess some values as well. Rubbing my arm with rubber eventually burns and hurts, so that’s a high value (0.85). Leather is a bit less (0.75), then comes burlap and wool (0.70), then the normal linen and cotton shirts (0.65), and then the smooth stuff, like flannel (0.55) and silk (0.50).

Static friction in Poser is entirely different from real life – as I understand it – , but we’ve got a graph now that shows theoretical and practical values in one. In literature, static friction values for cloth and skin (hardly to find, but nevertheless), ranged from 0.3 to 0.6. Similar literature however states that the Static value is a bit higher than for Dynamic, as can be expected from theory as well. Using the graph, 0.3 indicates a critical angle between 15° and 20° (red curve), which indicate a Poser value of say 0.05 (green curve). The same way, a literature value of 0.6 indicates a critical angle of about 30° which indicates a Poser value of say 0.15. And when I find a Dynamic friction (in literature) of say 0.7, and I expect the accompanying Static friction to be a bit higher (say 0.8) then this translates to a critical angle of 35°-40° and a Poser value of 0.4.

From this we can infer that the Poser default 0.5 is not too bad for cloth over a wooden table but could be reduced to say 0.35 for cloth over cloth or skin. But for smooth materials with Dynamic values as low as 0.5, the accompanying Static must be reduced to 0.15.

Another observation is that Static plays a role on moving cloth as low speeds. This is not what one expects from the books. And unfortunately, this “low speed range” (1-10 cm/frame) is very common in our clothing use of the Cloth Room. From getting seated very gracefully (0.5 cm/frame) to a speedy cartwheel (20 cm/frame), all normal animation fits in this range.

So, in order to improve image or animation results one not only has to take care of Dynamic but of Static also, while the meaning of the dial-values for both are very different. Since friction plays a relevant role in the interaction between cloth and figure, this is the place where our artistic / alien experience or gut feeling will creep in., and Real World should be considered overrated.

Finally, Cloth self-friction is a mystery to me as no change in value provides any effect on any result in animation or final image. I have no fit for even the concept.

Dynamics III – 4.3 Resistance

The Cloth Room Resistances for Fold, Shear and Stretch are determined by the fibers used, and the weave in which the fibers are combined.

The weaves – some of them shown in the next page figure – are hard to make calculations for. Fabric parameters themselves are published and available on the net, but only for the heavy duty / high performance ones. Look for fabrics for game sailing or surfing or parachutes, and you’ll find plenty. They even use different weaves for the different sails on the same ship. Look for fabrics for normal clothing, and you’ll find none. Google for “stretching jeans” and you’ll get lots of tips how to make you pants fit better. Google “folding cotton” and you’ll get a course in fancy towel folding.

http://www.bbc.co.uk/schools /gcsebitesize/design/textiles

Okay, some fiber mechanics then.Each fiber is made of some stuff that can be characterized by a Specific Weight, usually measured in grams / cm³ or the equivalent: tons (1000 kg) per m³.
Water has a value of about 1.0, Silk is 1.34

But fibers don’t come in m³. They have a thickness (diameter d) which translates to a cross-section (C= π d²/4), and a length L; their weight W = SW * C * L is expressed in tex (= grams per km) or denier (= grams per 9 km). So when SW=1 g/cm³, and C= 0.0001cm² (=0.01 mm², about 0.1mm thick) then the fiber will weight 10 tex (for 100,000 cm) or 90 denier (for 9 km).

The other way around, spider silk has a thickness of 3 micron = 0.003mm, good for 0.0134 tex. This implies that a fiber of 4,000,000,000cm (Earth circumference) weights only as much as 500gr. In threefold steps:

Cocoon silk = 10 micron => 0.1 tex.
Animal hair (sheep, wool) = 30 micron => 1 tex.
Cotton => 100 micron = 0.1mm => 10/15 tex (bedsheets)
T shirts do 20/30 tex.

Approximately, a m² of fabric contains 4/3d km of fibre (thickness d in mm), so the specific weight of fabric (in gr/m²) equals 4T/3d for fiber-weight T in tex while about: d = sqrt(T/1000). For cocoon silk, this means 4*0.1/3*0.01 = 13gr/m².

But that’s for single fiber, and usually fibers are spun into yarn, with thicker threads, thicker cloth and higher tex values. For various silk fabrics, values can be found like:

Gauze 12 to 20 gr/m²
Organza 15 to 25 gr/m²
Habutai 20 to 70 gr/m²
Chiffon 25 to 35 or 50 to 70 gr/m² (double thickness)
Charmeuze 25 to 125 gr/m²
Crepe de Chine 50 to 70 gr/m²
Raw Silk 150 to 175 gr/m²

Note 1: thickness affects appearance: 10 gr/m² is semi transparent, 25 gr/m² is translucent, 100 gr/m² is opaque.
Note 2: To put values in perspective: Poser default cloth density reads 0,005 gr/cm² = 50 gr/m².

Now we can start to pull the fiber. The result of that depends on the force per fiber-cross section, in Newton per m², or more practical: N / mm². But since fibers can come in various thicknesses, N/tex is the preferred material constant. And the amount of N/tex times the Specific Weight (in gr/cm³) of the material results in kN/mm²:

5 N/tex * 1.2 gr/cm³ = 5 N / ( 1 gr / 1,000,000 mm) * 1.2 gr/ 1000mm³ = 5 * 1,000,000 / 1000 * 1.2 N/mm² = 7 kN/mm².

At low forces, the elasticity or modulus of the material is the ratio between the stretch in % and the force in N/mm2. At high forces, a specific amount of N/mm2 will make the fibers snap. By doubling the thickness of the fiber (thread, yarn, cable) one can quadruple the strength of it.

Since measurements usually have a more scientific / engineering background instead of an industrial one, fiber strength is expressed in MPa (MegaPascal, 1 Pa = 1N/m²) or psi (pound/square inch).
1 N/mm² = 1 MPa = 146.25 psi; 1 kpsi= 6.84 MPa.

Values for polyester (www.ides.com):

Specific weight (aka “gravity”): 1.24 to 1.48 g/cm³.
Tensile Modulus (stiffness): about 300,000 to 400,000 psi or: 3000 to 4000 psi per % elongation
that’s 3000 to 4000/146.25 = 20 to 30 N/mm² per % (or say 25/1000/1.25 = 0.02 N/tex)
The breaking strength is 5000 to 9000 psi or about 35 to 60 N/mm² which is about twice as much, so polyester is not going to stretch very much.

Steel has about a similar stiffness (20 N/mm per % elongation) and due to its high specific weight (8 g/cm³) a low 0.0025 N/tex value: you’ll get less stretch resistance per pound of material. But it stretches nicely till say 200 MPa and breaks at 400 MPa. So steel can handle far larger forces than polyester, and stretches up to 10%.
Lead for instance is much more deformable, with 1.6 N/mm² per % elongation.

Teflon / PTFE: 2.2 gr/cm³, breaking strength 28 N/mm²; it’s a heavy, brittle material but it has an extremely low friction: 0.1 which makes it fine for surface coatings.

Another example: Kevlar (as in bullet proof vests): The modulus is about 60N/mm per % strain (twice as strong as polyester), but breaking at 2N/tex, and with a SW = 1.45 g/cm³ we get 2900 N/mm². That’s why it’s in bullet proof vests: the projectile has to break the fibers and that slows it down considerably, it’s say 10 times stronger than steel. Human skin breaks at say 20 N/mm² so that’s why we need the protection.

High performance cabling for shipping, like Astra: 0.97 gr/cm³ (so it floats on water), and with 0.15 – 0.20 N/tex it can handle about 150 N/mm² while stretching only 1%. And is can stretch a lot (30% or so) while not breaking. This is the stuff that keeps ships to the quay, especially when using over 30 mm thick cables.

Generally speaking, the stiffness for high performance cabling is about 100 N/mm² per % elongation. For most materials that we make to cover and protect our body, the stiffness is about 10. For natural clothing materials, (wool, cotton) it’s about 1. I needed tens of pages on the net to gather and combine information like the above. And I still have nothing reasonable for fabric itself, I’ll have to construct that.

