Why is this OpenGL ES code slow on iPhone?

Question

I\'ve slightly modified the iPhone SDK\'s GLSprite example while learning OpenGL ES and it turns out to be quite slow. Even in the simulator (on the hw worst) so I must be doing

Answer 1

(I know this is very late, but I couldn't resist. I'll post anyway, in case other people come here looking for advice.)

This has nothing to do with the texture size. I don't know why people rated up Nils. He seems to have a fundamental misunderstanding of the OpenGL pipeline. He seems to think that for a given triangle, the entire texture is loaded and mapped onto that triangle. The opposite is true.

Once the triangle has been mapped into the viewport, it is rasterized. For every on-screen pixel the your triangle covers, the fragment shader is called. The default fragment shader (OpenGL ES 1.1, which you are using) will lookup the texel that most closely maps (GL_NEAREST) to the pixel you are drawing. It might look up 4 texels since you are using the higher quality GL_LINEAR method to average the best texel. Still, if the pixel count in your triangle is, say 100, then the most texture bytes you will have to read is 4(lookups) * 100(pixels) * 4(bytes per color. Far far less than what Nils was saying. It's amazing that he can make it sound like he actually knows what he's talking about.

WRT the tiled architecture, this is common in embedded OpenGL devices to preserve locality of reference. I believe that each tile gets exposed to each drawing operation, quickly culling most of them. Then the tile decides what to draw on itself. This is going to be much slower when you have blending turned on, as you do. Because you are using large triangles that might overlap and blend with other tiles, the GPU has to do a lot of extra work. If, instead of rendering the example square with alpha edges, you were to render an actual shape (instead of a square picture of the shape), then you could turn off blending for this part of the scene and I bet that would speed things up tremendously.

If you want to try it, just turn off blending and see how much things speed up, even if the don't look right. glDisable(GL_BLEND);