Cloth dynamics in nature is a complex phenomenon and depends on the forces acting inside the cloth material. These forces depend on many many things - the material of the cloth threads, how they are interwoven, the age of the material etc etc. Further on, there are many cloth-like materials like leather and plastic which are not woven from separate threads but are one continuous piece.
In general, we don't know the internal cloth forces precisely and even if we did, they would probably be too complex to implement in a practical solution. However, we have noticed two main properties of cloth and cloth-like materials:
- they stretch to a different degree
- they bend to a different degree
Simulating those two properties is usually enough to give us plausible looking simulation of cloth and cloth-like materials. For example, silk stretches a little bit, but it bends easily and forms wrinkles. Paper, on the other hand, stretches and bends very little and quickly resumes its original form (we ignore phenomena like tearing and edge folding).
Additional forces like air resistance and wind help to make the simulation even more realistic.
This example demostrates the effect of stretching forces. The only difference between the cloth pieces is the stretch value. It should be noted that higher stretch values make the simulation slower, although the slowdown is not as great as when increasing the damping forces (see below).
Damping forces are essential for realistic cloth simulation. Cloth in nature is severly damped. Without the damping forces, the cloth appears too jumpy and ruberry. The only difference between the cloth pieces below is the stretch damping value. Note how low damping values make cloth more rubbery and restless. Higher damping values cause the cloth to go into a rest state faster. Higher damping values can also make the simulation substantially slower, as it becomes more difficult to find the correct solution to the dynamics equations.
The following animation demonstrates the effect of bend forces. Small bending forces cause the cloth to look soft and silky, while higher bend forces make it look like paper or metal sheet.
The following animation demonstrates the effect of bend damping forces. Again, low damping causes the cloth to look rubbery and springy, while higher damping forces make the cloth settle faster. Note that high damping forces can make the simulation considerably slower as the dynamics equations become more difficult to solve.
