CODEWITHSUNDEEP
openglopengl-esopengl-es-2.0
johnbakers
johnbakers

Reputation: 24771

Which is a larger performance drain: quantity of vertices in one draw call, or quantity of calls?

I am quickly finding that one of the organisational considerations you must make when preparing rendering in OpenGL is the type of topography and the arrangement of vertices.

Now there are some interesting methods out there for organising vertices into very long arrays, with nice uses of interleaved arrays, indexes, etc, so that you can pour a lot of geometry into one OpenGL call.

But it's much easier in some cases to simply iterate and perform multiple calls with smaller vertex arrays.

While I agree with the notion that premature optimization is somewhat wasteful, just how important of a consideration should it be to minimize OpenGL calls, especially if multiple calls would actually involve far fewer vertexes per call?

I can see that this is one of those decisions that is important early in the development process, since it forms a lot of the structure of how vertexes get created and organized.

Upvotes: 1

Views: 209

Answers (2)

kerim
kerim

Reputation: 2522

You can find a balance for your specific needs. But the thing is that there're many variables in the equation. And there's no simple solution (like "always make scene as one big single batch!"). TraxNet gave you a good advice though - always try to minimize api calls(whether drawing or state changes). But it hasn't to be just a few calls. On modern PC it could be thousands per frame, not so modern mobile phone, maybe, just a few hundred. Also TraxNet mentioned degenerate triangles(helping form strips) - though they're still triangles(kinda add to 'total' triangle count rendered) - they cost almost nothing still helping to minimize amount of draw calls.

Upvotes: 0

Trax
Trax

Reputation: 1910

There is an overhead for each command you send down to the GPU. By batching the vertices you minimize that overhead and also allows the driver to make small optimizations in you data before sending it to the hardware. It can make quite a difference and is the reason the glBegin and glEnd was completely removed from newer iterations of OpenGL.

You should try to avoid making many driver states changes and many drawing calls.

EDIT: Consider using degenerated vertices in you triangle strips (also helps in minimizing the number of vertices processed) so that you can just use one drawing call and render all your topology (unless you need to change some driver state between parts of the topology).

Upvotes: 1

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