Full screen quads

[Tom Payne]

Hi,

My WebGL app involves composing a number of full-screen layers. Each layer is drawn as two triangles that together cover the full screen.

I've just read

  https://hacks.mozilla.org/2013/04/the-concepts-of-webgl/

which says:

  "Small rendering operations are slow

GPUs are intended to be used to draw large batches of triangles at once. If you have 10,000 triangles to draw, doing it in one single operation (as WebGL allows) will be much faster than doing 10,000 separate draw operations of one triangle each. Think of a GPU as a very fast machine with a very long warm-up time. Better warm up once and do a large batch of work, than pay for the warm-up cost many times. Organizing your rendering into large batches does require some thinking, but it’s worth it."

So, my question is: instead of drawing my full screen layer as two very large triangles, would it be better to draw it as many smaller triangles?

Cheers,

Tom

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[Paul Lewis]

It shouldn't be, no. In the case you describe the warm up would be the same whether it's two large triangles or ten thousand triangles, but the actual work done by the GPU is much larger, as would the data transfer of vertex positions be. 

The point the article is trying to make is that whenever you switch state (shaders, buffers etc) that you need to consider that an expensive operation which you're looking to minimize. The typical approaches for this is to order models / objects by shader first then state. Essentially graphics developers look to minimize the number of switches that are needed.

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[Paul Lewis]

Slight correction, I think the warm up would actually be longer with more triangles since you'd have more vertices to buffer in. All the other overhead would, I guess, be similar, and that's what we'd like to minimize.

[Benoit Jacob]

I meant that if you have many triangles to draw, you are better off drawing them in few draw-calls than in many. But if you only have two triangles to draw, and you already draw them in one draw-call, then you are already optimal in this respect.

Benoit

2013/4/22 Tom Payne <tom....@camptocamp.com>

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[Tom Payne]

Many thanks for the responses Paul and Benoit.