1 meter of cloth required about 1000/(1.5 d) threads in one direction, and about the same in a perpendicular direction, using thread diameter d in mm. When a single thread has a stiffness S (eg 25 N/mm² per % elongation) then 1 m of cloth has a stiffness of (1000 S)/(1.5 d)*(3 d²/4) = S*d*500 (pi rounded to 3 as the 1.5 thread distance is a quesstimate anyway).

I also stated above that 1 m2 of fabric has a weight of 4T/3d, in gr/m², diameter d in mm, T in tex about equal to 1000 d², so the weight in gr/m² equals about 4.000 d/3 (d in mm). In Poser, I concluded earlier that the ratio Stretch Resistance to Cloth Density is typical for the material, an in this case it reads (S*d*500) / (4000 d/3) = 1.5 S / 4. In nature, that is say 10 and in Poser (default values) that is 50 / 0.005 = 10,000. Let’s check units again.

Assume d=1mm, then 1 m cloth takes 1000/1.5 = 667 threads, each having a 3/4 mm² cross section. Stiffness S = 25 N/mm² per % elongation then turns into 667 * 3/4 * 25 = 12,506 (N/mm² per %). The amount of fiber is 2* 667 m * 3/4 mm² = 1000 cm³, and hence weights 1340 gr (/m²). Which sounds okay, considering a 1mm thick thread resulting in a say 2mm thick cloth. The stiffness / density ratio is 12,506/1340 = 9.33 about 10.

In Poser, cloth density is not measured in gr/m² but in gr/cm². This makes a factor 10,000. Second, the default stretch resistance is known to produce far too elastic cloth in the simulations, 500 might do far more realistic and in line with the real values we’re using here (although it might elongate the sim itself). So, in Poser: 500 / (0.005 * 10,000) = 10 again.

In other words, the Poser Stretch Resistance is: (1.5/4 * 10,000 =) 3750 * S * D for stiffness S (in N/mm per %) as found in the literature, and cloth density D (in gr/cm² as set in Poser). Realistic densities range from 0.025 to 0.075 despite the 0.005 default representing silk. Realistic stiffness values are:

Rubber 0.3 – 1
Natural fabric 1 – 3 (wool, cotton)
Enhanced 3 – 10 (cotton / polyester etc)
Artificial 10 – 30 (nylon, …)

A reasonable cloth value is: 4 (enhanced cotton) * 0.03 (thin shirt) * 3750 = 450. And when that turns out to be too high for a proper sim, reduce both Resistance and Density in sync.

Since shearing is meaningful for cloth only, and not for individual fibers, and has less industrial implications and applications compared to stretch, there is far less research done and there are far less values available. So my suggestion is to use values below the Stretch Resistance. Just a bit, except for those special cases that hardly stretch but shear a lot, like chain mail vests. Then Shear Resistance really is smaller than the Stretch one.

Folding has a similar story. The industry wants to know how many times a fiber can be folded before it wears out and breaks. Resistance to folding is futile. Actually, when folding a fiber with thickness d (diameter), it experiences a local stretching of 100 * d/R. 100 for making the result into %, and R is the folding radius, much larger than d.

As can be seen in the figure, the core of the fiber makes a turn forcing the outer part to relatively stretch d/2R and the inner part to shrink d/2R as well.

A sharp fold means a small R and much local stretching in the fibers. And we discussed stretching. Thicker cloth means thicker threads so d/R goes up for the same sharpness of the folds: it takes more effort to make them. Which material is a better folder? Hard to say, a better question is: which material folds sharper for the same fiber thickness and the same force applied?

Rubber seems a bad folder as it’s hard to make the folds really sharp, but what if it’s equally thick as a cotton shirt? Personally, I tend to follow the stiffness values as presented above. Rubbers fold better than natural fabrics over enhanced fabrics over artificial materials, at the same cloth (mass) density.

Since folding only affects a portion of the cloth instead of the whole cloth, using a Folding Resistance which is a tenfold smaller than the Stretch Resistance (as demonstrated in the defaults) might be a good idea.

Dynamics IV – 1 In Perspective

Download this tutorial in PDF format (0.4 Mb).

As a decent mutual understanding is key in maintaining good relationships, I wrote Poser Features in Perspective to give you some historic background on various Poser functions that receive a lot of debate. Cloth Room, FireFly rendering and the evolution of the Vicky and Mike characters, for instance. Especially Poser users discussing the future might be interested in some history.

For those curious about the peculiarities of cloth simulation in general, I added Cloth Simulation in Perspective. It’s mainly about the behavior of 3D meshes for cloth, so especially Dynamic Garment Makers (virtual tailors) might be interested.

Dynamics IV – 2 Poser features in perspective

The history of Poser is rather well documented in Wikipedia. Just a brief overview:

1	1995	Fractal design
2	1996	Fractal design	Props, Animation
3	1998	MetaCreations	User Interface (the Bryce-like), Expressions, Hand poses
4	1999	MetaCreations	Sketch render, Transparency, Conforming clothes, Magnets
ProPack	2000	Curious Labs	Python scripting, custom rigging (now: setup room)
5	2003	Curious Labs	Firefly render, Collision detection, Dynamic hair & cloth
6	2005	e-Frontier	OpenGL, IBL, Cartoon render
7	2006	e-Frontier	Talk designer, multithreading, HDRI, Morphing brush, universal poses

MetaCreations was a merger of the people from Poser, from Bryce, from Painter, from RayDream/InfiniD, from Kai’s Power Tools and alike. Thanks to this, Bryce was able to handle Poser files from the early days on, and thanks to Kai Krause, they both have a similar user interface. When MetaCreations broke up, Bryce and Painter went to Corel who later sold Bryce to DAZ. InfiniD went to Eovia to become Carrara, which later ended up at DAZ as well. The Poser people continued as Curious Labs.

Establishing Poser 5 however turned into a shear disaster (well described by John (Vanishing Point) Hoagland himself in http://www.cocs.com/poser/poser5mess.htm) and e-Frontier took over. After a while they sold their entire American branch to Smith Micro (founded by William Smith Jr). The Japanese branch is still known for the 3D modeler “Shade” and still represents the Japanese version of Poser. From then on Poser is in the hands of Smith Micro, Poser Pro gets introduced, and functionality has increased a lot since.

For a better understanding of the advanced features like Cloth Room and FireFly, it pays off to elaborate a bit on that hectic period around the turn of the century. Let’s take a look from the other side.

Reyes

In January 1996 the writer/producer Jorge Martinez Reverte, Javier Reyes and Jose Maria De Espona formed a company REM Infografica, based in Madrid, Spain. REM was the acronym of Reyes Espona Martinez. Martinez became CEO, Reyes became Technical/R&D Director and De Espona became the Art & Production director for the 3D Models Databank. (source: www.deespona.com/personalweb).

A sequence of splits and mergers (1998) moved the modelbank to Viewpoint (later: Digimation), created Next Limit (RealFlow) and also Reyes Infografica, known from 3DS Max plugins like Cloth:Reyes, Cartoon:Reyes, NPR:Reyes and more. In 2000 the latter sold its plugin technology for use in Poser. Cartoon:Reyes became Poser Toon render (the Sketch render was already available in Poser 4), NPR:Reyes became the Poser FireFly render, Cloth:Reyes became Cloth Room and so on. Javier Reyes himself continued with Virtual Fashion, until that came to its own end as a product (say 2009).

Some background references:

http://www.cgarena.com/archives/interviews/3daliens/interview.html on the foundation of Next Limit
http://forums.cgsociety.org/archive/index.php/t-725233.html Reyes Infografica doing plugins for Max 2 (1998, I was into 3DS Max myself, those days).
3DS Max moves to Kinetix (Max 3, 1999) which then moves to Discreet (Max 4, 2000). From 2001 on, the Reyes plugins were not supported anymore
http://forums.cgsociety.org/archive/index.php/t-725233.html
http://forums.cgsociety.org/archive/index.php/t-1006.html
3DS MAX 5 introduced a completely renewed set of… rendering features.

