This change simplifies further modification of the code.
Explanation:
Considering that chunks are no longer used in the output format, it makes sense to also remove chunk related code from the intermediate processing. This modification also allows to use endpoint references from the leftmost block to the rightmost block in the previous scanline (wrapped reference to the left).
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 / 28.846 sec
Modified: 1494501 bytes / 25.628 sec
Improvement: 5.54% (compression ratio) / 11.16% (compression time)
[Compressing Kodak set with mipmaps]
Original: 2065243 bytes / 36.869 sec
Modified: 1945365 bytes / 33.497 sec
Improvement: 5.80% (compression ratio) / 9.15% (compression time)
This change significantly improves the compression ratio.
Explanation:
By default, the size of the endpoint and selector codebooks is calculated based on the number of blocks in the image and the quality parameter, while the actual complexity of the image does not affect the initial codebook size. So the target codebook size is selected in such a way, that even complex images can be approximated well enough. At the same time, normally, the lower is the complexity of the image, the higher is the density of the quantized vectors. Considering that vector quantization is performed using floating point computations, and the quantized endpoints have integer components, high density of quantized vectors will result in large number of duplicate endpoints. As the result, some identical endpoints are being represented by multiple different indices, which significantly affects the compression ratio. Note that this is not the case for selectors, as their corresponding vector components are rounded after quantization, but instead it leads to some duplicate selectors in the codebook being not used. In the modified version of the algorithm all the duplicate codebook entries are merged together, unused entries are removed from the codebooks, the endpoint and selector indices are updated accordingly.
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 / 28.835 sec
Modified: 1494630 bytes / 25.637 sec
Improvement: 5.54% (compression ratio) / 11.09% (compression time)
[Compressing Kodak set with mipmaps]
Original: 2065243 bytes / 36.875 sec
Modified: 1946533 bytes / 33.546 sec
Improvement: 5.75% (compression ratio) / 9.03% (compression time)
This change slightly improves compression ratio and compression time.
Explanation:
The efficiency of the Crunch encoding scheme depends on the similarity between the neighbour chunks. For this reason in original version of Crunch the order of chunks is reversed after each scanline, so that there is no jump from one side of the image to another at the image borders. The problem here is that inside of each chunk, the blocks are normally ordered in a usual up-to-down-left-to-right manner, regardless of the chunk scanning order. While on the forward scan we normally need to perform diagonal jumps (+1, +1) in order to get to the next chunk, on the reverse scan we normally need to perform much larger (-3, +1) jumps, which usually defeats the advantage of not having discontinuity at the image borders.
Note:
This modification alters the output 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 / 28.882 sec
Modified: 1579618 bytes / 28.743 sec
Improvement: 0.16% (compression ratio) / 0.48% (compression time)
[Compressing Kodak set with mipmaps]
Original: 2065243 bytes / 36.920 sec
Modified: 2061499 bytes / 36.833 sec
Improvement: 0.18% (compression ratio) / 0.24% (compression time)
Alexander Suvorov
richgel99@gmail.com