This change improves the compression speed for both DXT and ETC encodings.
Explanation:
On each iteration of the vector quantization algorithm, the leaf with the highest variance is selected for splitting. At the same time, each split operation adds at most 2 new leaves. Considering this, the search of the leaf with the highest variance can be performed more efficiently if all the leaves are stored in a priority queue (in order to guarantee that texture decompression gives identical result to the original version of Crunch, the node comparison operation also takes the node index into account).
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 / 28.844 sec
Modified: 1468204 bytes / 7.883 sec
Improvement: 7.21% (compression ratio) / 72.67% (compression time)
[Compressing Kodak set with mipmaps using DXT1 encoding]
Original: 2065243 bytes / 36.978 sec
Modified: 1914805 bytes / 10.490 sec
Improvement: 7.28% (compression ratio) / 71.63% (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: 15.165 sec
Average bitrate: 1.363 bpp
Average Luma PSNR: 34.050 dB
Alexander Suvorov
fbe3f6ca10