This change significantly improves the compression speed for DXT encoding.
Explanation:
The main ideas used for the DXT color endpoints computation optimization:
- When the DXT endpoint computation function is called from the qunatization algorithm, almost all of its input parameters (except the color metrics) are hardcoded in the quantization code. This allows to optimize the endpoint evaluation function (which is the bottleneck of the endpoint computation algorithm) for this specific set of parameters.
- In the original version of the evaluation function, selectors are computed each time when a new endpoint is evaluated. While in fact, this is not necessary, because some selector values are never used, so they can be computed lazily, based on the previously determined optimal endpoint values. This approach significantly reduces the amount of computations.
Other improvements:
- The original version of Crunch has a minor bug: the counter for the cached endpoint values does not get initialized. This results in nondeterministic DXT conversion of large textures, as the counter overflow can occur at a random moment. The issue is now fixed in the current branch.
DXT Testing:
The modified algorithm has been tested on the Kodak test set using 64-bit build with default settings (running on Windows 10, i7-4790, 3.6GHz). All the decompressed test images are identical to the images being compressed and decompressed using original version of Crunch (revision ea9b8d8).
[Compressing Kodak set without mipmaps using DXT1 encoding]
Original: 1582222 bytes / 28.893 sec
Modified: 1468204 bytes / 11.882 sec
Improvement: 7.21% (compression ratio) / 58.88% (compression time)
[Compressing Kodak set with mipmaps using DXT1 encoding]
Original: 2065243 bytes / 36.946 sec
Modified: 1914805 bytes / 15.628 sec
Improvement: 7.28% (compression ratio) / 57.70% (compression time)
ETC Testing:
The modified algorithm has been tested on the Kodak test set using 64-bit build with default settings (running on Windows 10, i7-4790, 3.6GHz). The ETC1 quantization parameters have been selected in such a way, so that ETC1 compression gives approximately the same average Luma PSNR as the corresponding DXT1 compression (which is equal to 34.044 dB for the Kodak test set compressed without mipmaps using DXT1 encoding and default quality settings).
[Compressing Kodak set without mipmaps using ETC1 encoding]
Total size: 1607858 bytes
Total time: 17.352 sec
Average bitrate: 1.363 bpp
Average Luma PSNR: 34.050 dB
Alexander Suvorov
6b3172f793