bc7enc16

by richgel999

richgel999 / bc7enc16

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

This project is basically a demo of some of the techniques we use in Basis BC7, which is Binomial's state of the art vectorized BC7 encoder. Basis BC7 is the highest quality and fastest CPU BC7 encoder available (2-3x faster than ispc_texcomp). It supports all modes and linear/perceptual colorspace metrics. Licensees get full ISPC source code so they can customize the codec as needed.

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.

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):

iscptexcomp slow vs. bc7enc16 uber4/maxpartitions 64 iscp_texcomp: 355.4 secs 48.6 dB bc7enc16: 122.6 secs 50.0 dB

iscptexcomp slow vs. bc7enc16 uber0/maxpartitions 64 iscp_texcomp: 355.4 secs 48.6 dB bc7enc16: 38.3 secs 49.6 dB

iscptexcomp basic vs. bc7enc16 uber0/maxpartitions 16 ispc_texcomp: 100.2 secs 48.3 dB bc7enc16: 20.8 secs 49.3 dB

iscptexcomp fast vs. bc7enc16 uber0/maxpartitions 16 iscp_texcomp: 41.5 secs 48.0 dB bc7enc16: 20.8 secs 49.3 dB

iscptexcomp ultrafast vs. bc7enc16 uber0/maxpartitions 0 iscp_texcomp: 1.9 secs 46.2 dB bc7enc16: 8.9 secs 48.4 dB

Non-perceptual (average RGB PSNR):

iscptexcomp slow vs. bc7enc16 uber4/maxpartitions 64 iscp_texcomp: 355.4 secs 46.8 dB bc7enc16: 51 secs 46.1 dB

iscptexcomp slow vs. bc7enc16 uber0/maxpartitions 64 iscp_texcomp: 355.4 secs 46.8 dB bc7enc16: 29.3 secs 45.8 dB

iscptexcomp basic vs. bc7enc16 uber4/maxpartitions 64 iscp_texcomp: 99.9 secs 46.5 dB bc7enc16: 51 secs 46.1 dB

iscptexcomp fast vs. bc7enc16 uber1/maxpartitions 16 ispc_texcomp: 41.5 secs 46.1 dB bc7enc16: 19.8 secs 45.5 dB

iscptexcomp fast vs. bc7enc16 uber0/maxpartitions 8 ispc_texcomp: 41.5 secs 46.1 dB bc7enc16: 10.46 secs 44.4 dB

iscptexcomp ultrafast vs. bc7enc16 uber0/maxpartitions 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 ispctexcomp fast vs. the latest version of bc7enc16 uber4/maxpartitions 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.

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