Determining optimal decompression buffer size

[heisters]

What is the best way to determine the size of the output buffer passed to HapDecode? Looking at HapDecode itself, it seems like there is no real way to know the correct size ahead of time, so one simply has to make a good guess. After a little experimentation, I settled on simply multiplying the size of the compressed buffer by a factor of 2, but that may fail with a different encoder (this source is FFMPEG), or a different Hap codec (this is Snappy + DXT5 YCoCg). Setting it too high obviously impacts performance, as the runtime needs to find too much contiguous memory.

https://github.com/heisters/libglvideo/blob/master/src/decoders/hap.cpp#L24

[bangnoise]

Usually (ie always for mov or avi) dimensions are stored independently in the container, so armed with those and the texture format (from HapGetFrameTextureFormat()) you can calculate it exactly (round dimensions up to a multiple of four, apply known DXT block size).

[bangnoise]

Also note that the texture format is constant for a file for most (all?) Hap encoders, so you should be able to allocate a buffer once and reuse it.

[heisters]

thanks!