hi all,

back again from the dungeons of Muppets Lab, Friction Research section :). From doing an awful lot of tilted plank and sliding cloth experiments, the physical well understood

a = Gsin(z) - Dcos(z)
for resulting acceleration a, gravity constant G = 1,09 (= 9.8 Earth value * 100 cm per meter / (30 frames per second)^2 ), tilt angle z and dynamic friction D

is a close enough approximation of the measured results. So, our Poser cloth parameter for Dynamic Friction can be set to real-life values for cloth and skin, which ranged from 0.65 (skin to metal) to 0.75 (skin to plastic).

For those who want to redo the experiments, a far better approximation of the results was found as a = G^2 * [ sin(z) - D*cos(z) ] for which however I don't have a proper physical understanding. But since Poser is not a science lab and G is about 1, the statement that we can do with reallife values is not that far off, and the best workable.

It was also observed that the formula did not hold very good at low cloth speeds, where also Static friction became a factor of influence. It was not at higher cloth speeds. It was observed too that no value for Dynamic friction could bring the cloth to a stand still, nor could prevent it from moving. Only Static friction could do the latter.

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 above intends to give some idea of this.
In the mixture, Static became noticeable only at low Dynamic values (< 0.1) or at Staitic values close to the critical one, where it prevents the cloth from moving at all.

The Static friction parameter could be measured by just tilting the plank until the cloth started moving. Higher friction means a larger angle, so each angle has a critical value for Static. Any value below that will not prevent the cloth from moving, then the speed increases, the effects of Static fades out while the effects of Dynamic kicks in.

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

1. low speeds at low Dynamic friction is very hard to do measurements on. 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.
2. unfortunately, the low speed 1-10 cm/frame range is the one for clothes under normal moving conditions. Bad luck.

The values I found did not resemble any physical meaning to me.

angle theory found

 5 0.0875 0.0105
10 0.1763 0.0295
15 0.2679 0.0400
20 0.3640 0.0550
25 0.4663 0.0700
30 0.5774 0.1278
35 0.7002 0.2000
40 0.8391 0.5757
45 1.0000 0.8625
50 1.1918 0.9480 

This means that at a tilt angle of 30 degrees the cloth started moving when the Static friction value came below 0.1278, while physics theory says that at that angle the friction value is 0.5774 (=tan(z)). And since the thoeretical values hold quite well for the Dynamic friction, we could use this table to bring both in line.

For instance, a smooth material with Dyn friction = 0.2 might have s Static value which is in nature a bit higher. in the table we can find that at say 15 degrees (value 0.2679) where we read that we should set the Poser Static friction to 0.04.

Same way, for skin and cloth Dynamic is about 0.7, and at 35 degrees we find that the equivalent Static is 0.2. But we can also see that at that point it rises quite sharp, so a slightly higher Static value might quite well be 0.5 or so. Which is the Poser default.

Hence, the Poser Dynamic friction does resemble real life good enough but the 0.1 default is far too low for skin and cloth. The Static friction does not resemble anything in real life to my current knowledge, but the default is not that bad for skin and cloth. But to stay in sync with smooth materials and quite lower Dynamic friction situations, Static is dropping to zero awfully fast.

= = =

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.
On top of that, the great SM example 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.

Besides all that, I noticed that the cloth and the plank always had a 2" distance after dropping / draping, whatever settings I used (especially for calculation offset and depth) wherever in the program.

Now back to the dungeons, focus on the stiffness parameters.

- - - - - 

Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though