hi all,

from about Oct to Dec 2010 we ran a great thread on Dynamics anything, me and Bagginsbill and others doing some research.

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2815804

Somewere halfway I did the details on windforce (page 10, Dec 22 11;12 am).

Quoting myself:

gravity acceleration in Poser is 9,8 m/s2 ( = 9,8 * 100 / (30*30) ) = 1,089 cm/frame^2. It is slightly different from the one used in the Gravity script from the Scripts menu, so take care when mixing results from both.

density in Poser is gram/cm2, so the default 0,0050 means 50 grams/m2, about half the value for office paper and good for light linen. The max 1.0 means 10 kg/m2 which is about a sheet of lead of 1 mm thick.

air damping in Poser is gram/cm2 per second (not per frame!). An object that feels a force will accelerate, the airdamping will increase with velocity, and this results in a maximum speed for the cloth relative to the wind/atmosphere. So when out figure is wearing a gown, and some body parts are moving at about or above this speed limit, we can expect the cloth sim to break or to show that the other forces have to work hard to make it possible.

For a force of 1G (gravity) this speed is v = d.g/a for the Poser dial values d (density) and a (air damping). So our default light linen reads: v = 0,005 * 9.8 / 0,02 =  2,45 m/s (= 2,45*100/30) = 8,16 cm/frame. Which is not that much, and does have considerable effects on the speed of our animations, and the amount of frames needed to establish a decent simulation result.

windforce amplitude in Poser is m/s, that is: amplitude 1 applied horizontally to a cloth of default material (light linen) pulled down vertically by gravity, will bring the cloth into a 45 degrees angle. That is: 3,46 m/s. For amplitudes below 1 the dial behaves in a linear way, so 0.5 implies half the windspeed. For windspeeds above 1, doubling the dial value quadruples the windspeed. This translates into: 

 - amplitude 1,0 is the upper limit of Beaufort 2, light breeze. 
 - amplitude 0,5 is the upper limit of Beaufort 1, light air 
 - amplitude 0,1 is the upper limit of Beaufort 0, calm 
 - amplitude 2,0 is the upper limit of Beaufort 6, strong breeze
 - amplitude 3,0 is the lower limit of Beaufort 12, hurricane

Relevant for those who want to combine poses, moves, cloth sims and falling props into one believable shot. For instance: at windforce 2, the wind speed is 3,46 * 4 = 13,84 m/s. At default air damping, this will exercise a force of 13,84 / 2,45 (see above) = 5,65 G's on the cloth. This requires a figure working very hard to walk slowly forward against the wind, and also requires large stretch resistance values to prevent the gown from being ripped in pieces.

So I do hope that some understanding not only enables you to make believable dynamic clothes, but also to make believable dynamic pictures.

End quote.

And by the way, the wind generator is limited to some distance. You get full speed within that range, and no speed at at outside it. the sudden drop-down will give weird effects on large pieces of cloth.

happy posing.

- - - - - 

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