OpenGL - skybox and frustum clipping

Question

I want to create skybox, which is just textured cube around camera. But actually i don't understand how this can work, because the camera viewing volume is frustum and the skybox is cube. According to this source:

http://www.songho.ca/opengl/gl_projectionmatrix.html

Note that the frustum culling (clipping) is performed in the clip coordinates, just before dividing by wc. The clip coordinates, xc, yc and zc are tested by comparing with wc. If any clip coordinate is less than -wc, or greater than wc, then the vertex will be discarded.

vertexes of skybox faces should be clipped, if they are outside of frustum. So it looks for me that the cube should be actually a frustum and should match exactly the gl frustum faces, so my whole scene is wrapped inside of that skybox, but i am sure this is bad. Is there any way how to fill whole screen with something, that wraps whole gl frustum?

Answer 1

The formulation from your link is rather bad. It is not actually vertices that get clipped, but rather fragments. Drawing a primitive with vertices completely off-screen does not prevent the fragments that would intersect with the screen from getting drawn. (The picture in the link also actually shows this being the case.)

That having been said, however, it may (or may not, depending on the design of your rendering code) be easier to simply draw a full-screen quad, and use the inverse of the projection matrix to calculate the texture coordinates instead.