   D3DFVF_XYZRHW what rhw stand for and used for?
  Hey guys,I'm really confused that what rhw value is used for when I set the FVF D3DFVF_XYZRHW. I modified the rhw value of the vertices to 1.0f or 2.0f or 3.0f, and then I got the same result on the screen. I google this question, I find an answer about that. It said that "1/W (the fourth coordinate) is useful to do perspective correct interpolation. Without it, color, texcoord and depth buffer values wouldn't be reliable." But I'm not sure this mean. Can someone interpret it? Thanks a lot.   
  It stands for "reciprocal of homogeneous W".  What it means is that the X, Y, Z, and W coordinates have already been multiplied by the reciprocal of homogeneous W (AKA divided by W) after transformation, which means that the X and Y coordinates are ready to be used as screen coordinates.  When you're dealing with untransformed vertices and you're transforming them in the vertex shader, you need the W value because you need to multiply X, Y, and Z by its reciprocal after interpolation.   ;  Thanks for your answer, Matt. But I also want to know why I change the W value, it's no effect to the final result. The color interpolation result is same. Whether I misunderstand something or not? I want to know how the value of W affect my result. Thanks a lot!   