Reducing Texture Size Means Fewer Pixels to Be Fetched

So what else could be changed? Something that is not all that obvious. Our Bob instances are 32x32 units in size. We use a projection plane that is 320x480 units in size. On a Hero, that will give us pixel-perfect rendering. On a Nexus One or a Droid, a single unit in our coordinate system would take up a little under a pixel. In any case our texture is actually 128x128 pixels in size. We don't need that much resolution, so let's resize the texture image bobrgb888.png to 32x32 pixels. We'll call the new image bobrgb888-32x32.png. Using this smaller texture, we get the following FPS for each device:


12-10 04:48:03.940: DEBUG/FPSCounter(629): fps: 23

12-10 04:48:04.950: DEBUG/FPSCounter(629): fps: 23

12-10 04:48:05.860: DEBUG/dalvikvm(629): GC freed 21812 objects / 524256 bytes in 134ms

12-10 04:48:05.990: DEBUG/FPSCounter(629): fps: 21

12-10 04:48:07.030: DEBUG/FPSCounter(629): fps: 24


12-10 04:51:11.601: DEBUG/FPSCounter(9191): fps: 56

12-10 04:51:12.609: DEBUG/FPSCounter(9191): fps: 56

12-10 04:51:13.625: DEBUG/FPSCounter(9191): fps: 55

12-10 04:51:14.641: DEBUG/FPSCounter(9191): fps: 55

Nexus 0ne:

12-10 04:48:18.067: DEBUG/FPSCounter(2238): fps: 53

12-10 04:48:19.077: DEBUG/FPSCounter(2238): fps: 56

12-10 04:48:20.077: DEBUG/FPSCounter(2238): fps: 53

12-10 04:48:21.097: DEBUG/FPSCounter(2238): fps: 54

Wow, that makes a huge difference on the second-generation devices. It turns out that the GPUs of those devices hate nothing more than having to scan over a large amount of pixels. This is true for fetching texels from a texture as well as actually rendering triangles to the screen. The rate at which those GPUs can fetch texels and render pixels to the framebuffer is called the fill rate. All the second-generation GPUs are heavily fill-rate limited, so we should try to use textures that are as small as possible (or map our triangles only to a small portion of them), and not render extremely huge triangles to the screen. We should also look out for overlap: the fewer overlapping triangles, the better.

NOTE: Actually, overlap is not an extremely big problem with GPUs such as the PowerVR SGX 350 on the Droid. These GPUs have a special mechanism, called tile-based deferred rendering, that can eliminate a lot of that overlap under certain conditions. We should still care about pixels that will never be seen on the screen, though.

The Hero did only slightly benefit from the decrease in texture image size. So what could be the culprit here?

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