切变是坐标系的变换，非均匀的拉伸。切变时候，角度变化，但是面积或体积不变。也可以理解为坐标轴间的角度变化，造成的扭曲。

如下图，这是x坐标根据y坐标的切变，机器人的y坐标没有变化，只有x坐标变化了，变化后的坐标x`可以理解为将y坐标乘以切变因子s与原坐标x的和：x` = x + sy。如果是3D则增加z坐标的切变因子t： x` = x + sy，y` = y + tz

切变因子通过切变角度计算。

那么构建根据y坐标切变x坐标的代码为：

float s = tan(D3DXToRadian(30));
D3DXMATRIX matShear(1.0f,0.0f,0.0f,0.0f,
s,1.0f,0.0f,0.0f,
0.0f,0.0f,1.0f,0.0f,
0.0f,0.0f,0.0f,1.0f);
g_device->SetTransform(D3DTS_WORLD,&matShear);