This change improves compression ratio.
Explanation:
In the original algorithm the relative position of the block, used for prediction of the selector index for the currently decoded block, depends on the position of the current block in the chunk. It can be a horizontal neighbour or a diagonal neighbour. Using left nearest neighbour for selector index prediction for each block (except the blocks at the image borders) minimizes the average distance to the prediction block and therefore usually improves the selector index prediction. Similarly to the endpoint index processing, the selector ordering histogram in now generated based on the selector index prediction order.
Note:
This modification alters the output file format and makes it incompatible with the previous revisions.
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.
[Compressing Kodak set without mipmaps]
Original: 1582222 bytes / 28.869 sec
Modified: 1561622 bytes / 28.522 sec
Improvement: 1.30% (compression ratio) / 1.20% (compression time)
[Compressing Kodak set with mipmaps]
Original: 2065243 bytes / 37.038 sec
Modified: 2033151 bytes / 36.407 sec
Improvement: 1.55% (compression ratio) / 1.70% (compression time)
Alexander Suvorov
125536a3b5