OpenGL: Is it more efficient to use GL_QUADS or GL_TRIANGLES?

Question

I know that OpenGL deprecated and got rid of GL_QUADS in the newer releases. I have heard this is due to the fact that modern GPUs only render with triangles so calling a quad would just make the GPU work harder to break it into two triangles (what I have heard anyway, I am not much of an expert on any of this topic).

I was wondering whether or not it is better (assuming the average person's CPU is faster, relatively, than their GPU) to just manually break the rendering of quads into two triangles yourself or to just let the GPU do it itself. Again, I have absolutely no real experience with OpenGL as I am just starting. I would rather know which is better for most machines these days so I could focus my attention on either rendering method*. Thanks.

*Yet I will probably utilize the 'triangle method' for the sake of it.

Answer 1

Even if you feed OpenGL quads, the triangularization is done by the driver on the CPU side before it even hits the GPU. Modern GPUs these days eat nothing except triangles. (Well, and lines and points.) So something will be triangulating, whether it's you or the driver -- it doesn't matter too much where it happens.

This would be less efficient if, say, you don't reuse your vertex buffers, and instead refill them anew every time with quads (in which case the driver will have to retriangulate every vertex buffer), instead of refilling them with pretriangulated triangles every time, but that's pretty contrived (and the problem you should be fixing in that case is just the fact you're refilling your vertex buffers).

I would say, if you have the choice, stick with triangles, since that's what most content pipelines put out anyways, and you're less likely to run into problems with non-planar quads and the like. If you get to choose what format your content comes in, then use triangles for sure, and the triangulation step gets skipped altogether.

Answer 2

Any geometry can be represented with triangles, and that is why it was decided to use triangles instead of quads. Another reason is two triangles do not have to be co-planar, which is not true for quad.

Yes, you select to render quads, but the driver will converting the quad into two triangles.

Therefore, by choosing to render a quad will not make GPU work less, but will make your CPU work more, because it has to do the conversion.