A Simple Cloth Simulator

Selected Topics in Computer Graphics

A Simple Cloth Simulator

Authors

Yair Yarom (irush at cs.huji.ac.il)
Ely Levy (elylevy at cs.huji.ac.il)

Getting the simulator to work

Get the tar ball and open it
Use make to compile
- Use make opt for optimized version
- compiles on Mac OS X/FreeBSD/Linux
Usage: sim <cloth file>

Controlling the simulator

Mouse Control

Use the left button to control the arcball and rotate the cloth.
Use the right button to control the arcball and rotate the light source.

Keyboard control

Key	Description
m	Change cloth movement algorithm (none, Euler, midpoint, Runga-Kutta)
x	Toggles adaptive step on/off
p	Pause
space	Moves the cloth a single step
d	Switch particles drawing method (none, dots, lines)
c	Toggles cloth drawing on/off
o	Switch obstacle drawing method (none, wire, solid)
n	Toggles normal drawing on/off
b	Toggles blending on/off
f	Focus on the cloth
a	Toggles auto-focus on/off
r	Resets the cloth's parameters (position and velocity)
l	Loads a different cloth file
k	Reloads the last cloth from file
t	Resets the cloth's and light's arcballs
s	(Un)stuck the stationary points in place
q	Quit

Cloth file format

Each line is of the format
<attribute>: <space delimited parameter list>
The colon is optional
Lines starting with # are ignored (comments)
Some attributes may appear more then once (pos, stuck, hole, sphere)
The other attributes, if appears more then once, the last one will be considered

Attribute	Parameters	Description	Default value
Size	int u,v	The cloth's size: uxv	20 20
Mass	double m	The mass of each particle	2
Base	double x,y,z	The base position of the cloth (particle 0,0 that the cloth is positioned by)	0 0 0
Direction	({x,y,z},{x,y,z})	The direction to the u,v axis of the cloth. i.e for standing cloth Direction y x	x z
Distance	double d	The starting distance between each two neighboring particles (up,down,left,right)	1
Breaking	double d	The maximal stretch between two neighboring particles before the cloth tears. To disable tearing set to 0	0
Step	int s	The number of milliseconds between each simulation step	20
Time	double t	The number of seconds that had passed between each step (delta(t))	0.02
G	double g	The gravity constant	20
KS	double KS	KS value for all 3 spring types
KD	double KD	KD value for all 3 spring types
Near	double KS,KD	The KS and KD of the near spring (up,down,left,right)	3000 50
Far	double KS,KD	The KS and KD of the far spring (up up,down down,left left,right right)	500 10
Cross	double KS,KD	The KS and KD of the cross spring (up left, up right,down left, down right)	2000 10
Drag	double d	The drag force constant (0 to disable)	0.2
Wind	double x,y,z,f	The wind's direction (x,y,z) and power p (set power 0 to disable)	1 0 0 0
Pos	int u,v double x,y,z	Sets the starting position of particle (u,v) to (x,y,z)	Per particle according to grid
Stuck	int u,v	Makes particle (u,v) stationary	(non stationary by default)
Hole	int u,v	Disconnects particle (u,v) from all its neighbors	(no holes by default)
Color	{front, back, background} double r,g,b,a	Sets the color of the cloth (two sides) and the color of the background	front 0.2 0.2 1.0 1.0 back 0.2 1.0 0.2 1.0 background 0 0 0 1
Blending	none	Begins simulation with blending turned on	(blending is off by default)
Movement	{null, euler, midpoint, runga-kutta}	The movement algorithm to use	runga-kutta
Sphere	double x,y,z,r	Places a sphere obstacle with the center at (x, y, z) and with radios r	(No spheres by default)
Adaptive	none	Begins simulation with adaptive step turned on	(adaptive step is off by default)

An example cloth file with the default values is available.

Technical details, implementation, special features, etc.

Cloth

The cloth is represented by an array of particles (particle_t). Each particle (besides the position, velocity, forces and mass) has 4 pointers to its neighbors (left, bottom, right and up). Using this implementation, it was easy to rip the cloth apart by simply disconnecting far away particles (originally used as a debugging tool), or to make holes in the cloth (or splitting it completely).