So now we know where those Poser modules came from, and why creating Poser 5 took so much trouble: a lot of (Spanish?) 3DS max plugins had to be integrated in the Poser 4 framework. In the meanwhile, sometimes some confusion passes by.

The rendering module
The NPR:Reyes (NPR = Non Photoreal Render) aka FireFly render is based on the initial Renderman REYES technology, where REYES is an acronym for Render Everything Your Eyes (can) See. The Reyes from Infografica is just a normal Spanish name. In the NPR:Reyes product, the two came together.
REYES was a serious improvement over the scanline rendering (Poser 4, 3DS Max) without the enormous calculation times from the raytracers in those days, and with the ability to chop rendering tasks in pieces to support large scale projects. It did not support extensive raytracing (*) but offered speedy alternatives (environment mapping) instead. Later on, Pixar enhanced the Renderman techniques by adding explicit raytracing to create their own PhotoRealistic Renderman toolkit.
So it might come as a shock to you, but FireFly is not raytracing very well. By design.

(*) for the tech’s amongst you: classical raytracing techniques scatter rays of light throughout the scene, and must consider the scene, and the objects, as a whole to do so. The REYES algorithm on the other hand chops a scene into chunks (buckets) and then into very tiny pieces (micropolygons) in an early stage. So both treat the scene geometry in opposite ways. As a result, REYES based renderers have problems with raytracing, and raytracing renderers have problems with performance.

A view on characters

So, the introduction of Poser 5 was a milestone in its history, and a heavy one. Next to the software and its functions, it had impact on content and future developments as well. Up till then, DAZ had created the Vicky and Mike characters (versions 1 and 2) based on the P4 female and male, which as such were Zygothe products. The turnover to Poser 5 brought some serious licensing issues between MetaCreations and DAZ, and the latter decided to go their own way.

This own way introduced the unimesh geometry, the wish to create all characters from one single mesh. Vicky and Mike 3 introduced that concept, soon to be followed by Stephanie 3 which was a real-person body scan translated to the unimesh. DAZ also had their own view on the software, which brought us Daz Studio consisting of a free base with paid plugins – just a completely different marketing concept compared to Poser.
Boths DAZ views developed, and after a successful rebuild of Vick and Mike 4 using the new modo modeler, and after successfully turning Girl 4, Steph 4 and so on into morphs of Vickys mesh, DAZ launched Genesis – the new generation unimesh, with all known figures (Vick and Mike 5 included) provided as morphs of that base mesh, and including a serious Poser contender Daz Studio 4, at the turn of 2011/2012. The weight-mapping mechanisms for smoother posing forced Smith Micro into similar features for Poser 9 / Pro 2012. But mainly, the full features of the new DAZ characters could not be deployed in Poser at the fullest, you needed DS4 for that.
This came as a serious chock to the Poser community. Being “Poser-compatible” was not the default anymore.

There is always more (on clothing)

Around the turn of the century, Serge Marck published his site www.poserfashion.net (before it got hacked, you can find the Original in Internet’s WayBackMachine). He published clothes for Poser 4 (conforming), Poser 5 (some dynamic for V3) and Poser 6 (dynamic, V4).He was into cloth simulation for some time already, and discussed SimCloth, Cloth:Reyes, ClothFX and Maya/Cloth. Apparently the relevant products in those days.

SimCloth is still around as a Max plugin. It’s opensource, so just go http://www.spot3d.com/simcloth/ and get the code, if you like. That is, if you want to find out about the internals. It’s created by Vladimir Koylazov (Vlado) in 2000 – 2002, and got a serious update in 2005. Vlado himself works at the Chaos Group, home of the VRay renderer. Small world.

ClothFX is announced (Jan 2004, www.thefreelibrary.com) as “the” cloth simulator for 3DS MAX, and referred to as the new name for Stitch. This one gets a Stitch Lite equivalent “compatible to Max4, Max5 and Stitch” (Feb 2003) and owned by Digimation. Didn’t they get the REM Infografica Modelbank also? During the issue of 3DS Max 7, ClothFX was distributed for free and since Max 8 it’s part of the program stack. According to the current 3Ds Max manuals, ClothFX is a trademark from Size8 software. Their single page website refers to TurboSquid where the free ClothFX was distributed.

Cloth:Reyes ended up in Poser Cloth Room. According to the credits for Poser, the cloth simulation was written by Size8 Software also. Note that Virtual Fashion was a garment maker, like Marvelous Designer. Note that ClothFX introduced a garment maker into 3DS Max. And Poser got the simulation part only, and lacks a garment maker.

According to various posts on the net, and forum posts in CGSociety, debates around REM Infografica and Reyes Infografica were around patents on fluid and cloth dynamics. Apparently, not all the clothing guys went with Javier Reyes… perhaps Size8 is just another part of that original team. We’ll never know.

In the meantime, life goes on and so does cloth simulation. Halfway 2012 I found:

Kinect Virtual Fashion – by MicroSoft. It uses MS Kinect / XBOX to measure up your body, and then you can dress up virtually. On Youtube: http://www.youtube.com/watch?v=s0Fn6PyfJ0I
Cisco StyleMe Virtual Fashion Mirror, does about the same in retail environments, it’s a shopping business product
ImageTwin, http://www.imagetwin.com/ – use Kinect to obtain your body shape.
TC2, www.tc2.com – the guys behind the body scan technology

Now, given the body scan thing, where do you think that Virtual Fashion clothing comes from, since Javier Reyes teamed up with MicroSoft in various fashion conferences, and the software disappeared from the consumers software market in 2009?

Conclusion?

Now perhaps we can appreciate why FireFly is not that good in raytracing, and is very unlikely to become so in the near future. Till then we have to manage with translators (like Pose2Lux, or a Reality plugin for Poser perhaps?) into third party raytracers like LuxRender.

We also can appreciate why Cloth Room is very unlikely to undergo any drastic transformations. Till then we have to manage with a decent understanding of the tools at hand.

We also can appreciate why DAZ is demonstrating that Poser-compatibility is not an industry standard any more.

And finally, let’s not point at Smith Micro for the things they just inherited a few years ago.

Dynamics IV – 3 Cloth Simulation in Perspective

When I started to Google on cloth simulation, I stumbled into
http://www.animares.com/book/Secondary-Motion/Cloth/Hintergrund.html.
Its in German, on issues with cloth simulation. The page mainly noted to watch out for parameter settings, conversion and scale.
In short, if the parameters are determined for seconds, meters, kilograms etc and the mesh is defined in inches or internal (poser) units, then you’re off. I’ll let this alone, the Poser parameters are in semi-metrics (cm, s, gram), and I already mentioned the gravity matching the Earth surface conditions. Every now and then in this tutorial you’ll see me juggle to end up right. But for users of Imperial units (inches etc) there is a health warning in here: watch your units.
Also, cloth in real life does not scale, and neither do cloth simulators. Sheets of 1×1 mm, 1×1 mtr and 1×1 km really do behave different. Thread thickness plays a role, and so does the thread pulling strength and loads. You cannot fold the 1x1mm piece, and a 1km fibre will break under its own weight. Things like that.

The webpage also noted that although regular meshes do fine in most areas of 3D, cloth simulation is not one of them. Cloth Simulation benefits from a “Delaunay-Method” which gives irregular triangulation of meshes. It’s implemented in Sim Cloth / Clothmesh, in Cloth Reyes / Hexamesh, in ClothFX / Cloth Panel and in Maya / Cloth. So they said. A familiar collection.

Since ClothFX ended up in 3DSMax (see Poser features in perspective) it’s the Hair&Cloth chapter from 3DSMax that presents clear examples. With a few splines the contours of a garment are defined, and the Garment Maker creates the irregular mesh from it. For a shirt in development:

And for a pocket attached to it (just to illustrate what an irregular mesh looks like):

Since ClothReyes is implemented as Poser Cloth Room, let’s see what we can find out. Google is our friend.
Well, I end up at a program called Virtual Fashion (Pro 1.5), available in Spanish and English, July 2009, with the ability to export Quad and Hexa Meshes to Poser. Doesn’t it sound familiar (Poser features in perspective)?
A Mesh/Garment Maker apparently was not included when Cloth:Reyes was included into Poser, I can’t see it different. Poser got the simulator only.
In the meantime, Hexamesh itself is a toolkit doing fine in CAD/CAM environments, like the fabrication of (e.g. medical) components – which is not on our route.

