Cloth Body

• BodyCloth class

As it was already mentioned, each particle is a sphere with a set radius. That brings the following:

• Cloth particles use continuous collision detection, so higher values are preferable for more robust behaviour. Collisions between the particles are not calculated and should not be considered when setting a radius. Be careful, however, as particles too big in diameter can provide incorrect interaction with environment (twitching or blowing up of the cloth). Too low radius results in poor collision handling.
• As cloth particles are approximated with spheres, the cloth never lies flat on the ground or tightly adheres to the surfaces. There is always some gap that corresponds in size to set radius.

The number of iterations allow to control the accuracy of the solution of cloth inner joints. For example, it may help to avoid twitching of cloth.

• The minimum value of 1 means the joints are solved once per physics frame. Solving is performed in a random order to provide more predictable stretching results.
• Low number of iterations results in faster simulation. At the same time, it is more prone to stretching and gives cloth more elastic appearance.
• The higher the iteration count, the more accurately constrains are solved and the stiffer the simulated cloth is. Increased number of iterations is considerably expensive and at some point ceases to bring a noticeable benefit of increased accuracy, so it is reasonable to remain within cost-effectiveness limit.

Linear restitution determines how far the cloth particles can be stretched apart from each other. It enforces cloth joints to restore the distance that was between the vertices of the original mesh:

• By the maximum value of 1, the particles spring back with great force and the cloth is hard to stretch. It gives the effect of stiff non-stretch cloth, e.g. linen or tweed.
• The lower the value, the more easily the particles are moved away from each other and the more stretchable and elastic the cloth is. For example, nylon or spandex.
• 0 and near zero values are not allowed because they cause unstable simulation and blowing up of the cloth.

Angular restitution determines the possible angle between cloth triangles that are formed by particles. It constrains the folding of the cloth by enforcing joints to keep the angle that was between triangles of the original mesh:

• By the maximum value of 1, the angles tend to be retained and the cloth resists folding. The cloth appears to be stiff and wrinkle-free (for example, paper). Be careful, however, the maximum value may provide unsteady behaviour.
• By the minimum value of 0, the cloth can be easily folded and bent in any direction, regardless of the original topology of the mesh.

Linear threshold sets the distance limit for stretching the cloth. When two particles move away from each other further than this limit, joints that connect them break and the tear appears.

• If set to infinity (inf), the cloth is stretched without tearing.

Same as linear threshold, angular threshold represents a maximum angle to fold the cloth relative to its initial state. If cloth triangles are bent any further, the joint break and triangles are separated along the adjacent edge.

• It is reasonable to set the angular threshold no higher than 180 degrees.
• If set to infinity (inf), the cloth is folded without tearing.

Convincing simulation of the clothing on a skinned character requires a different approach. To follow bones transformations, each vertex of the cloth that is found in the pin joint area is mapped to the nearest skinned mesh vertex (up to the distance specified by the Threshold parameter). However, it is not recommended to attach the cloth directly to the skinned character, because difference in topologies may result in visual artifacts. Instead of it, it is better to create an identical cloth surface on the skinned mesh character, make it invisible and attach a physical cloth to it.

For example, we need to create a cape that is glued to the shoulders of skinned character while the rest flaps and folds loosely. It is done in the following steps:

1. When creating a skinned mesh, add a cape surface identical to the cape that the character would wear. It can be either a whole cape, or only its clipped part that needs to be pinned. The latter is preferable when complex clothing is simulated, which requires more flexible control over which parts to simulate physically and which ones to move with skinned character. In our case, it's cloth part over the shoulders.
2. Add skinned mesh that has a rag doll body assigned. Take notice, the cape surface should be enabled.
3. Add a separate dynamic cloth mesh and synchronize its position with skinned character. Turn off physical simulation and assign cloth body.
4. Pin cloth body to rag doll body. If the Threshold distance is set low enough, the physical cape will be automatically pinned only to the cape surface (i.e. shoulders). After that, the cape surface is simply disabled and does not provide any load at all.