Opengl Depth buffer and Culling

Question

Whats's the difference between use back face culling and a buffer of depth in OpenGL?

Answer 1

A given triangle has two sides, the front face and the back face. The side you are looking at is determined by the order the points appear in the vertex list (also called the winding). Typically lists of triangles have alternating winding so that you can reuse the preceding two points but the facing of a given triangle in the strip doesn't alternate. Back face culling is the optimization step where in triangles in the scene which are oriented away from the view are removed from the list of triangles to draw.

A depth buffer (z-buffer) is used to hang onto the closest thing (the depth is relative to the view) that has already been rendered. If the thing that comes up next in the draw list is behind something that I've drawn already (ie, it has a depth that places it farther away) I can skip drawing it, as it is obstructed. If the new thing to draw is closer, I draw it and I update the depth buffer with the new, closer value.

Answer 2

Backface culling is when OpenGL determines which faces are facing away from the viewer and are therefore unseen. Think of a cube. No matter how your rotate the cube, 3 faces will always be invisible. Figure out which faces these are, remove them from the list of polygons to be drawn and you just halved your drawing list.

Depth buffering is fairly simple. For every pixel of every polygon drawn, compare it's z value to the z buffer. if it's less than the value in the z buffer set the z buffer value as the new z buffer value. If not, discard the pixel. Depth buffering gives very good results but can be fairly slow as each and every pixel requires a value lookup.

In reality there is nothing similar between these two methods and they are often both used. Given a cube you can first cut out half the polygons using culling then draw them using z buffering.

Culling can cut down on the polygons rendered, but it's not a sorting algorithm. That's what Z buffering is.