This change improves the compression speed for DXT encoding.
Explanation:
When performing per-component endpoint optimization, it is not necessary to go through all the source pixels on every iteration in order to calculate the total weighted squared error for a specific trial endpoint. The computation can be optimized in the following way:
sum(w(i) * (x - p(i)) * (x - p(i))) = sum(w(i) * x * x) - sum(w(i) * 2 * x * p(i)) + sum(w(i) * p(i) * p(i)) = sum(w(i)) * x * x - sum(2 * w(i) * p(i)) * x + sum(w(i) * p(i) * p(i))
The values of sum(w(i)), sum(2 * w(i) * p(i)), sum(w(i) * p(i) * p(i)) can be precalculated for each of 4 selectors, and only have to be updated when the solution improves. This way the error computation on each iteration can be performed using 12 multiplications instead of 2 * N (where N is the number of pixels in the processed cluster).
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 / 28.811 sec
Modified: 1468204 bytes / 10.520 sec
Improvement: 7.21% (compression ratio) / 63.49% (compression time)
[Compressing Kodak set with mipmaps using DXT1 encoding]
Original: 2065243 bytes / 36.936 sec
Modified: 1914805 bytes / 13.902 sec
Improvement: 7.28% (compression ratio) / 62.36% (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: 17.121 sec
Average bitrate: 1.363 bpp
Average Luma PSNR: 34.050 dB
Alexander Suvorov
4bd4355683