Movement

Three movements algorithms are implements: Euler (forward), midpoint and Runga-Kutta (4^th order). In addition, the adaptive step algorithm was implemented, and can be activated on each of the movement algorithms. We also read about higher order Runga-Kutta and about a method to do adaptive step Runga-Kutta using 6^th order. But we decided that it wasn't that interesting to implement as even Euler with adaptive step was stable and fast enough on most clothes.

Forces

Each force type, calculates it's affect on a particle, and add the value to the particle's total force.

Gravity:
Simple gravity force: mass * g, in direction -Y
Springs (near, far and cross):
Implemented using the formula: dx * ((ks * |dx| - dist) + (kd * (dx ^. dv) / |dx|)) / |dx|
Where dist is resting length of the spring.
Drag:
The drag force is calculated so that the particle will gain a velocity which will decrease it's current velocity by the drag constant in the direction of the particle's normal. Thus, the drag constant should be in the range [0, 1], where 0 disables the drag and 1 mean very heavy drag. Also, a vertical cloth will fall faster than a horizontal one.
Wind:
The wind adds force to the particle in the direction of it's normal with size equal to the dot product between the normal and the wind's direction, multiplied by the power parameter. This way, the wind has full effect to perpendicular cloth and almost none to a parallel cloth (parallel to the wind's direction). Also, a random value of up to 1% of the power parameter is added in each direction of the wind. This is done to add some noise so that the wind will have some affect on cloths that are parallel to the wind's direction. Also one can add random noise without wind by specifying wind direction of 0 0 0 and as much power as he wants.

Obstacles

Currently there is only one obstacle - a sphere. This is (very) simply implemented, by simply checking the position of the particles as they move, and throwing them outside the sphere if they get inside. This isn't a very good simulation of the real world, but it acts nicely on our cloths.

Example clothes

Every exploding/riping cloth is best viewed with dots drawing enabled (the 'd' key).

To show how our exercise behaves we chose some clothes with interesting features,and some which don't behave exactly as one might have wanted but still are fun to see. (at least for us...).

	Most of the cloth simulation work is about finding the right parameters but until you do, you can expect some unstable behavior. For example while trying to stabilize the 50x50 hanging.cloth we made it too heavy causing it to rip (rip.cloth), or we chose too strong springs creating a nice vibration effect (wavy.cloth) or making it explode all together (shredded.cloth better viewed with dots enabled). And last but no least tearUp.cloth which is not related to the hanging but still has nice tearing effect.The biggest cloth we tried which is a good example for the power of adaptive step is huge.cloth. Disabling the adaptive step will cause a shower of cloth particles (With dots on of course).
	Another set of parameters we got to play with was the wind and drag constants. We placed 3 clothes, with no stationary points, and let them fall. In addition, the edge of each of them was a bit tipped over. in falling.cloth the cloth starts to fall left because of the drag, although the wind is to the right. In falling2.cloth, the edge is tipped stronger, letting the wind have more effect. in falling3.cloth, there is no wind at all.
A feature we found very interesting was the hole feature. We added a simple ability to disconnected particles. This came up after adding the tearing effect and seeing that it created some cool clothes. We took for example flagbig.cloth (which got bigger since) and with a few holes created flag.cloth, which is a different type of flag shape. Another example for the hole feature is four.cloth we actually have 4 different clothes (with dots on you can see the disconnected particles). And of course we can use the holes to make, well, holes in the cloth like in holes.cloth.
	We didn't have time to make all the obstacles we planned, for example we didn't get to add a floor, but we felt that we must have some obstacle, so we used a sphere. A simple example will be our sphere.cloth and sphere2.cloth which are just a cloth falling on a basic sphere.We of course added the ability to add more than one sphere (as seen in sphere2.cloth). A fun thing we noticed was the impact of the spheres on the cloth when the sphere is small enough and the speed is high enough. We created bullet.cloth and then by the long tradition of violent games created bullets.cloth....
Last some wind experiments: simulating a sail in sail.cloth, few nice strips in the wind in strip2.cloth and strip3.cloth and attempt at water.cloth simulation using wind and playing with the springs constants.