This page summarizes the major functional and performance changes in each release of the 4.x series.

All performance data on this page is measured on an Intel Core i5-9600K clocked at 4.2 GHz, running astcenc using AVX2 and 6 threads.

Status: May 2024

The 4.8.0 release is a minor maintenance release.

• General:
  • Bug fix: Native builds on macOS will now correctly build for arm64 when run outside of Rosetta on an Apple silicon device.
  • Bug fix: Multiple small improvements to remove use of undefined language behavior, to improve support for deployment using Emscripten.
  • Feature: Builds using Clang can now build with undefined behavior sanitizer by setting -DASTCENC_UBSAN=ON on the CMake configure line.
  • Feature: Updated to Wuffs library 0.3.4, which ignores tRNS alpha chunks for type 4 (LA) and 6 (RGBA) PNGs, to improve compatibility with libpng.

Status: January 2024

The 4.7.0 release is a major maintenance release, fixing rounding behavior in the decompressor to match the Khronos specification. This fix includes the addition of explicit support for optimizing for decode_unorm8 rounding.

Reminder - the codec library API is not designed to be binary compatible across versions. We always recommend rebuilding your client-side code using the updated astcenc.h header.

• General:
  • Bug fix: sRGB LDR decompression now uses the correct endpoint expansion method to create the 16-bit RGB endpoint colors, and removes the previous correction code from the interpolation function. This bug could result in LSB bit flips relative to the standard specification.
  • Bug fix: Decompressing to an 8-bit per component output image now matches the decode_unorm8 extension rounding rules. This bug could result in LSB bit flips relative to the standard specification.
  • Bug fix: Code now avoids using alignas() in the reference C implementation, as the default alignas(16) is narrower than the native minimum alignment requirement on some CPUs.
  • Feature: Library configuration supports a new flag, ASTCENC_FLG_USE_DECODE_UNORM8. This flag indicates that the image will be used with the decode_unorm8 decode mode. When set during compression this allows the compressor to use the correct rounding when determining the best encoding.
  • Feature: Command line tool supports a new option, -decode_unorm8. This option indicates that the image will be used with the decode_unorm8 decode mode. This option will automatically be set for decompression (-d*) and trial (-t*) tool operation if the decompressed output image is stored to an 8-bit per component file format. This option must be set manually for compression (-c*) tool operation, as the desired decode mode cannot be reliably determined.
  • Feature: Library configuration supports a new optional progress reporting callback to be specified. This is called during compression to to allow interactive tooling use cases to display incremental progress. The command line tool uses this feature to show compression progress unless -silent is used.

Status: November 2023

The 4.6.1 release is a minor maintenance release to fix a scaling bug on large core count Windows systems.

• General:
  • Optimization: Windows builds of the astcenc command line tool can now use more than 64 cores on large core count systems. This change doubled command line performance for -exhaustive compression when testing on an 96 core/192 thread system.
  • Feature: Windows Arm64 native builds of the astcenc command line tool are now included in the prebuilt release binaries.

Status: November 2023

The 4.6.0 release retunes the compressor heuristics to give improvements to performance for trivial losses to image quality. It also includes some minor bug fixes and code quality improvements.

Reminder - the codec library API is not designed to be binary compatible across versions. We always recommend rebuilding your client-side code using the updated astcenc.h header.

• General:
  • Bug-fix: Fixed context allocation for contexts allocated with the ASTCENC_FLG_DECOMPRESS_ONLY flag.
  • Bug-fix: Reduced use of reinterpret_cast in the core codec to avoid strict aliasing violations.
  • Optimization: -medium search quality no longer tests 4 partition encodings for block sizes between 25 and 83 texels (inclusive). This improves performance for a tiny drop in image quality.
  • Optimization: -thorough and higher search qualities no longer test the mode0 first search for block sizes between 25 and 83 texels (inclusive). This improves performance for a tiny drop in image quality.
  • Optimization: TUNE_MAX_PARTITIONING_CANDIDATES reduced from 32 to 8 to reduce the size of stack allocated data structures. This causes a tiny drop in image quality for the -verythorough and -exhaustive presets.

Status: June 2023

The 4.5.0 release is a maintenance release with small image quality improvements, and a number of build system quality of life improvements.

