Doomsday Engine

I checked the history of the sky renderer. It appears the current behavior (depth test disabled) goes all the way back to 1.9.7. In Beta6.9, sky models were rendered with depth testing enabled, but the depth buffer was then cleared after the models had been drawn.

Nowadays we can use a separate depth buffer for rendering this kind of "alternate layer" stuff (already being used with HUD models), so I will try the same mechanism for the sky models: depth test enabled, but in a separate Z buffer.

The drawing order has always been the same as the order of the definitions.

However, there is a problem. The depth and stencil buffers are stored on a single texture. The stencil buffer is used for determining which parts are sky, so switching the depth/stencil texture to an alternate one will cause this information to be lost, and sky clipping will malfunction.

(If the renderer was using shaders, one could check stencil values from the main FB texture. However, this would still require a bit of trickery because the sky model would need to use a shader that also does a stencil check. Perhaps it would be possible to set up a render target that uses stencil from the main depth/stencil texture but still uses its own depth texture.)

An adequate solution at this point is to simply clear the depth buffer after rendering the sky models. I'm still considering applying a more wholesale update of all depth test/mask state changes to be done via GLState.