Saturday, 23 January 2010

N900/Maemo5 and different texture formats is providing an nice tool for benchmarking the different texture formats that the meamo5, SGX based device supports. The tool GLMemPerf is meant for measuring the texture memory bandwidth performance of an OpenGL ES 2.0 implementation with a set of different kinds of precompressed textures. As an output it blit the different textures from its /data folder to the screen and provides an nice set of information - how the different formats and texture processing methods effect to the overall performance.

From the results the compressed textures formats (IMG specific PVRTC and ETC1) are clearly giving an performance benefit - as one could expect. The IMG PVR Texture Compression format has the 2-bits-per-pixel and 4-bits-per-pixel options create PVRTC data that encodes source pixels into 2 or 4 bits per pixel. This meaning that each texel is encoded into fewer bits than the equivalent uncompressed texture, which of course is meaning less load for memory bandwidth.
The downside with the compression is that the overall quality of the render will suffer. The quality hit is still depending from the compressed content so might be worth of checking how 2 vs 4 bits per pixel will effect.

The other thing to note is how the textures are stored and accessed from the memory.
SGX as many other GPUs benefit greatly from the usage of using power-of-two sized textures - which improves the implementation of the texture mapping when converting the texture coordinates to texel coordinates. It might be worth of rounding up the textures to the nearest power-of-two to get the benefit. Note also that the PVRTC formats require that the textures are in power-of-two size.
The other thing related accessing the memory is the texture twiddling. The twiddling is by default done for all the SGX uploaded textures. What the twiddling does is that it re-arranges the texture samples to optimize the memory access for different operations. One thing to note is that with pixmaps you will not get this benefit as in a case of pixmaps you don't upload the textures to the video memory but instead source the pixels directly.

How about generating and processing your textures? For this IMGTec is providing an nice tool called PVRTexTool. The tool has an version for command line usage and also an GUI based tool for seeing in more detailed the output of your changes - for ex when wanting to do comparison between the different compression methods. The PVRTexTool supports your normal RGB/ARGB formats + the texture compression format PVRTC and ETC.

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