Hi, I'm trying to implement graphic pipeline in software level. I have some problems with clipping and culling now.

Basically, there are two main concerns:

Super quick answer, when I did this I clipped in screen space (2D) after back-face culling (also in screen space using winding).

You reconstruct the triangles with interpolation but you need to multiply/divide by w to account for the perspective correction.

While screen space might not be optimal, it is very easy, plus you only have to transform each vertex only once.

Good luck ;)

Thanks for the reply. I think I might try that out if there is no better suggestion.

When I did the occlusion software rasterizer for Urho3D, I remember encountering some precision/overflow issues when clipping in screen space against the near plane. Basically, the X/Y coordinates could become huge if the near clipping plane was near 0. Therefore I switched to clipping in homogenous coordinates before perspective divide. When you reconstruct triangles in homogenous coords, just interpolate all of x,y,z,w after you find where an edge intersects with the clip plane in question. Note that clipping against all frustum planes can theoretically generate a large amount of new vertices & triangles.

If you want to check, the file in question is OcclusionBuffer.cpp.

Thanks, I just implemented clipping only with near plane which cares only about z value. As for x, y value, my workaround is to clamp the bounding-box in screen space for every triangle passed into rasterizer.

A confusion about reconstructing triangle in homogeneous coordinate. How do you know the clipping in homogeneous coordinate? I mean I have information of view frustum before projection transformation, but after that vertices are transformed into clipping coordinate, should I also do projection transformation on view frustum?

Sorry for my poor English, even myself no sure what I mean.

After projection transform, the view volume is fixed (in Direct3D convention, left clip plane at X -1.0, far clip plane at Z 1.0, near clip at Z 0.0) so there should be no need to transform the frustum.

Ah, I took a bit of a shortcut. What I should have said that after projection & perspective divide the vertices sit inside the fixed view volume. But the end result is, that when you're clipping homogenous coordinates using 4-dimensional dot products, you can use those fixed clipping planes.

After all these days, I still can't get clear about the clipping and reconstruction of triangle. Say triangle Tp has been left multiplied by perspective matrix but not yet divided by its fourth component w. Should I now do the clipping and reconstruction? if I do clipping and reconstruction in this stage, what clipping plane should I clip against?