This change significantly improves compression ratio for ETC1 encoding.
Note: This modification alters the output file format for ETC1 encoding and makes it incompatible with the previous revisions.
Explanation:
Previously, for simplicity, endpoint references for ETC1 encoding have been only computed withing the tiling area. Now endpoint references are computed for all the ETC1 subblocks. This means that endpoints can now be inherited from the surrounding ETC1 blocks, which significantly improves the compression ratio.
Endpoint references for ETC1 subblocks are encoded in the following way:
- The first ETC1 subblock has the reference value of 0 if the endpoint is decoded from the input stream, the value of 1 if the endpoint is copied from the second subblock of the left neighbour ETC1 block, and the value of 2 if the endpoint is copied from the first subblock of the top neighbour ETC1 block.
- The second ETC1 subblock has the reference value of 0 if the endpoint is copied from the first subblock, the value of 1 if the endpoint is decoded from the input stream and the corresponding ETC1 block is split horizontally, and the value of 2 if the endpoint is decoded from the input stream and the corresponding ETC1 block is split vertically.
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 / 28.901 sec
Modified: 1473711 bytes / 13.353 sec
Improvement: 6.86% (compression ratio) / 53.80% (compression time)
[Compressing Kodak set with mipmaps using DXT1 encoding]
Original: 2065243 bytes / 36.997 sec
Modified: 1920600 bytes / 18.096 sec
Improvement: 7.00% (compression ratio) / 51.09% (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: 1639063 bytes
Total time: 17.421 sec
Average bitrate: 1.389 bpp
Average Luma PSNR: 34.057 dB
Alexander Suvorov
7402f3d4f3