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

Your texture is 512*512*4 bytes per pixel. That's a megabyte of data. If you render it 200 times per frame you generate a bandwidth load of 200 megabytes per frame.

With roughly 4 fps you consume 800mb/second just for texture reads alone. Frame- and Zbuffer writes need bandwidth as well. Then there is the CPU, and don't underestimate the bandwidth requirements of the display as well.

RAM on embedded systems (e.g. your iphone) is not as fast as on a Desktop-PC. What you see here is a bandwidth starvation effect. The RAM simply can't handle the data faster.

How to cure this problem:

• pick a sane texture-size. On average you should have 1 texel per pixel. This gives crisp looking textures. I know - it's not always possible. Use common sense.

• use mipmaps. This takes up 33% of extra space but allows the graphic chip to pick use a lower resolution mipmap if possible.

• Try smaller texture formats. Maybe you can use the ARGB4444 format. This would double the rendering speed. Also take a look at the compressed texture formats. Decompression does not cause a performance drop as it's done in hardware. Infact the opposite is true: Due to the smaller size in memory the graphic chip can read the texture-data faster.