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Read ZBuffer

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Is there a way to read from the zbuffer? I'm trying to do pointsprite visibility testing for a neato effect, but can't find anyway to do it.

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You can get the screen coordinates by multiplying the vertex by the ModelviewProjection matrix. To obtain the transformed z position of your vertex, multiply the vertex by the Modelview matrix.

z position makes no sense when considering screen coordinates.

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float mtx[16];
glGetFloatv( GL_MODELVIEW_MATRIX, mtx );
tpos[0] = mtx[0]*ptcl->pos[0] + mtx[1]*ptcl->pos[1] + mtx[2]*ptcl->pos[2] + mtx[3];
tpos[1] = mtx[4]*ptcl->pos[0] + mtx[5]*ptcl->pos[1] + mtx[6]*ptcl->pos[2] + mtx[7];
tpos[2] = mtx[8]*ptcl->pos[0] + mtx[9]*ptcl->pos[1] + mtx[10]*ptcl->pos[2] + mtx[11];
glGetFloatv( GL_PROJECTION_MATRIX, mtx );
tpos[0] = mtx[0]*tpos[0] + mtx[1]*tpos[1] + mtx[2]*tpos[2] + mtx[3];
tpos[1] = mtx[4]*tpos[0] + mtx[5]*tpos[1] + mtx[6]*tpos[2] + mtx[7];
tpos[2] = mtx[8]*tpos[0] + mtx[9]*tpos[1] + mtx[10]*tpos[2] + mtx[11];

float fBuffValue;
glReadPixels( tpos[0], tpos[1], 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &fBuffValue );

if ( fBuffValue < tpos[2] )
ptcl = ptcl->nextParticle;

good thing you can 'try' and edit other peoples posts =)

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