Skybox Render

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Skyboxes are often frame-buffer-bandwidth bound optimize them:
(1) render them last, reading (but not writing) depth, and allow the early-z optimizations along with regular depth buffering to save bandwidth.
(2) render the skybox first, and disable all depth reads and writes. Which option will save you more bandwidth is a function of the target hardware.

If a large portion of the skybox is obscured, the first technique will likely be better; otherwise, the second one may save more bandwidth.

reference: http://http.developer.nvidia.com/GPUGems/gpugems_ch28.html

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