I’m presenting this meshing story, because simulation results appear to be dependent on mesh structures and people ask (in forums etc) about “best mesh structures” and “what to do with existing mesh files”. We don’t need to re-invent wheels here. At least a serious part of this tutorial is devoted to mesh structures.
So I might need a tool to turn regular (quad, tri, …) meshes into irregular ones.

Since people are fond of Marvelous Designer as a cloth making toolkit, the Show Mesh example in the manual shows:

That’s not just simple triangles, that’s Delaunay Triangulation. Don’t look further!

I continued my strolling around the Net and in the end I found myself buried deep down in a worldwide scientific community on cloth simulation, US patents included, which evolved from about the mid ’80s. That was about the moment I stopped in the scientific arena after getting my MSc on applied math and experimental physics. I never looked back. This means to me that I will stop looking further into this area on the vertex level. Back to the front! Let’s make fun images. But I’ll give you what I found on the ways forth and back.

From www.poserfashion.net (before it got hacked, you can find the Original in Internet’s WayBackMachine) I learned that the author (Serge Marck) was into cloth sim long before Poser started doing it. He states that all cloth sim routines (those days) were based on the modeling of physical fibre behaviour in cloth. He also flagged and demonstrated that because of this modeling, the simulation results depend on non-virtual things as real world cloth size and structure. In effect, he showed differences (from Cloth Room) based on cloth size at constant vertex density, on vertex density at constant cloth size, and on mesh type (quad vs tris) at constant density and size.

His main source was a tutorial by P.S. Karthikeyan. It’s nowhere nowadays, I only could find a Russian translation of one chapter out of it (Google Translate is my friend too), on the modeling fundamentals. This said that all started in the ’80s from the well understood behaviour of cables (hanging bridges, telegraph cables) and the effects of fibre parameters, the effects of fibre thickness on those parameters, and the effects of gravity and wind. Cloth then is interpreted as a two-way weave of cloth fibres each described with cable behaviour.

It also said that – at those days – P.S. Karthikeyan was a 3rd year student on the Aerospace Institute in Madras, India. I did not find any other references to him related to cloth sim, after that. But I did find that the tutorial must have been very good, because it’s quoted in about every paper on this matter from those days on. Google rocks.

One of the papers I ran into looked worth downloading and sharing, it looked readable, an overview on fundamentals and implementations in the popular cloth sim software mentioned above and in various forumposts. Recommended.
This researchpaper can be downloaded here as well (10Mb, use Right-click Save As…), as it seems to be disappeared from its original location. This original location is the RISC – Research Institute for Symbolic Computation, deep down the science dungeons. Get me out!

With respect to all of this, Cloth Room definitely is a different beast than Materials Room or alike. It’s not a Room at all, it’s the Magic Kingdom Castle, or the complete Disneyland Park around it. I’m going back to the front gate. See you there.

Dynamics V – 1 Crash Course on Math and Physics

Understanding cloth and having a mental model of the physics involved is necessary but not sufficient to make effective and efficient simulating systems. Creating those systems is a world in its own right. Especially those with an engineer’s way of looking at things might be interested in this mini tour through the deep down dungeons. Crash Course on Math & Physics (this page) and Crash Course on Sims & Settings (next)really are the math and physics loaded chapters. Any use of the underground escape tunnel brings you into Muppets Lab “where the future is made today”. You have been warned.

Download this tutorial in PDF format (0.4 Mb).

Some people immediately zap away when they see even a two-character formula coming along, others insist in getting the rough engineering treatment to enhance their understanding. This chapter is for the latter part of the audience, although I can recommend the conclusions to everyone.

I’ll start with some math first because 1) it simplifies handling the formulas and 2) in the forums people have stated to have some issues handling and grasping it. Second I’ll do the physics, from an electrical point of view. This gives the proper resulting formulas in an easy way, but it’s too far away from our actual simulations. So third, I’ll present the physics from a mechanical point of view. Similar formulas but much closer to our actual cloth simulation.

Then, fourth, I’ll consider the simulation algorithms themselves. Finally, I’ll present some conclusions that help you further in handling the Poser parameters and cloth simulation.

Complex Calculations

In order to handle straightforward object motion, steady currents and flows, I can simply deal with real numbers. But handling disruptions, oscillations, alternating current and more becomes a lot easier when I extent my repertoire to imaginary and complex numbers.
In that area, the magic bullet is called ‘i’ with the property i² = -1. I don’t have to understand it, it’s there, period. Multiplying a real number b by i turns it into the imaginary i*b (or: ib for short), and repeating the operation turns it into i*i*b = i²b = -b which brings me back into the real world again. When I mix a real number a and an imaginary ib I get a complex number a+ib, and I can do all adding and subtraction while keeping the real and imaginary worlds separate:
(a+ib) + (c+id) = (a+c) + i(b+d).
Like I can represent real numbers on a straight line, I can represent complex numbers in a simple plane. I can use dots or arrows to them, and adding and subtraction behave like simple adding and subtracting those arrows as in vector algebra.

Complex numbers however are not very friendly in multiplication and handling powers, exponentials and logarithms. As you can see in
(a+ib)*(c+id) = ac + ibc + iad + i²bd = (ac-bd) +i(bc+ad)
real and imaginary parts of the numbers get mixed, which might turn longer calculations into a sheer mess.

The vector approach however gives us an alternative. Instead of considering the x and y components of the vector, we can consider its length R and angle q with the horizontal axis. This makes a+ib equivalent to R*[cos(q)+isin(q)]. Adding and subtracting from this approach becomes a nightmare, but multiplication, division and more becomes easy. Multiplication is: multiply the lengths and add the angles, which follows in a straightforward way from classic geometry (I’ll save you the details).

To simplify our tool even more, I use natural exponents and logarithms to rewrite R to e^r (or r=ln(R) ), and especially (cos(q) + isin(q)) to e^iq. In other words, a+ib becomes e^(r+iq) where r=ln(sqrt(a²+b²)) and q= arctan(b/a). This is handy, since multiplication of values is equivalent to the addition of exponents, and taking powers is equivalent to multiplying exponents, so (e^(r+iq) * e^(s+it) => e^{[(r+s)+i(q+t)]} and [a+ib]^N => [e^(r+iq)] ^N => e^N(r+iq).

So, with this dirty trick, the issues of complex multiplication and power-lifting are reduced to addition and simple multiplication. That’s a gain.

This is the basic tool, now the application of it in physics. Alternating currents, oscillating movements and the like can be described as x(t) = Rcos(wt) where R is the amplitude, and w=2 pi f, for oscillating frequency f. Each time wt makes a multiple of 2 pi, or each time ft makes a whole number, the oscillation starts anew. Now it has become a small step to state that the oscillation we see is just the real world part of a complex movement, which also has an imaginary component in a world we can or cannot imagine J.
But as a whole, its described with x(t) = Re^iwt or even with x(t) = e^(r+iwt).

The main reason for doing this, is that we need time-derivatives (and integrals) like dx/dt to get speed, acceleration and more. But the classical approach presents us a plethora of sine and cosine functions, and we really need a lot of geometry to get understandable results. In the new approach, we don’t.

When x(t) = e^(r+iwt) then dx/dt = iw * e^(r+iwt), so d2x/dt2 = -w2 * e^(r+iwt), from position to velocity to acceleration,
but also ∫xdt = 1/iw e^(r+iwt) and so ∫∫xdt2 = -1/w2 e^(r+iwt), from acceleration to velocity to position.

Going Electrical

In abstraction, basic physics considers a ‘field’ and a ‘thing’, and moving the thing against the field requires energy (from us or from the thing itself), while moving the thing along the field returns that energy back. One unit of field times one unit of thing make 1 Joule of energy, and if I do so every second it requires or generates 1 Joule per second or: 1 Watt of power.