• General:
  • Bug-fix: Improved handling compiler arguments in CMake, including consistent use of MSVC-style command line arguments for ClangCL.
  • Bug-fix: Invariant Clang builds now use -ffp-model=precise with -ffp-contract=off which is needed to restore invariance due to recent changes in compiler defaults.
  • Change: macOS binary releases are now distributed as a single universal binary for all platforms.
  • Change: Windows binary releases are now compiled with VS2022.
  • Change: Invariant MSVC builds for VS2022 now use /fp:precise instead of /fp:strict, which is is now possible because precise no longer implies contraction. This should improve performance for MSVC builds.
  • Change: Non-invariant Clang builds now use -ffp-model=precise with -ffp-contract=on. This should improve performance on older Clang versions which defaulted to no contraction.
  • Change: Non-invariant MSVC builds for VS2022 now use /fp:precise with /fp:contract. This should improve performance for MSVC builds.
  • Change: CMake config variables now use an ASTCENC_ prefix to add a namespace and group options when the library is used in a larger project.
  • Change: CMake config ASTCENC_UNIVERSAL_BUILD for building macOS universal binaries has been improved to include the x86_64h slice for AVX2 builds. Universal builds are now on by default for macOS, and always include NEON (arm64), SSE4.1 (x86_64), and AVX2 (x86_64h) variants.
  • Change: CMake config ASTCENC_NO_INVARIANCE has been inverted to remove the negated option, and is now ASTCENC_INVARIANCE with a default of ON. Disabling this option can substantially improve performance, but images can different across platforms and compilers.
  • Optimization: Color quantization and packing for LDR RGB and RGBA has been vectorized to improve performance.
  • Change: Color quantization for LDR RGB and RGBA endpoints will now try multiple quantization packing methods, and pick the one with the lowest endpoint encoding error. This gives a minor image quality improvement, for no significant performance impact when combined with the vectorization optimizations.

Status: March 2023

The 4.4.0 release is a minor release with image quality improvements, a small performance boost, and a few new quality-of-life features.

• General:
  • Change: Core library no longer checks availability of required instruction set extensions, such as SSE4.1 or AVX2. Checking compatibility is now the responsibility of the caller. See astcenccli_entry.cpp for an example of code performing this check.
  • Change: Core library can be built as a shared object by setting the -DSHAREDLIB=ON CMake option, resulting in e.g. libastcenc-avx2-shared.so. Note that the command line tool is always statically linked.
  • Change: Decompressed 3D images will now write one output file per slice, if the target format is a 2D image format.
  • Change: Command line errors print to stderr instead of stdout.
  • Change: Color encoding uses new quantization tables, that now factor in floating-point rounding if a distance tie is found when using the integer quant256 value. This improves image quality for 4x4 and 5x5 block sizes.
  • Optimization: Partition selection uses a simplified line calculation with a faster approximation. This improves performance for all block sizes.
  • Bug-fix: Fixed missing symbol error in decompressor-only builds.
  • Bug-fix: Fixed infinity handling in debug trace JSON files.

Key for charts:

• Color = block size (see legend).
• Letter = image format (N = normal map, G = grayscale, L = LDR, H = HDR).

Relative performance vs 4.3 release:

Status: January 2023

The 4.3.1 release is a minor maintenance release. No performance or image quality changes are expected.

• General:
  • Bug-fix: Fixed typo in -2/3/4partitioncandidatelimit CLI options.
  • Bug-fix: Fixed handling for -3/4partitionindexlimit CLI options.
  • Bug-fix: Updated to stb_image.h v2.28, which includes multiple fixes and improvements for image loading.

Status: January 2023

The 4.3.0 release is an optimization release. There are minor performance and image quality improvements in this release.

Reminder - the codec library API is not designed to be binary compatible across versions. We always recommend rebuilding your client-side code using the updated astcenc.h header.

• General:
  • Bug-fix: Use lower case windows.h include for MinGW compatibility.
  • Change: The -mask command line option, ASTCENC_FLG_MAP_MASK in the library API, has been removed.
  • Optimization: Always skip blue-contraction for QUANT_256 encodings. This gives a small image quality improvement for the 4x4 block size.
  • Optimization: Always skip RGBO vector calculation for LDR encodings.
  • Optimization: Defer color packing and scrambling to physical layer.
  • Optimization: Remove folded decimation_info lookup tables. This significantly reduces compressor memory footprint and improves context creation time. Impact increases with the active block size.
  • Optimization: Increased trial and refinement pruning by using stricter target errors when determining whether to skip iterations.

Key for charts:

• Color = block size (see legend).
• Letter = image format (N = normal map, G = grayscale, L = LDR, H = HDR).

Relative performance vs 4.2 release:

Status: November 2022

The 4.2.0 release is an optimization release. There are significant performance improvements, minor image quality improvements, and library interface changes in this release.

Reminder - the codec library API is not designed to be binary compatible across versions. We always recommend rebuilding your client-side code using the updated astcenc.h header.

