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https://github.com/richgel999/bc7enc16

Fast single source file BC7/BPTC texture encoder with perceptual metric support
https://github.com/richgel999/bc7enc16

Last synced: 30 days ago
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Fast single source file BC7/BPTC texture encoder with perceptual metric support

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bc7enc16 - Fast, single source file BC7/BPTC GPU texture encoder with perceptual colorspace metric support



Note: Since this repo was created, we've released two new codecs with better BC7 encoders:

https://github.com/richgel999/bc7enc_rdo

https://github.com/BinomialLLC/bc7e



bc7enc16 purposely only supports modes 1 and 6. This is a strong opaque texture encoder, with basic

support for alpha channels (using mode 6). The intended use case is opaque textures, or opaque textures 

with relatively simple alpha channels. It also acts as a relatively simple to understand example.



If alpha is highly correlated compared to RGB, or alpha is relatively simple

(think simple masks where lots of blocks are either all-transparent or

all-opaque), it should work great. For complex alpha channels more modes (such

as 4, 5 or maybe 7) are necessary.



This codec supports a perceptual mode, where it computes colorspace error in

weighted YCbCr space (like etc2comp), and it also supports weighted RGBA

metrics. It's particular strong in perceptual mode, beating the current state of

the art CPU encoder (Intel's ispc_texcomp) by a wide margin when measured by

Luma PSNR, even though it only supports 2 modes and isn't vectorized.



Why only modes 1 and 6?

Because with these two modes you have a complete encoder that supports both

opaque and transparent textures in a small amount (~1400 lines) of

understandable plain C code. Mode 6 excels on smooth blocks, and mode 1 is

strong with complex blocks, and a strong encoder that combines both modes can be

quite high quality. Fast mode 6-only encoders will have noticeable block

artifacts which this codec avoids by fully supporting mode 1.



Modes 1 and 6 are typically the most used modes on many textures using other

encoders. Mode 1 has two subsets, 64 possible partitions, and 3-bit indices,

while mode 6 has large 4-bit indices and high precision 7777.1 endpoints. This

codec produces output that is far higher quality than any BC1 encoder, and

approaches (or in perceptual mode exceeds!) the quality of other full BC7

encoders.



Why is bc7enc16 so fast in perceptual mode?

Computing error in YCbCr space is more expensive than in RGB space, yet bc7enc16

in perceptual mode is stronger than ispc_texcomp (see the benchmark below) -

even without SSE/AVX vectorization and with only 2 modes to work with!



Most BC7 encoders only support linear RGB colorspace metrics, which is a

fundamental weakness. Some support weighted RGB metrics, which is better. With

linear RGB metrics, encoding error is roughly balanced between each channel, and

encoders have to work *very* hard (examining large amounts of RGB search space)

to get overall quality up. With perceptual colorspace metrics, RGB error tends

to become a bit unbalanced, with green quality favored more highly than red and

blue, and blue quality favored the least. A perceptual encoder is tuned to

prefer exploring solutions along the luma axis, where it's much less work to find

solutions with less luma error. bc7enc16 is, as far as I know, the first BC7

codec to support computing error in weighted YCbCr colorspace.



Note: Most of the timings here (except for the ispc_texcomp "fast" mode timings at the very bottom)

are for the *original* release, before I added several more optimizations. The latest version of 

bc7enc16.c is around 8-27% faster than the initial release at same quality (when mode 1 is enabled - 

there's no change with just mode 6).



Some benchmarks across 31 images (kodim corpus+others):



Perceptual (average REC709 Luma PSNR - higher is better quality):



iscp_texcomp slow vs. bc7enc16 uber4/max_partitions 64

iscp_texcomp:   355.4 secs 48.6 dB

bc7enc16:       122.6 secs 50.0 dB



iscp_texcomp slow vs. bc7enc16 uber0/max_partitions 64

iscp_texcomp:   355.4 secs 48.6 dB

bc7enc16:       38.3 secs 49.6 dB



iscp_texcomp basic vs. bc7enc16 uber0/max_partitions 16

ispc_texcomp:   100.2 secs 48.3 dB

bc7enc16:       20.8 secs 49.3 dB 



iscp_texcomp fast vs. bc7enc16 uber0/max_partitions 16

iscp_texcomp:   41.5 secs 48.0 dB 

bc7enc16:       20.8 secs 49.3 dB



iscp_texcomp ultrafast vs. bc7enc16 uber0/max_partitions 0

iscp_texcomp:   1.9 secs 46.2 dB

bc7enc16:       8.9 secs 48.4 dB 



Non-perceptual (average RGB PSNR):



iscp_texcomp slow vs. bc7enc16 uber4/max_partitions 64

iscp_texcomp:   355.4 secs 46.8 dB 

bc7enc16:       51 secs 46.1 dB



iscp_texcomp slow vs. bc7enc16 uber0/max_partitions 64

iscp_texcomp:   355.4 secs 46.8 dB

bc7enc16:       29.3 secs 45.8 dB



iscp_texcomp basic vs. bc7enc16 uber4/max_partitions 64

iscp_texcomp:   99.9 secs 46.5 dB

bc7enc16:       51 secs 46.1 dB



iscp_texcomp fast vs. bc7enc16 uber1/max_partitions 16

ispc_texcomp:   41.5 secs 46.1 dB

bc7enc16:       19.8 secs 45.5 dB



iscp_texcomp fast vs. bc7enc16 uber0/max_partitions 8

ispc_texcomp:   41.5 secs 46.1 dB

bc7enc16:       10.46 secs 44.4 dB



iscp_texcomp ultrafast vs. bc7enc16 uber0/max_partitions 0

ispc_texcomp:   1.9 secs 42.7 dB 

bc7enc16:       3.8 secs 42.7 dB



DirectXTex CPU in "mode 6 only" mode vs. bc7enc16 uber1/max_partions 0 (mode 6 only), non-perceptual:



DirectXTex:     466.4 secs 41.9 dB 

bc7enc16:       6.7 secs 42.8 dB



DirectXTex CPU in (default - no 3 subset modes) vs. bc7enc16 uber1/max_partions 64, non-perceptual:



DirectXTex:     9485.1 secs 45.6 dB 

bc7enc16:       36 secs 46.0 dB



(Note this version of DirectXTex has a key pbit bugfix which I've submitted but

is still waiting to be accepted. Non-bugfixed versions will be slightly lower

quality.)



UPDATE: To illustrate how strong the mode 1+6 implementation is in bc7enc16, let's compare ispc_texcomp 

fast vs. the latest version of bc7enc16 uber4/max_partitions 64:



Without filterbank optimizations:



                Time       RGB PSNR   Y PSNR

ispc_texcomp:   41.45 secs 46.09 dB   48.0 dB

bc7enc16:       41.42 secs 46.03 dB   48.2 dB



With filterbank optimizations enabled:

bc7enc16:       38.78 secs 45.94 dB   48.12 dB



They both have virtually the same average RGB PSNR with these settings (.06 dB is basically noise), but 

bc7enc16 is just as fast as ispc_texcomp fast, even though it's not vectorized. Interestingly, our Y PSNR is better, 

although bc7enc16 wasn't using perceptual metrics in these benchmarks. 



This was a multithreaded benchmark (using OpenMP) on a dual Xeon workstation.

ispc_texcomp was called with 64-blocks at a time and used AVX instructions.

Timings are for encoding only.