Performance and background images for OpenGL ES/iPhone

Question

I'm developing a 2D game for the iPhone using OpenGL ES and I'd like to use a 320x480 bitmapped image as a persistent background.

My first thought was to create a 320x480 quad and then map a texture onto it that represents the background. So... I created a 512x512 texture with a 320x480 image on it. Then I mapped that to the 320x480 quad.

I draw this background every frame and then draw animated sprites on top of it. This works fine except that the drawing of all of these objects (background + sprites) is too slow.

I did some testing and discovered that my slowdown is in the pixel pipeline. Not surprisingly, the large background image is the main culprit. To prove this, I removed the background draw and everything else rendered very fast.

I am looking for advice on how to keep my background and also improve performance.

Here's some more info:

1) I am currently testing on the Simulator (still waiting on Apple for the license)

2) The background is a PVR texture squeezed down to 128k

3) I had hoped that there might be a way to cache this background into a color buffer but haven't had any luck with that. that may be due to my inexperience with OpenGL ES or it just might be a stupid idea that won't work :)

4) I realize that the entire background does not always have to refresh, just the parts that have been drawn over by the moving sprites. I started to look into techniques for refreshing (as necessary) parts of the the background either as separate textures or with a scissor box, however this seems less than elegant.

Any tips/advice would be greatly appreciated...

Thank you.

Answer 1

Thanks to everyone who provided info on this. All of the advice helped out in one way or another.

However, I wanted to make it clear that the main issue here turned out to be the behavior of simulator itself (as implied by Andreas in his response). Once I was able to get the application on the device, it performed much, much better. I mention this because, prior to developing my game, I had seen a lot of posts that indicated that the device was much slower than the simulator. This might be true in some instances (e.g. general application logic) but in my experience, animation (particularly 3d transformations) are much faster on the device.

Answer 2

1. Do not do performance testing on the simulator. Ever!
   The differences to the real hardware are huge. In both directions.
2. If you draw the background every frame:
   Do not clear the framebuffer. The background will overdraw the whole thing anyway.
3. Do you really need a background texture ?
   What about using a color gradient via vertex colors ?
4. Try using the 2bit mode for the texture.
5. Turn of all render steps that you do not need for the background.
   E.g.: Lighting, Blending, Depth-Test, ...

If you could post some of your drawing code it would be a lot easier to help you.

Answer 3

If you're making a 2D game, is there any reason you aren't using an existing library? Specifically, the cocos2d for iPhone may be worth your time. I can't answer your question about how to fix the issue doing it all yourself, but I can say that I've done exactly what you're talking about (having one full screen background with sprites on top) with cocos2d and it works great. (Assuming 60 fps is fast enough for you.) You may have your reasons for doing it yourself, but if you can, I would highly suggest at least doing a quick prototype with cocos2d and seeing if that doesn't help you along. (Details and source for the iPhone version are here: http://code.google.com/p/cocos2d-iphone/)

Answer 4

I dont have much experience with OpenGL ES, but this problem occurs generally.

Your idea about the 'color buffer' is good intuition, essentially you want to be storing your background as a frame buffer and loading it directly onto your rendering buffer before drawing the foreground.

In OpenGL this is fairly straight forward with Frame Buffer Objects (FBO's). Unfortunatly I dont think OpenGL ES supports them, but it might give you somewhere to start looking.

Answer 5

you may want to try using VBOs (Vertex Buffer Objects) and see if that speeds up things. Tutorial is here

In addition, I just saw, that since OpenGL ES v1.1, there is a function called glDrawTex (Draw Texture) that is designed for

fast rendering of background paintings, bitmapped font glyphs, and 2D framing elements in games

Answer 6

1. You could use frame buffer objects similar to the GLPaint example from Apple.
2. Use a texture atlas to minimize the number of draw calls you make. You can use glTexCoordPointer for setting your texture coordinates that maps each image to its correct position. Remember to set your vertex buffer too. Ideally one draw call will render your entire 2D scene.
3. Avoid enabling/disabling states where possible.