Less abstract, in electricity the ‘field’ is electric potential (char: V) determined in Volt and the ‘thing’ is electrical charge (char: Q) determined in Coulomb. When such an amount of charge passes a measuring point within 1 second, we’ve got an electrical current of 1 Coulomb per second, or: 1 Ampere.
When I move such a charge to a place with a 1 Volt higher potential it requires 1 Joule (char: E, in formula: E=V*Q), and when I do so every second it requires a power of 1 Watt (char W, in formula: W = V*I). Especially this last formula on electrical power might hang somewhere in your memory, resulting from a physics class or so.

When I move a charge from one place to another, bridging a potential, it becomes harder for the next portion of charge to do the same. The source of the charges becomes short due to the displacement, or: anti-charged and as we know opposite charges pull each other. The destination of the charges gets a surplus or: gets charged, and similar charges push each other. In other words, by moving a charge across a field, it gets less interesting to continue, the field itself gets reduced. The ratio between these is understood as: electrical capacity (char C, measured in Farad).
In formula: dQ = C dV, when I move a charge dQ the field changes dV. When the device that includes the source and the destination of the charge is said to have a large capacity, this means that the displacement of large charges have little effect on the field.

Even when moving the charges hardly effects the field, the charge elements (e.g. electrons) have to bridge the gap between source and destination, collide to each other and to loads of molecules in between, and dissipate some heat. This is understood as electrical resistance (char R, measured in Ohm).
In formula: V = I*R or I=V/R, Ohm’s law. A large resistance is like having air (or less) between two poles of a battery, they are electrically isolated, there is hardly any current and no energy loss. A small resistance is like short-circuiting the battery with a copper wire. Lots of current, and the heat might even melt the wire. As current I = dQ/dt we see that first we had a relationship between ‘field’ V and ‘thing’ Q, and now we’ve got a relationship between V and the first time-derivate of the ‘thing’: dQ/dt.

So the next step is to investigate what happens with that second time-derivative, or: dI/dt. This just means: alternating current. When alternating current is applied onto a coil, it generates an electric field. This is understood as electrical induction, the fundament under dynamos, generators, turbines and the like. Char: F (after Faraday), measured in Henry (both did the discoveries at the same time).
In formula: V = F*dI/dt or dI/dt = V / F, a large induction generates a strong field from the alternating current.

Now, let me bring all the elements together. I’ve got a device with two poles, I apply an alternating voltage onto the poles, and the charges and currents in the device start working in parallel:

Working in parallel implies that I have to add the individual currents, so: (I1+I2+I3) = I total = Voltage / “Impedance” (Impedance is some kind of resistance for alternating currents and voltages, taking capacity and induction into account).

Alt. Voltage Vexp^{iwt}
Capacity I = dQ/dt = C * iw * Vexp^{iwt}
Resistance I = 1/R * Vexp^{iwt}
Induction I = 1/F * 1/iw * Vexp^{iwt}
In parallel I = [ C * iw + 1/R + 1/F*1/iw ] * Vexp^{iwt} = 1/IMP * Vexp^{iwt}
So IMP = 1/[ iwC + 1/R + 1/iwF ], after taking the inverse on both sides
= iwR / [-w2 RC + iw + R/F], after multiplying with iwR on both sides
= iwR [ (R/F-w2RC) – iw]/[ (R/F-w2RC)2 + w2], after multiplying with [ (R/F-w2RC) – iw]
= w * [w/R + i (1/F-w2C)] / [(1/F-w2C)2 +(w/R)2]
If R very small (short circuit), then 1/R becomes dominant and IMP => (w2/R) / (w2/R2) = R2/R = R
If R very large (isolation), then IMP = iw / (1/F-w2C) and becomes quite large when 1/F-w2C => 0,
that is: w2 => 1/FC. Then the impedance => R again.
This w = sqrt(1/FC) represents the resonance frequency of the system, it behaves at a minimal energy loss.

Now we’ve got the essential formula, let’s apply them on a mechanical system.

Going Mechanical

The mechanical route follows the same approach. The ‘field’ is force (char F, measured in Newton) and the ‘thing’ is displacement (say X, measured in meter). Applying a force of 1 N over 1 m requires 1 J of energy, and applying that force over a route with a speed of 1 m/s requires 1 W of power. Note the similarities with electrical.

A spring pulls and pushes, proportional to its stretch: F = S * X. The stronger the spring, the more force is required to stretch it (with the same amount of displacement). Spring strength does not have a specific name, it’s measured in N/m.
Spring strength is not related to any mass, but when a mass is attached to a string, the force will generate an appropriate acceleration: F = M * a, Newtons Law, where a is d² X/dt², the second time derivative of the ‘thing’ we’re considering. M is mass, in kilograms.
So we’ve seen forces proportional to distance X (the spring) and to its second time-derivative acceleration A (gravity like). So all we need to make it like electrical, is a force that is proportional to the first time-derivative of distance, which is: speed, or velocity v. This introduces: damping D as in F = -D*v. The faster you move, the more the force will work against you. Air-resistance is one example of them.
Now we can make the step from electrical to mechanical, just by substituting the equivalent concepts
- So electrical capacity C relates to 1/S spring strength
- So electrical resistance R relates to 1/D damping
- So electrical induction F relates to M mass
And so w = sqrt(S/M) represents the resonance frequency of the system. The resonance becomes noticeable when there is only little damping. Strong springs with little mass with make high frequencies, loosening the springs and/or raising the mass will reduce the resonance frequency. Damping will kill the oscillations and will produce a stable result faster.

RESULT: IMP = w * [wD + i (1/M-w²/S)] / [(1/M-w²/S)² +(wD)2], times S²M² at both sides:
= wSM * [wDSM + i(S-w²M) ] / [(S-w²M)² + (wDSM)²]
=> 1/D at the resonance frequency w = sqrt(S/M)

Mechanical systems and electrical systems do compare, but to a certain extent. Electrical components can be made quite ideal: capacitors with a good isolation and no leakage (extreme resistance), resistors with hardly capacity or induction by their own, inductors with metal kernels and so on. And those systems are pretty linear over a large range of values, from electron beams in a CRT monitor to outdoor flashes in a thunder storm, the same laws keep applying.

Mechanical components are far less ideal: springs do have mass and damping by themselves, and the stretching is far from linear. When the spring is made of an iron wire coil, there are upper and lower limits to its expansion or compression depending on the length and thickness of the wire used. So in fact the formula should read F = S₀ X + S₁ X³ + S₂ X⁵ + …

Odd powers only, as all forces work in one direction: against the displacement.
The same holds for damping, as can be found in any aero/fluid-dynamics class or textbook. For low speeds it’s proportional, but at higher speeds the additional terms kick in quickly: F = D₀ X + D₁ X³ + …

All this means that the math still can be done but that the final resulting formulas become longer and less friendly.

Conclusions

By looking at complex numbers in addition to normal, real numbers, I got tools for expressing fluctuations in a more elegant way. By applying them to electrical circuits with alternating current, I got the formulas for the “impedance” of a system responding to alternating inputs. By applying those formulas to a mechanical context, I got a descriptor for a mechanical system, responding to external forces.

I found that such a system is characterized by

Mass (M), making it harder for a force to accelerate an object
Damping (D), making it easier for a force to reduce an objects speed, and making the object lose energy faster
Spring Strength (S), making it harder for a force to displace an object
Resonance, the tendency to vibrate with a frequency sqrt(S/M). This is reduced by Damping.

Dynamics V – 2 Crash Course on Sims and Settings

Understanding cloth and having a mental model of the physics involved is necessary but not sufficient to make effective and efficient simulating systems. Creating those systems is a world in its own right. Especially those with an engineer’s way of looking at things might be interested in this mini tour through the deep down dungeons. Crash Course on Math & Physics (previous page) and Crash Course on Sims & settings (this one) really are the math and physics loaded chapters. Any use of the underground escape tunnel brings you into Muppets Lab “where the future is made today”. You have been warned.

In this chapter, I’ll consider the simulation algorithms themselves. Finally, I’ll present some conclusions that help you further in handling the Poser parameters and cloth simulation.

