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projecting and unprojecting

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Iv'e got a video card that doesnt support screenbuffer objects. What I am trying to is take a small capture of the screen of some pre-selected sqare size centered at one point, Then reproject this square point at that depth back into the scene using a shader. Now, here's what im trying: I do a gluProject(m_Pos[0],m_Pos[1],m_Pos[2],pMod,pProj,pView,&dpx,&dpy,&dpz) All of the non-matrix arguments are doubles, and I know that this is correct by drawing a shape at that point on the screen. dpx and dpy are in coordinates between 0 and my resolution dimensions if the point is on the screen. What I try next, is GLdouble nul[3]; GLdouble nlr[3]; gluUnProject(dpx-64,dpy-64,dpz,pMod,pProj,pView,nul+0,nul+1,nul+2); gluUnProject(dpx+64,dpy+64,dpz,pMod,pProj,pView,nlr+0,nlr+1,nlr+2); nul and nlr appear to give me two coordinates centered around the original point (m_Pos) that I want. This is true, except they are always aligned with a certain axis. I get a projected square like i want, just not one that is facing my camera. What is wrong with my logic? Also, are there any better ways of doing this?

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