tex2D vs. tex2Dproj
texCoord(  texX, texY, texZ , texW ) means texture coordinate after transforming from the texture matrix [ position goes through world, view, projection, and texture ] .    We can use this texCoord to fetch pixels from a texture, sampler :  1.  float4 color = tex2D( sampler,  float2( texX / texW, texY/texW );  2.  float4 color = tex2Dporj( sampler, float4( texX, texY, texZ, texW ) );   The above two methods will have the same result because tex2Dproj  operator supports divide w in its interface.    http://www.gamedev.net/community/forums/topic.asp?topic_id=408894  http://bbs.gameres.com/showthread.asp?threadid=104316