Going Sim

Creating a computer simulation of a mechanical system requires the build of a ball-and-spring network. Like a mesh, the vertexes are small balls with carry all the mass, while the edges connecting the vertices are small (mass-less) springs which do the pushing and pulling. Balls and springs both take their share in damping, and the balls pick up the external forces and limitations like gravity, attached weights, air-damping, collision and friction to objects.

In order to make the simulation work like the real thing, various technical (constructive) aspects require attention. Building goods simulations is a serious profession. To name a few:

Resonance
mentioned already as a ‘natural aspect’ of electrical and mechanical circuits. What does it do?
Say, the resonance frequency is: 100. This means that a vertex is oscillating around its proper place to be, at 100 times a second, real world time (as the values for Damping, Spring strength, Mass, Object size etc. are real world values). If the simulation recalculates the positions of the vertices in steps of 1/1000 of a second, the oscillation would be clearly visible: a predictable instability. The oscillation is samples ten times a period.
If the simulation recalculates the vertex position each 1/60 of a second, the neat oscillation would appear as a random jumping around, as a noise, an unpredictable instability. The oscillation is samples less than once a period.
In all cases, an instability which makes that the sim needs longer calculation times to arrive at a steady result. In other words; we have to wait till the oscillation-energy is faded away. The best way to speed that up, is to increase the damping, as this stands for energy loss per second.
Mesh density
in how many subparts do I chop the system? In cloth, which can be considered a surface, halving sizes in two dimensions imply quadrupling the amount of balls and springs, and calculation time. In Poser I’ve seen pieces of cloth chopped into pieces of 5x5cm (divides 1 m² into 400 balls) to 1x1cm (10.000 balls), but a cotton fibre is as thin as 0.1mm (100 million balls per m²) and no one is building a sim like that. But by not doing so, one runs the risk of “finite element artifacts”.
Non linearity
as long as the mechanics, the systems and the forces considered are linear, increasing mesh density has no effect. For instance, the cloth has a mass density of 1kg/m2, and whether I chop it into 10.000 or 100 million balls will not have effect on the effects of gravity or air damping on the cloth. And when the cloth is 1 m x 1 m and I chop 1 horizontal fibre into 100 little springs, or 10.000, will not have effect on its sheer stretching behavior. But when things are non-linear, mesh density makes a difference.
- Resonance.
  as discussed in the electrical and mechanical sections, each system has some kind of resonance frequency for which the damping is minimal. If I give the system a kickstart, it will send all sorts of signals around in all sorts of frequencies (noise like) but the resonance one stays alive the longest. For mechanics, it’s proportional to sqrt(S/M) for spring strength (say: stretch resistance) S and mass M.
  Now I chop the cloth into pieces, at mesh density d. For instance, d=100 chops the 1x1m cloth into fibers of 1 cm (with stretch resistance S/100) and pieces of 1x1cm (with mass M/10.000).
  As a result, the resonance frequency goes up a tenfold (sqrt(d) to be precise). As stated above, this affects the noisiness, the randomness of the vertex positions.
- Folding
  one can imagine that making an angle between two adjacent polys in a mesh is easy for very small distortions but becomes quite hard for the larger angles. Each extra degree of bending requires more and more force to accomplish. So if the 1x1m cloth is divided into 10x10cm pieces and I need a 30° between two adjacent pieces, or the cloth is divided into 1×1 cm pieces and I need 3° between two adjacent pieces but for 10 pieces in a row, the latter requires less force to accomplish.
  In Poser terms: with the same fold-resistance, finer meshes will fold easier. Mesh density makes a difference.
Edge effects

You might have experienced it when making waves with a heavy rope or garden hose (if not, give it a try). When the other end is loose it jumps up end down wildly. Actually, when your waves in the rope have some amplitude, the loose end will show you the double. You can create some constant wave in the rope if you manage to pick a frequency that creates a specific length of the wave: such that there are ¼ , ¾ , 1 ¼ etc waves in the length of the rope.
When you fix the other end, to a wall or a pole, the other end will be still at any time. You can still make those standing waves but at different frequencies, such that there are ½, 1, 1 ½ etc waves in the length of the rope.
Not only ropes and garden hoses, but simulations as well suffer from edge effects. At the end of the ball/spring network, balls have less springs attached. Does that require heavier balls, more strength for the remaining springs attached to those balls, or just the other way around? Does the sim require fixation (like the rope at the wall) or does it allow for jumping around (like the open end case)?
Anyway, as the image illustrates, the vertices at the edge of a mesh behave different because they are missing forces, as the springs (in between edges) are not there.
All this is up to the sim builder, and there is no real escape. No sims without edge effects, unless one can make calculations on an edgeless world (a globe, e.g.). For cloth and clothes, one might give the edges specific treatment, like is done with real clothes. A rim, with higher mass / density, less stretching and less folding.
Calculation order
There are various ways to get the results of a simulation. Iterating step by step is always required, but one can take a ball/spring network in iteration N, and use that information only to calculate the next iteration N+1. A layered approach, so to speak. Another, more continuous approach, is to take a vertex with the information from iteration N, plus the already derived information for surrounding vertices from iteration N+1, and calculate the iteration N+1 for that vertex.
Again, this is up to the sim builder and there is no escape. The first approach requires more memory (as two nets have to be kept in store) and requires more iterations (as less info is used per step), but is easier to implement as a multi-threaded routine, it will be less prone to edge effects, and if any these will be distributed quite evenly along all edges. The second approach requires less resources, less iterations, but is typically a single-thread routine which will add directional effects to the edges. That is; it will calculate the vertices in a specific order, like top to bottom left to right so edge effects will typically appear in the top left or bottom right corner.

First, there is gravity only. But having the first ball moved will skew the forces on the second, and so on.

Then, you will get this, a piece of cloth skewing to the right-bottom corner when let loose.

I suspect modern simulators like Marvellous Designer to be of the first kind, and Poser Cloth Sim of the second kind as it stems from the Poser 5 period (even before) and has hardly seen a methodical update after then.

Unsimulated aspects
Aspects of cloth which are absent in the sim, are: thread thickness, and weave density. Thicker threads make thicker cloth, with more weight per square inch, are harder to fold especially over large angles, are harder to stretch, and might increase the friction between threads. This friction between threads by itself affects shear and folding, affects air damping, and so on. A high weave density (threads x thickness per stretching meter) has similar effects.
Modern advanced cloth simulations take threads and weave aspects into account. Poser cloth room limits itself to global cloth descriptors which include named details. From that, Poser simulates non-woven materials as well; leather, fleece, rubber and the like. This implies a simulation mechanism which is further away from physical details in the real world.

Perhaps you’re familiar with software creation. Then you’re aware that only 20% of the effort really goes to the program’s functionality, and 80% is about the user interface, preventing people from inputting erroneous values or clicking buttons at the wrong moment or selecting impossible combinations of options; that is: making the software user aware. For sims, things are about similar. 20% is about the simulation itself and 80% is about handling the resource requirements, the artifacts from resonance and edges and non-linearities of nature.
This implies that you might have to upgrade yourself from a Poser User to a Poser Sim User.

Going Poser Cloth Room

Offering a cloth simulator to end users introduces a guaranteed set of shortcomings which are not well understood, by those users at least. Not simulating various aspects of cloth (thread thickness, weave density, weave structure) makes it harder to translate the real world to sim parameters, and impatience combined with insufficient resources introduces algorithms with additional artifacts (like skewing cloth).

Mesh structures, mesh densities and various resistance and damping settings will affect the presence or absence of edge effects and resonance, which – at least from an end users perspective – introduce seemingly random vertex movements and floppy edges. Increasing mesh densities, resistance and damping as well as longer simulation times (more frames) kill those effects eventually. Sometimes raising the damping might just make things worse instead of better. But for most people it’s unclear that they have to cope with simulation artifacts that have nothing to do with cloth behavior or with their use of Cloth Room.

In the end, it’s clear not what is meant by the various kinds of parameters:

Resistance, against pulling or torque, is like electrical capacity: it stores energy in the cloth and releases it later on.
Damping is like electrical resistance: it turns cloth movements into energy loss and brings the system to a standstill.
Gravity, Friction and Wind make the forces (via Mass, or: Cloth Density), and so generate the energy for Resistance to store and for Damping to lose. Which effectively make these the drivers of the simulation.

