1. 缩放矩阵是对角矩阵，对角线上每个值表示缩放的比例
2.
2D Shear
Shear transformations produce a shape distortion.
(old coordinates are (x, y) and the new coordinates are (x', y'))
X-Direction Shear is given by the following matrix:
(1 0 0)
(SHx 1 0)
(0 0 1)
( 1 0 0)
(x' y' 1) = ( x y 1) * (SHx 1 0)（这里使用齐次坐标系表示的，所以多了一个1）
( 0 0 1)
Which produces a shearing along x that is proportional to y:
x' = x + SHx * y
y' = y
1 = 1
Y-Direction Shear is given by the following matrix:
(1 SHy 0)
(0 1 0)
(0 0 1)
(1 SHy 0)
(x' y' 1) = ( x y 1) * (0 1 0)
(0 0 1)
Which produces a shearing along y that is proportional to x:
x' = x
y' = x * SHy + y
1=1
shear的过程中x和y中一定有一个是保持不变的。
3. So if you have a location on the object which is say the center of mass plus some displacement,rotating that is equivalent to rotating the center of mass and then rotating the displacement.
旋转是线性操作，在2D中是可以交换的，但在3D中不可以。
旋转的矩阵表达可以通过极坐标系进行推理。