I also implemented clipping in my software rasterizer a while ago and can definitely sympathize! (Although I've written several simple scanline rasterizers in m

I also implemented clipping in my software rasterizer a while ago and can definitely sympathize! (Although I've written several simple scanline rasterizers in my life, this was the first time I actually bothered to implement proper clipping. I actually reinvented Sutherland–Hodgman from scratch which was pretty fun.) The problematic part is actually only the near plane due to how projective geometry works. At z=0 there's a discontinuity in real coordinates after z division, which means there can be no edges that cross from negative to positive z. Z division turns such an edge [a0, a1] into an "inverse" edge (-∞, a0'] ∪ [a1', ∞) which naturally makes rendering a bit tricky. In projective/homogenous coordinates, however, it is fine, because the space "wraps around" from positive to negative infinity. All the other planes you can clip against in screen space / NDC space if you wish, but I'm not sure there are good reasons to split the job like that.