（unfinished）
Prerequisite: Knowledge about mechanics of materials(helpful link)
Elastic materials are important because:
- Cool visual effects
- Not too hard to implement
- Base of other materials
Deformation: A mapping function from rest material position to deformed material position.
Deformation gradient F:
Deform/rest volume ratio $J=det(\textbf{F})$
Hyperelastic materials: materials whose stress-strain relationship is defined by a strain energy density function.
- Stress: Internal elastic forces to let material resotre original shape.
- Strain: Just replace it with deformation gradients F for now.
The Finite Element Method(有限元)
Linear tetrahedral(triangle) FEM
affine transform
F is constant within an element