Basis SIMD BC7 Texture Encoder v1.18
Copyright (C) 2018-2020 Binomial LLC, All rights reserved

For questions or problems, please contact rich@binomial.info.

Note the very latest version of bc7e.ispc, with determinism fixes and improvements to solid color block encoding, is here: https://github.com/richgel999/bc7enc_rdo. This repo is an archive of the original release of bc7e.

-- Legal Stuff

This release of bc7e.ispc is covered by the Apache 2.0 license - see LICENSE.

-- Special Thanks

A huge thanks to the folks at Activision for enabling us to open source this code.

-- Quick Intro

This package contains bc7e.ispc (or just BC7E), our fast and high quality SIMD
BC7 encoder. This encoder was designed to compete directly against the best
available open source BC7 encoders.

In practice, this means we compete directly against Intel's "ispc_texcomp"
library, as the other available CPU encoders are impractically slow. BC7E is
usually around 2-3x faster vs. ispc_texcomp at the same average quality (as
measured by PSNR or SSIM), and up to ~8x faster vs. ispc_texcomp's "slow"
profile. Note that BC7E is not a derivative of ispc_texcomp - it's a totally
new encoder using our own algorithms.

BC7E currently supports Euclidean distance or perceptual (scaled YCbCr)
colorspace metrics. 

BC7E doesn't use approximate (and non-deterministic/ill-defined) SSE rsqrt() or rcp() instructions, unlike ispc_texcomp.

-- Building It

Currently, the included example compiles under Windows, but it should be easy to compile 
under other platforms. We tested this release with Visual Studio 2019. We'll add CMake files next.

To build it, you'll need a copy of Intel's ISPC compiler for your platform:

https://ispc.github.io/downloads.html

The package contains the latest pre-built version of ispc.exe (v1.10.0) as of
3/24/19. We've tested BC7E under Windows, Linux and OSX. We've tested mostly
using ispc v1.82, and less with v1.10.0.

bc7e.ispc is built like this (for max perf on all CPU's):
ispc -g -O2 "bc7e.ispc" -o "$bc7e.obj" -h "$bc7e_ispc.h" --target=sse2,sse4,avx,avx2 --opt=fast-math --opt=disable-assertions

"--opt=fast-math" is optional. In some quick testing, it didn't make a noticeable difference.

If determinism across Intel/AMD CPU's is important in your usage case, you'll want to probably limit the targets to only SSE and disable fast math.

In the .ispc source code, we've prefixed all scatter/gathers with "#pragma ignore warning(perf)", as they just cloud the compiler's output and aren't performance critical.

-- API

The C-style API is modeled after ispc_texcomp's and is very simple. Note that
(just like with ispc_texcomp) you must do all multithreading on your own - BC7E
just encodes blocks on a single thread using SIMD instructions.

First, before doing anything with BC7E, call ispc::bc7e_compress_block_init() a
single time (and preferably only a single time). This method computes some
lookup tables used to accelerate encoding. If you fail to call this method, the
encoder will always return all-zero BC7 block data. (This is different from
ispc_texcomp.)

Next, call one of these functions in the BC7E ispc code to select the encoding
profile you want to use:

ispc::bc7e_compress_block_params_init_ultrafast(struct bc7e_compress_block_params * p, bool perceptual) (mode 6 only for non-alpha blocks)
ispc::bc7e_compress_block_params_init_veryfast()
ispc::bc7e_compress_block_params_init_fast() 
ispc::bc7e_compress_block_params_init_basic() (a good default profile)
ispc::bc7e_compress_block_params_init_slow()

The fastest mode is ultrafast on opaque blocks, which selects an optimized code path that only supports mode 6.

Note these functions are calibrated to compete against ispc_texcomp's similarly
named profiles. The "slow" profile is significantly faster than ispc_texcomp's
(by around 8x). Also, unlike ispc_texcomp BC7E automatically determines if each
block has any pixels using alpha, so there's no need to select an alpha specific profile.

These two profiles are slower than "slow", but have higher quality, and are still faster than ispc_texcomp's "slow" profile:
ispc::bc7e_compress_block_params_init_slowest()
ispc::bc7e_compress_block_params_init_veryslow()

It's possible to customize the codec yourself by tweaking one of these basic profiles.

Each of these init functions takes a pointer to an encoding params struct
(ispc::bc7e_compress_block_params), and a bool "perceptual" parameter. If you
know the source pixels will be in sRGB space, enabling perceptual mode will
noticeably improve the Y PSNR/SSIM, possibly allowing you to use a faster
encoding profile.

These init function are all thread safe: they just fill in the params struct you provide with internal codec settings.

Finally, call ispc::bc7e_compress_blocks() with an array of 4x4 pixel blocks. You should
always call this function with a multiple of 8-16 blocks. Try to call it with at
least 32-64 blocks. Note that this function wants a pointer to an array of 16
pixel blocks, one block after the other, which is slightly different from
ispc_texcomp's input. This function is thread safe.

If you call this function without calling bc7e_compress_block_init() first, the
encoder will return blocks filled with all 0's (or assert() if you build the
ispc file in debug).

-- Optional support for encoding textures with decorrelated alpha channels

BC7's is weakest with textures containing decorrelated alpha channels. This can
lead to noticeable blockiness in either RGB or A with every encoder we've tried.
By default, the encoder doesn't do anything special vs. other encoders to handle
this scenario. It normally optimizes for lowest overall RGBA error, which can
cause the encoder to select correlated alpha modes that cause either RGB or A to
appear overly blocky (but still leading to overall lowest error).

We've added an optional mode 6/7 specific error metric weighting vector, which
allows you to nudge the encoder to use the correlated alpha modes less often.

To use this feature, after you call one of the profile selection functions
(bc7e_compress_block_params_init_basic() etc.), you can optionally set the
values in the "m_alpha_settings.m_mode67_error_weight_mul[]" array. 

This array contains a per-component error weight multiplier that's
only used in modes 6/7. This allows you to deemphasize the usage of the
correlated alpha modes (6/7). These modes can cause blockiness in either RGB or
A on highly uncorrelated textures containing complex alpha channels. To use
this, I would first start with setting the RGB (first 3 array values) to 3,3,3
or 5,5,5 and test the results. 

Setting these values higher than 1 will cause the encoder to use modes 4/5 more
often on alpha blocks. This will result in higher overall PSNR/SSIM error, but
hopefully less blockiness.

-- bc7enc example

We've included a very simple C++ command line example of how to use BC7E. It uses
OpenMP for threading. It's a simple tool that packs images to DX10 BC7 .DDS
files. (Note that note all DDS viewers support these newer files.)

It should build on Windows/Linux/OSX (although we didn't get a chance to include
the CMake files in v1.18). We've built it with VS2015 and VS2017.

-- Possible future improvements

Currently we're thinking along the lines of faster encoding by more closely
tuning the code generation of the key inner loops for SSE4/AVX, more
decorrelated alpha improvements, and custom error metrics. We have done a Pareto
analysis of the codec, which has given us some interesting directions to look
at. Any suggestions are welcome.

-- Bug reports/questions

If you have any bugs or suggestions about this codec, don't hesitate to contact Rich
Geldreich: rich@binomial.info or richgel99@gmail.com.