I want to know how to make polygons that are just masks, and then I put still images in the mask to create a landscape effect, like the sky in Doom. Any way to do that?

They are called skyboxes.

Basically, you need a texture (oh well, you probably need at least 5) and some triangles to slap the texture on.

Draw your world.

Turn off z writing.
You may want to apply some transformation to make the skybox look immobile - infite. I usually reset all the transformations and apply some rotations to get the skybox as it should.

Draw the skybox.

Turn z writing on again

Next frame / other things...


You can also draw the skybox BEFORE anything else but this would be less efficient on most cards (most of the time a lot of skybox''s fragments can be discarded thanks to early z-culling).

My two cents.

I hope this is wath you were searching for.