Cloth Body

• BodyCloth class
• An example illustrating the use of cloth body and particles joint
• Fragment of the video tutorial on physics illustrating simulation of cloth using Cloth body

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 behavior. 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 controls the accuracy of the solution of cloth inner joints. This number indicates how many times the joints are solved per physics frame. Joints are solved in a random order to provide more predictable stretching results.

• Low number of iterations results in faster simulation. However, in this case the cloth is more prone to stretching and looks more elastic. The minimum value is 1.
• High number of iterations provides more accurate solution of constraints. In this case the cloth lookes stiffer. The maximum value is 16.
  Notice

  Increased number of iterations is considerably expensive and at some point ceases to bring a noticeable benefit, so it should be kept within a reasonable 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 easier the particles are moved away from each other and the more stretchable and elastic the cloth is, e.g. nylon or spandex.
  Notice

  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).
  Notice

  The maximum value may provide unsteady behavior.
• 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. This value is set by default.

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 breaks and triangles are separated along the adjacent edge.

• If set to infinity (inf), the cloth is folded without tearing. This value is set by default.
  Notice

  It is recommended to keep the angular threshold lower or equal to 180 degrees.

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 particles joint area is mapped to the nearest skinned mesh vertex (up to the distance specified by the Threshold parameter of the particles joint).

For example, we need to create a cape that is glued to the shoulders of a 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 ragdoll body assigned. Make sure that the cape surface is enabled.
3. Add a separate dynamic cloth mesh and synchronize its position with skinned character. Turn off physical simulation (CTRL + SPACE) and assign a cloth body.
4. Attach cloth body to ragdoll body. If the Threshold distance of the particles joint is set low enough, the physical cape will be automatically attached only to the cape surface (i.e. shoulders). After that, the cape surface is simply disabled and does not provide any load at all.