   Editing Depth Buffer
  I have a very large voxel model that is getting raytraced to provide pixel data.  Included in this pixel data is the z depth of each pixel.  I want to put this pixel data on a quad, and then be able to bisect it with polygon data (ie, I render a sphere model to a texture with z depth data, put it on a textured quad, and then bisect it with a triangle; the triangle should be clipped correctly with the sphere, not the quad).  The only way I''ve thought of being able to do this is to edit the depth buffer to replace the depth of the textured quad with the depth of the pixels from the file.  I''m not experienced enough with OpenGL to know if this is possible.  Is it?If so, then here are the problems I''ve come up with so far:1.  The camera orientation needs to be the same as the camera from the renderer.2.  Clipping planes must be the same as the renderer (so the depth buffer values make sense in context with the pixel depth values).3.  There will need to be some scaling routine for the depth values if the camera moves in or out (or maybe not allow this at all).Can anyone think of any additional things to worry about?Thanks for any input!!Kelly 
  Sounds like nailboards (an extension of imposters) but other than that I havn''t a clue how they''re actually implemented. This seems to describe what you need. ;  The fastest way to do nailboards is to tweak depth values in a fragment program.  Of course, one of the downsides to doing this is that it overrides some z-buffer optimizations that video cards possess.  ATI has a D3D demo that does this, using PS1.4.http://www.ati.com/developer/samples/DepthExplosion.html 