Unsupported aspects

When I take the backdoor to enter the stage Cloth Room behind the scenes, that is: I take a look at the Python manual, I find some parameters which go unsupported by the properties dials in the 4^th panel of the cloth room. These are all of them:

Airdamping Dial: Air Damping
Clothclothforce always 10, no way to set it manuall (python script required), unclear what is does
Clothfriction Dial: Cloth Self Friction
Dampingstretch Dial: Stretch Damping
Density Dial: Cloth Density (grams per cm2, 1g/cm2 = 10kg/m2)
DynamicFriction Dial: Dynamic Friction
FrictionFromSolid Checkbox: Collision Friction. If checked, the Object parameter are used instead of the Cloth parameters. So either cloth shows a different behavior over rough and smooth object surfaces but the same for all cloth elements, OR cloth shows a different behavior for various cloth elements but the same for all collision objects. In one sim one cannot have both, various cloth elements over various surfaces.
FrictionvelocityCutoff always 30 (cm/s), no Dynamic Friction below this speed, no way to set it manually
ShearResistance Dial: Shear Resistance
SpringResistance no way to set it manually, might be related to EdgeSprings, which are OFF (0)
StaticFriction Dial: Static Friction
Thickness is derived from Collision Depth and Collision Offset, which define distances between the outside of the object and the outside of the cloth, which are the parts we see. These collision parameters BTW are aspects of the objects, not of the cloth so in one sim all cloth elements have the same thickness for a specific object, and another thickness or another object. So Cloth Thickness is not very well represented. I guess it doesn’t do anything
U-BendRate not used, no way to set it manually
U-BendResistance Dial: Fold Resistance
U-Scale purpose is unclear, no way to set it manually
UseEdgeSprings 0/1 whether to use Edge Springs calculations. No way to set it, though. It’s OFF (0). Here you have the ability of Cloth Room to manage edge effects, as discussed in earlier chapters.
U-StretchResistance Dial: Stretch Resistance
V-Bendrate V-values are not used
V-BendResistance ditto
V-Scale ditto
V-StretchResistance ditto

File layout

Cloth info about a cloth prop or cloth figure is stored in the pp2/ppz (prop) and cr2/crz (figure) respectively, and looks like:


    vertsGroup _default_
        {
        v 0 
        v 1 
        … 
        v 6003 
        v 6004 
        stitchVertsGroupProperties 
            { 
            U_Bend_Resistance 30.000000 
            V_Bend_Resistance 5.000000            \ no dial 
            U_Stretch_Resistance 40.000000 
            V_Stretch_Resistance 50.000000        \ no dial 
            Shear_Resistance 50.000000 
            U_Scale 1.000000                \ no dial 
            V_Scale 1.000000                \ no dial 
            Density 0.010000 
            Thickness 0.000000                \ no dial 
            Spring_Resistance 5000.000000 
            Air_Damping 0.020000 
            Dynamic_Friction 0.100000 
            Static_Friction 0.500000 
            Friction_Velocity_Cutoff 30.000000    \ no dial 
            Cloth_Cloth_Force 10.000000        \ no dial 
            U_Bend_Rate 0.000000            \ no dial 
            V_Bend_Rate 0.000000            \ no dial 
            Cloth_Cloth_Friction 0.000000 
            Damping_Stretch 0.010000 
            get_friction_from_solid 0 
            Use_Edge_Springs 0                \ no setting 
            anisotropic 0                    \ no setting 
            } 
        } 
    vertsGroup _choreographed_ 
        { 
        } 
    vertsGroup _constrained_ 
        { 
        v 3 
        v 6 
        … 
        v 5752 
        v 5753 
        } 
    vertsGroup _softDecorated_ 
        { 
        } 
    vertsGroup _rigidDecorated 
        { 
        }

CaseStudy: Hairy Stuff (1 Intro)

As long as your hair is styled properly …

Download this tutorial in PDF format (2.0 Mb).

Introduction

This Case Study is a beginners guide into Poser Hair Room, and will be about assigning fur or short hair to objects, clothes and animals and about assigning facial and body hair to humans.
To deploy the basic techniques I’ll put fur on a purse and on a jacket, and play a bit with hair coloring.

Then we’ll have a look at a pre-made, commercial product: Snow Bride. After that, I’ll consider more elaborate figures like a toon kitten or puppy and investigate under water scenes by considering a long-haired fish as well. Long hair on animals brings up horse manes and tail, so I’ll give that some attention too. The step to humans is a small one then, as long as I start with facial and body hair.

Long hair on human heads will be dealt with in #Photo Shoot#. Have a good hairday.

CaseStudy: Hairy Stuff (2 Purse)

In this chapter I’ll put fur on a simple ladies purse (from the Noctia outfit).

I switch the preview to Smooth Lined (or Flat Lined) to see the polygons, open the Hair Room, ensure I’ve got the Purse figure as well as the Purse bodypart of it selected, I click [New Growth Group] in panel 1 and assign “PurseFur” as the name for the new hairdo.

In order to select the entire surface into my hair group, I click [Edit Growth Group] which opens the Group Editor, and [Add All] adds all polys from the Purse\Purse (=figure\part) to the hair group.

The option Hide Other Objects helps me to concentrate on the Purse object, and when I check Show Multigrouped faces I get the entire purse, as all polygons are part of the group Purse as well as part of the group PurseFur.

For more details on the Group Editor, just consult the Poser Reference Manual. I can close the editor now, and click [Grow Guide Hairs] in panel 2.

Now I realize that I don’t need hairs on the back, I’m only going to render the front side. So I [Edit Growth Group] again, tick the [-] option to subtract faces from the group, position the camera so I can view the purse from aside. While selecting polygons or just clicking them, and by clicking [Grow Guide Hairs] regularly, I can check when I’ve got the proper surface. If I see any hair growing out of unwanted places, I still have to deselect a poly.

Growing Hair
Next, I estimate the purse at about 20cm wide and 10cm high, that makes 200cm2. When the fur comes from an animal with a dense coat I guess I’ll need 400 hairs per cm2 (human heads do about 200), so that will be 80.000 hairs. In this tut, I’ll far less. Given the size of the purse, the hairs can be 1-2cm long, and 1-2mm thick as animal hair. These short, thick hairs covering the purse are not going wild in wind, gravity or any styling so I do not need high mesh resolutions.

So on panel 2: Hair Length = 0,015 and Length Variance = 0,005 (my Poser is set to Meter units, for inch-units you can use about 0,6 and 0,2 respectively). On panel 3: Hair Density = 400,000, Tip Width=1, Root Width=2, Verts per Hair=4. For clarity, I zero out all other parameters in panels 2 and 3 (Kink Scale has a minimum of 1).

Important note: Hair Length and Variance are presented in user units, in my case: meters. My 0.015m = 1.5cm = 0.6″. Hair Density is presented in (hairs per) square user units, in my case: hairs per m2. My 400,000/m2 = 40 /cm2 = 258/inch2.

As all hairs are standing straight out, as I haven’t done anything dynamic and as there is a serious color contrast between the purse and the hairs, the rendered result looks like a porcupine. But I’m not done yet.

Since animal fur grows quite flat onto the skin, I can use the Pull Down dial to establish that effect. There is nothing to Pull back or Pull Side, so I leave these alone (at 0.0).

As the left image (shot from aside) reveals, I need a large value (0.005) and the hairs bend quite sharp. This is due to the lack of resolution, a hair can only bend at a vertex. So I turn the 4 Verts per hair up to 32. And now I get a much smoother result, at a much lower value (0,00004) – right image. And as I don’t need clumpiness or kinks in this straight animal fur, I decide that this should do.

From this I can identify some areas for improvement.

The hairs are the bottom should be shortened (shaved).
I need a coloring of the purse and the hair, and blend them together
If then still needed, I might need a higher hair density. This is simply a matter of raising the value and turning the appropriate dial. Twofold, Fourfold, I’ll see.

