切變是坐標系的變換，非均勻的拉伸。切變時候，角度變化，但是面積或體積不變。也可以理解為坐標軸間的角度變化，造成的扭曲。

如下圖，這是x坐標根據y坐標的切變，機器人的y坐標沒有變化，只有x坐標變化了，變化后的坐標x`可以理解為將y坐標乘以切變因子s與原坐標x的和：x` = x + sy。如果是3D則增加z坐標的切變因子t：?x` = x + sy，y` = y + tz

切變因子通過切變角度計算。

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那么構建根據y坐標切變x坐標的代碼為：

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);

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