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Comment by Jasper_ | Hacker News Reader

Vertex/triangle clipping is quite rare, and mostly used for clipping against the near plane (hopefully rare in practice). Most other implementations use a guard band as a fast path (aka doing it in screen space) -- real clipping is only used where your guard band doesn't cover you, precision issues mostly.

I'm not sure what issues you're hitting, but I've never found clipping to be that challenging or difficult. Also, clip control and clip space aren't really specifically about clipping -- clip space is just the output space of your vertex shader, and the standard "clip control" extension just controls whether the near plane is at 0 or -1. And 0 is the correct option.

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Guard band clipping is only really applicable to "edge function" type rasterizers. For the classic scanline-based algorithm, sure, you can easily clip to the right and bottom edges of the viewport while rasterizing, but the top and left edges are trickier. Clipping in clip space, before rasterization, is more straightforward, given that you have to frustum cull primitives anyway.

> clipping against the near plane (hopefully rare in practice)

I am not sure I understand why this would be rare. If I am intending to construct a rasterizer for a first-person shooter, clipping is essentially mandatory for all but the most trivial of camera arrangements.

Clipping or culling? I expect it’s mostly the latter unless your camera ends up intersecting the geometry.

Both. You almost always need both.

Clipping deals with geometry that is partially inside the camera. Culling (either for backfaces or entire instances) is a preliminary performance optimization that can be performed in a variety of ways.

As an example, if you're writing a shooter then the floor might be a large square that will almost definitely be intersecting the near plane. You absolutely need clipping here.

This is precisely the first place I realized I needed proper clipping. Wasted many hours trying to hack my way out of doing it the right way.

Yes, of course, I was definitely imagining you were struggling to get simpler scenes to work. But also, proportionally few of your triangles in any given scene should be near-plane clipped. It's OK to have a slow path for it, and then speed it up later. I've never felt the math for the adjusted barycentrics is too hard, but it can take a bit to wrap your head around. Good luck :)

You and the GP have different rasterization algorithms in mind I think. The GP, I presume, is talking about a classic scanline-based rasterizer rather than an edge function "am I inside or not" type rasterizer that GPUs use.

Even with a guard band don’t you need to at least test the polygons for Z clipping prior to the perspective divide?

Clipping in X and Y is simpler at least, and again the guard band hopefully mostly covers you.

> Even with a guard band don’t you need to at least test the polygons for Z clipping prior to the perspective divide?

Yes. Guard band is an optimization that reduces the amount of potential clipping required. You still need to be able to clip for fundamental correctness.

If you totally reject a vertex for a triangle without determining precisely where it intersects the desired planes, you are effectively rejecting the entire triangle and creating yucky visual artifacts.

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