title: optimizing iPhone OpenGL ES fill rate
body: <p>I have an Open GL ES game on the iPhone. My framerate is pretty sucky, ~20fps. Using the Xcode OpenGL ES performance tool on an iPhone 3G, it shows:</p>
  
  <p>Renderer Utilization: 95% to 99%</p>
  
  <p>Tiler Utilization: ~27%</p>
  
  <p>I am drawing a lot of pretty large images with a  lot of blending. If I reduce the number of images drawn, framerates go from ~20 to ~40, though the performance tool results stay about the same (renderer still maxed). I think I'm being limited by the fill rate of the iPhone 3G, but I'm not sure.</p>
  
  <p>My questions are: How can I determine with more granularity where the bottleneck is? That is my biggest problem, I just don't know what is taking all the time. If it is fillrate, is there anything I do to improve it besides just drawing less?</p>
  
  <p>I am using texture atlases. I have tried to minimize image binds, though it isn't always possible (drawing order, not everything fits on one 1024x1024 texture, etc). Every frame I do 10 image binds. This seem pretty reasonable, but I could be mistaken.</p>
  
  <p>I'm using vertex arrays and glDrawArrays. I don't really have a lot of geometry. I can try to be more precise if needed. Each image is 2 triangles and I try to batch things were possible, though often (maybe half the time) images are drawn with individual glDrawArrays calls. Besides the images, I have ~60 triangles worth of geometry being rendered in ~6 glDrawArrays calls. I often glTranslate before calling glDrawArrays.</p>
  
  <p>Would it improve the framerate to switch to VBOs? I don't think it is a huge amount of geometry, but maybe it is faster for other reasons?</p>
  
  <p>Are there certain things to watch out for that could reduce performance? Eg, should I avoid glTranslate, glColor4g, etc?</p>
  
  <p>I'm using glScissor in a 3 places per frame. Each use consists of 2 glScissor calls, one to set it up, and one to reset it to what it was. I don't know if there is much of a performance impact here.</p>
  
  <p>If I used PVRTC would it be able to render faster? Currently all my images are GL_RGBA. I don't have memory issues.</p>
  
  <p>One of my fullscreen textures is 256x256. Would it be better to use 480x320 so the phone doesn't have to do any scaling? Are there any other general performance advice for texture sizes?</p>
  
  <p>Here is a rough idea of what I'm drawing, in this order:</p>
  
  <p>1) Switch to perspective matrix.
  2) Draw a full screen background image
  3) Draw a full screen image with translucency (this one has a scrolling texture).
  4) Draw a few sprites.
  5) Switch to ortho matrix.
  6) Draw a few sprites.
  7) Switch to perspective matrix.
  8) Draw sprites and some other textured geometry.
  9) Switch to ortho matrix.
  10) Draw a few sprites (eg, game HUD).</p>
  
  <p>Steps 1-6 draw a bunch of background stuff. 8 draws most of the game content. 10 draws the HUD.</p>
  
  <p>As you can see, there are many layers, some of them full screen and some of the sprites are pretty large (1/4 of the screen). The layers use translucency, so I have to draw them in back-to-front order. This is further complicated by needing to draw various layers in ortho and others in perspective.</p>
  
  <p>I will gladly provide additional information if reqested. Thanks in advance for any performance tips or general advice on my problem!</p>
  
  <p><strong>Edit:</strong></p>
  
  <p>I added some logging to see how many glDrawArrays calls I am doing, and with how much data. I do about 20 glDrawArray calls per frame. Usually about 1 to up to 6 of these has about 40 vertices each. The rest of the calls are usually just 2 vertices (one image). I'm just using glVertexPointer and glTexCoordPointer.</p>