unity，UNITY_PROJ_COORD和tex2Dproj

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