Shaving
Of course I can remove the bottom area from the PurseFur hair group, and create a new one with shorter hairs. But now I’m going to alter individual hairs instead, and so I click [Style Hairs…] in panel 3. The Select+ option is on by default, and I start selecting all hair(tips) that I want to alter.

Then I use the Lengthen dial (rotate left) to shrink the selected hairs. When done I [Clear Selection], spot any hairs I missed and give them the same treatment.

Notes:

There is no way to save selections for later use. Once done, I’ve got to reselect them.
Any touching of button, dial of value field in panel 2 will completely undo all styling for all hairs in the hair group at hand. This means that I must be sure about Length and Pull settings before starting to style.

Dye Hair
In order to change to coloring of the purse and the fur, I step into Material Room, Advanced tab, and select the PurseFur prop from the objects list. This prop contains all hairs in that group, but not the purse surface itself.

The fur material is set to its Blonde default, and the first thing to do is to increase the Root Softness. This blends the root of the hair into the surface by making the hair increasingly transparent from the outside in. But in this case, the contrast between haircolor and surface color is so strong that I really need recoloring. Since the purse is dark teal/blue-green I selected Root = RGB(31,33,32), Tip = RGB(71,85,99) and Specularity somewhat in between, and set Root Softness to 0.5.

The result was not that satisfying, so I severely reduced the Specularity (and Alt_Specularity) of the purse itself, but reducing the brightness or their color swatches. And – as stated in the improvement notes above – I drastically increased haircount at the cost of longer render times and higher memory consumption. From 400,000 to 1,280,000.

To have more fun, I turned the purse into Marsupilami, the comic animal by Franquin (www.marsupilami.com).

So I altered the material for the purse itself into a simple one.

And I copied the Spots into the material of the PurseFur object, in between the original Noise and Hair nodes.

Since I’m using the same Spots in all places, any differences come from the filtering by the colorswatches. The Diffuse and Specular in the purse material itself both are the darkest, while Specular has a reduced value (0.1) as well. Only metals have a high specularity. My idea on finished animal furs is that the skin itself has the darkest tone. In the PurseFur hair material, the Root color is the next darkest, the Tip color is just white and the specular for the hair is somewhat in between with a slight brownish hue. The effect of the (already present) Noise node is very visible in the spots now.

And since the hair matches the cloth of the purse, I can do with a reduced hair density. So I lowered it from 1,280,000 to 320,000, and got after rendering:

CaseStudy: Hairy Stuff (3 Jacket)

This is the jacket from the Witch Hunter for Vicky4.

It has some decorated parts on the sleeves and the front, and I’m going to “furrify” the smooth parts. This is not very different from handling the purse, but let’s look before we leap.

From a fur point of view, the jacket has a Left Sleeve with the hairs growing sideways (positive X, to our right), a Right Sleeve with the hair growing sideways (negative X, to our left) and a Body. Each of these is built from three mesh components:

Left Sleeve = Left Collar + Left Shoulder + Left Forearm
Right Sleeve = Right Collar + Right Shoulder + Right Forearm
Body = Hip + Abdomen + Chest

As I found out by clicking around on the jacket. In the Materials Room I found portions for

A Pen, that’s some jewelry sticking upon the left front of the jacket
Sleeves, that are the ultimate ends at the arms
Jacket, everything else

Each sleeve – from arm pit to wrist – measures about 50cm long x 40cm circumference = 200cm2, the body measures about 70cm high x 100cm circumference = 7,000cm2, minus 20% for the open front minus 20% for the decorated parts make say 4,000cm2. From the 7,000cm2, half of it sits at the back of the body, which leaves 4,000 – 3,500 = 500cm2 for the front side of the jacket. We’ll need this info to set the haircount to reasonable values.

Setup
I enter the Hair Room, select the Right Collar (mesh group / subobject / bodypart), start [New Growth Group], name it (“RightArm1” for instance), click [Edit Growth Group] to collect the polygons for the group, and just [Add All] to finish. Perhaps I’ll need to do some more detailing later on, but this gives me a start.

I select the Right Shoulder in preview, repeat the procedure (“RightArm2”, select Right ForeArm, repeat the procedure (“RightArm3”) but instead of using [Ad All] I click [Add Material], as I want to exclude the decorated ends. Those ends were assigned to a Sleeves material group, so I select this, and then click [Invert] because I just want the other part.

While the jacket has object-groups and material-groups which technically are independent, I must create hair-groups for each separate object-group, and I can use the material-groups for further selection within that object-group. Like above, Sleeves addressed both left and right end but while working on the right arm object, I could address the right part of the material group only.

And I cannot make one single “Right Arm” hair group, I end up with three parts. When I select the Right ForeArm in preview, then hairgroup RightArm3 is accessable only.

This way I also build my LeftArm1/2/3 hair groups, and also make my JackBody1/2/3 groups out of Hip, Abdomen and Chest. For defining JackBody1 from the Hip etcetera, I manually select the relevant portions as I’m avoiding the decorated parts and – for the moment – I don’t need hair on the back for my current renders.

Using the mouse for selections requires some practice. Adding, deleting and turning cameras is a must for the fine details. Sometimes it pays to make larger selections, and exclude (using the [-] option) some polys later. Some people might prefer a tablet for this kind of work.

Sometimes handling logic brings a way out. Excluding the Pin on the chest part could be done easily, by selecting the jacket part completely, then inverting (to everything but…), then adding all polys with the “Pen” material like I did with the Sleeves material before, and Inverting again.

Time to save now.

Growing fur
My next step is to grow fur on one area (say: Chest = JackBody3) , adjust it till I like it, and then (manually) copy my findings to the other areas. So I select that part of the jacket (preview), as well as the hairgroup (panel 1), and deploy panel 2 to [Grow Guide Hairs] with a length of 3.5cm, a Variance of 0.5cm (so: hairs will be 3-4 cm), no Pull back or Pull Side but a serious Pull Down supported by enough Verts per Hair. Kinks and Clumpiness could be zeroed out, and I vary the hairs between 2 (root) to 1 (tip) mm.

It renders like this:

From this I learn that it’s better to exclude the polys from the hair groups that do not face the front, but face up, down or sideways instead. The hairs from the side do not contribute to a nice result.

When re-selecting the bodypart for re-editing, be sure that you’ve got the object part (here: Chest) and not the hair-prop JackBody3. The first one contains the Hair Growth Group including all settings, the second contains the (guide) hairs that result from that. Check Hide Other Objects in the group editor to get rid of the hairs while (re)selecting.

Then I need to color the hair, and I need to set the hair density to a proper value. The full front of the jacket was estimated 500cm2, so this third part might be 180cm2 and with 200 hairs/cm2 that makes 36,000 hairs. To be halved or doubled depending on coloring.

Handling the material
I selected the Root color to match the purple surface RGB=(60,20,60) while making spots of darker tips RGB=(30,10,30) and added the same spots to a bright specularity RGB=(120,40,120), all in the same hue.

Once I like it, and once I’ve made another hair group, I can copy this material to the other hair groups:

With the triangle at the upper right corner: Select All
Copy
Then select any other hair Prop (the prop, not the growth group body part),
And with the same triangle-menu: Paste
Then, from the list of buttons: [Remove Detached Nodes] to clear the original hair material
Select another hair prop, paste again, remove original nodes, and so on.

Details galore
Hair was quadrupled (144,000 for this area) and I’m still convinced that the top inner side, and the top outer side can use a shave. This is due to the high density of small polygons to make a smooth bend in the jackets form. With a guide hair per vertex, and populated hairs per guide hair, many small polygons mean: many hairs in that area.

Now I’ve got my settings I can do the other parts of the jacket.

Hair length 0,0350
Variance 0,0050
Pull 0,00012
Density (72.000 to 144.000)
Tip 1
Root 2
Verts 32

The arms have the hair pulled sideways instead of down, require far more pull and mind the +/- sign for the proper direction.

This still leaves room for perfection:

Around the Right and Left Collar or: RightArm1 / LeftArm1 elements, hair (polys) should be deleted and/or hairs should be shaved using the Hair Editor, and hair Pull and Density have to be looked into. Plus the details for shaving mentioned already for the upper/inner portions of the jacket. As usual, the devil is in the details, and so is our time.