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Mapping coordinates (gluUnproject - gluProject)

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Hello fellows, I'm doing some shadow mapping and i'm rendering from the light's point of view, get the depth buffer and save it to some array using glReadpixels. Later i render from the camera's point of view, do another glReadpixels and for every pixel i need to find its position in the previously saved array (where the depth map is stored). So i thought, ok 1) for every pixel in my array (x,y) i do a glUnproject(x,y,depth[x,y], camView, camProj, viewport, &wx, &wy, &wz) 2) glProject(wx, wy, wz, lightView, lightProj, viewport, &lx, &ly, &lz) but i'm getting some garbage, i.e. negative coordinates or off-limits like -1322, 673, etc assuming the viewport is 0,0,512,512 this should not happen... does it? or maybe i'm totally wrong in my approach... comments? thank you

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well, i figured it out.
For some reason i don't know after a glReadPixels(0,0,width,height,pixels) where pixels has been declared as:
GLubyte pixels[width][height][3]

I was reading this using a nested for loop like:
for(x=0; ...)
for(y=0; ...)
gluUnproject(x,y,...)

but x and y were inverted so now I use:
gluUnproject(y,x,...) and i'm done (wtf?)

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