看ProjectorMultiply.shader，有这么一句：
fixed4 texS = tex2Dproj (_ShadowTex, UNITY_PROJ_COORD(i.uvShadow));
查了一下：
UNITY_PROJ_COORD：given a 4-component vector, return a texture coordinate suitable for projected texture reads. On most platforms this returns the given value directly.
参考：https://docs.unity3d.com/412/Documentation/Components/SL-BuiltinMacros.html
tex2Dproj:either by explicitly dividing x and y
by w
or by using
tex2Dproj.
参考：https://en.wikibooks.org/wiki/Cg_Programming/Unity/Projectors