This change improves the compression speed for both DXT and ETC encodings.
Explanation:
When a node is split during the quantization step, all of its vectors are split between the child nodes, and new memory is allocated to store each new set of vectors. At the same time, the set of vectors of the parent node is no longer accessed after the split. Considering that the sets of vectors of the child nodes do not intersect, it is possible to reuse the memory allocated for the parent set of vectors, to store the child sets of vectors. This can be achieved in the following way. All the source vectors are initially stored in an array. Let's assume that it is possible to reorder this common array of vectors in such a way, so that vectors of each node would form a continuous block within this array. Then it would be sufficient to store only two indices for each node (pointing to the first and to the last node vectors in the common array of vectors) in order to describe the complete set of vectors of this node. This assumption is correct for the root node, which has initial vector indices pointing to the first and to the last elements of the complete vector array. When a node is split, let's reorder its vectors (stored in a continuous block within the common array of vectors) in such a way, so that vectors of the left child node are put in front, and then followed by the vectors of the right child node (the indices of the first and last vectors of the child nodes should be set accordingly). This way each child node will also have its vectors stored in a continuous block within the common array of vectors, defined by two indices, and the split iteration can be repeated. Note that the memory, which is used to store the sets of vectors for all the nodes, now needs to be allocated only once.
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 / 28.872 sec
Modified: 1468204 bytes / 8.276 sec
Improvement: 7.21% (compression ratio) / 71.34% (compression time)
[Compressing Kodak set with mipmaps using DXT1 encoding]
Original: 2065243 bytes / 36.971 sec
Modified: 1914805 bytes / 10.944 sec
Improvement: 7.28% (compression ratio) / 70.40% (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: 15.364 sec
Average bitrate: 1.363 bpp
Average Luma PSNR: 34.050 dB
Alexander Suvorov
11a89d25ed