Large scrolling background in OpenGL ES

Question

I am working on a 2D scrolling game for iPhone. I have a large image background, say 480×6000 pixels, of only a part is visible (exactly one screen’s worth, 480×320 pixels). What is the best way to get such a background on the screen?

Currently I have the background split into several textures (to get around the maximum texture size limit) and draw the whole background in each frame as a textured triangle strip. The scrolling is done by translating the modelview matrix. The scissor box is set to the window size, 480×320 pixels. This is not meant to be fast, I just wanted a working code before I get to optimizing.

I thought that maybe the OpenGL implementation would be smart enough to discard the invisible portion of the background, but according to some measuring code I wrote it looks like background takes 7 ms to draw on average and 84 ms at maximum. (This is measured in the simulator.) This is about a half of the whole render loop, ie. quite slow for me.

Drawing the background should be as easy as copying some 480×320 pixels from one part of the VRAM to another, or, in other words, blazing fast. What is the best way to get closer to such performance?

Answer 1

That's the fast way of doing it. Things you can do to improve performance:

• Try different texture-formats. Presumably the SDK docs have details on the preferred format, and presumably smaller is better.
• Cull out entirely offscreen tiles yourself
• Split the image into smaller textures

I'm assuming you're drawing at a 1:1 zoom-level; is that the case?

Edit: Oops. Having read your question more carefully, I have to offer another piece of advice: Timings made on the simulator are worthless.