   Can we multiply the inverse projection matrix?
  it seems that the projection is not invertable. Or do we need some other information to do such kind of stuff.   
  What kind of projection matrix are you referring to?  The OpenGL/D3D projection matrices are invertible, while a typical projection matrix in the mathematical sense is not.   ;  What type of projection does the matrix encode?If it is a parallel projection with znear and zfar in the same distance, the matrix cannot indeed be inverted. Though if this is the case, it should be trivial to derive the inverse of x and y coefficients by hand.   ;  1. My application render a quad with perspective projection onto a rendertarget.2. Then I render the previous pass onto a fullscreen quad whose z = 0.  I need to get the quad position of the 1st step from pixel shader in the 2nd step. So I thought about invert the projection matrix, and get worldview coordinate back. Is it possible?I thought after projection, z axis is lost. and it seems not to be invertible.   ;  invert to what exactly , the pipeline goes like thisObjectspace -> WorldSpace -> ViewSpace -> ProjectionSpace;  I try to invert projectedspace to worldspace.   ;  I dont think you can?, it cant be the inverse of view*proj i dont think....   ;  I dont think so either. So have figured something else to go around this problem, which I used ray plane intersection method inside my shader.Thank you everyone.   ;  This type of thing is quiote common in defered rendering so its definatly done. If you have the scren space coordinates and the depth then it should be doable. Have a look at some defered rendering stuff and see if you can find how its done.   ;  If v = (x,y,z,1) is in NDC coordinates and P is the direct3d projection matrix, then v*inverse(P) takes you to view space.  Of course, you will need to divide by w after since inverse(P) will change the w coordinate to not be 1.   