Are triangles a gpu restriction or are there other rendering pathways?

Question

To preface this question, I have a competent understanding of OpenGL and the maths behind it, and while I have never touched anything related to DirectX I imagine the concepts are similar.

There is plenty of information around about why triangles are used for 3D graphics (they are necessarily planar, are indivisible except into smaller triangles, etc). However, I would like to know if triangles are merely a convenient way of storing and manipulating 3D data (simpler maths regarding interpolation, etc), or if there is a hardware limitation in the graphics card that only realistically allows the rendering of triangles (e.g. instructions that can essentially ONLY be applied to triangles).

Following on from this, is there any way to achieve pixel-by-pixel control of graphics rendering (as outlined briefly by the answer to this question). While I appreciate direct control over individual pixels is done through a driver, is there any way I can get this kind of control over a rendering environment? Is there away to 'avoid triangles' completely?

Answer 1

Yes and no. Kind of.

Current GPUs are designed to render triangles because triangles are nice to work with. And because current GPUs are designed to work with triangles, people use triangles and so GPUs only need to process triangles, and so they're designed to process only triangles.

As you say, triangles just have advantages that make them convenient to use. GPUs can be made (and have been made) to render other primitives natively, but it's just not really worth it. If you tell a modern GPU to render a quad, it splits it up into two triangles and renders those.

Not because there's a technical reason why a GPU can't render quads natively, but because it's not worth spending transistors on. It's much more useful to focus the GPU on doing triangles as fast as possible, and then just emulate other primitives if they're needed.

So yes, modern GPUs have hardware limitations so they don't work with quads, for example, but not because it's impossible to design a GPU which works with quads. It'd just be less efficient to do so. :)

As for "avoiding triangles", sure, that's basically what the fragment shader does: it fills in one single pixel. The GPU just runs it a few million times in parallel to fill in the entire screen. You could draw two big triangles, which form a quad filling the entire screen, and then just specify a fragment shader which fills that with the content you like.

If you want more control over the process, do it in software instead: paint one pixel at a time to a memory surface, and then load that as a texture on the GPU. But it's slow. :)