AstroVis

Depth-test is made after fragment shader so it will potentially perform tasks on fragments that wont be visible.

Early-z: http://www.gpgpu.org/w/index.php/Code_Examples#Early-z

Using fragment kill (Cg discard) or modifying the depth value in a fragment program will disable early-z. http://www.opengl.org/discussion_boards/ubbthreads.php?ubb=showflat&Number=214828

This might help for the blending problems: http://www.opengl.org/wiki/index.php/Alpha_Blending#Alpha_blending_and_the_Z_buffer