• General:
  • Bug-fix: Compression for RGB and RGBA base+offset encodings no longer generate endpoints with the incorrect blue-contract behavior.
  • Bug-fix: Lowest channel correlation calculation now correctly ignores constant color channels for the purposes of filtering 2 plane encodings. On average this improves both performance and image quality.
  • Bug-fix: ISA compatibility now checked in config_init() as well as in context_alloc().
  • Change: Removed the low-weight count optimization, as more recent changes had significantly reduced its performance benefit. Option removed from both command line and configuration structure.
  • Feature: The -exhaustive mode now runs full trials on more partitioning candidates and block candidates. This improves image quality by 0.1 to 0.25 dB, but slows down compression by 3x. The -verythorough and -thorough modes also test more candidates.
  • Feature: A new preset, -verythorough, has been introduced to provide a standard performance point between -thorough and the re-tuned -exhaustive mode. This new mode is faster and higher quality than the -exhaustive preset in the 4.1 release.
  • Feature: The compressor can now independently vary the number of partitionings considered for error estimation for 2/3/4 partitions. This allows heuristics to put more effort into 2 partitions, and less in to 3/4 partitions.
  • Feature: The compressor can now run trials on a variable number of candidate partitionings, allowing high quality modes to explore more of the search space at the expense of slower compression. The number of trials is independently configurable for 2/3/4 partition cases.
  • Optimization: Introduce early-out threshold for 2/3/4 partition searches based on the results after 1 of 2 trials. This significantly improves performance for -medium and -thorough searches, for a minor loss in image quality.
  • Optimization: Reduce early-out threshold for 3/4 partition searches based on 2/3 partition results. This significantly improves performance, especially for -thorough searches, for a minor loss in image quality.
  • Optimization: Use direct vector compare to create a SIMD mask instead of a scalar compare that is broadcast to a vector mask.
  • Optimization: Remove obsolete partition validity masks from the partition selection algorithm.
  • Optimization: Removed obsolete channel scaling from partition avgs_and_dirs() calculation.

Key for charts:

• Color = block size (see legend).
• Letter = image format (N = normal map, G = grayscale, L = LDR, H = HDR).

Relative performance vs 4.0 and 4.1 release:

Status: August 2022

The 4.1.0 release is a maintenance release. There is no performance or image quality change in this release.

• General:
  • Change: Command line decompressor no longer uses the legacy GL_LUMINANCE or GL_LUMINANCE_ALPHA format enums when writing KTX output files. Luminance textures now use the GL_RED format and luminance_alpha textures now use the GL_RG format.
  • Change: Command line tool gains a new -dimage option to generate diagnostic images showing aspects of the compression encoding. The output file name with its extension stripped is used as the stem of the diagnostic image file names.
  • Bug-fix: Library decompressor builds for SSE no longer use masked store maskmovdqu instructions, as they can generate faults on masked lanes.
  • Bug-fix: Command line decompressor now correctly uses sized type enums for the internal format when writing output KTX files.
  • Bug-fix: Command line compressor now correctly loads 16 and 32-bit per component input KTX files.
  • Bug-fix: Fixed GCC9 compiler warnings on Arm aarch64.

Status: July 2022

The 4.0.0 release introduces some major performance enhancement, and a number of larger changes to the heuristics used in the codec to find a more effective cost:quality trade off.

• General:
  • Change: The -array option for specifying the number of image planes for ASTC 3D volumetric block compression been renamed to -zdim.
  • Change: The build root package directory is now bin instead of astcenc, allowing the CMake install step to write binaries into /usr/local/bin if the user wishes to do so.
  • Feature: A new -ssw option for specifying the shader sampling swizzle has been added as convenience alternative to the -cw option. This is needed to correct error weighting during compression if not all components are read in the shader. For example, to extract and compress two components from an RGBA input image, weighting the two components equally when sampling through .ra in the shader, use -esw ggga -ssw ra. In this example -ssw ra is equivalent to the alternative -cw 1 0 0 1 encoding.
  • Feature: The -a alpha weighting option has been re-enabled in the backend, and now again applies alpha scaling to the RGB error metrics when encoding. This is based on the maximum alpha in each block, not the individual texel alpha values used in the earlier implementation.
  • Feature: The command line tool now has -repeats <count> for testing, which will iterate around compression and decompression count times. Reported performance metrics also now separate compression and decompression scores.
  • Feature: The core codec is now warning clean up to /W4 for both MSVC cl.exe and clangcl.exe compilers.
  • Feature: The core codec now supports arm64 for both MSVC cl.exe and clangcl.exe compilers.
  • Feature: NO_INVARIANCE builds will enable the -ffp-contract=fast option for all targets when using Clang or GCC. In addition AVX2 targets will also set the -mfma option. This reduces image quality by up to 0.2dB (normally much less), but improves performance by up to 5-20%.
  • Optimization: Angular endpoint min/max weight selection is restricted to weight QUANT_11 or lower. Higher quantization levels assume default 0-1 range, which is less accurate but much faster.
  • Optimization: Maximum weight quantization for later trials is selected based on the weight quantization of the best encoding from the 1 plane 1 partition trial. This significantly reduces the search space for the later trials with more planes or partitions.
  • Optimization: Small data tables now use in-register SIMD permutes rather than gathers (AVX2) or unrolled scalar lookups (SSE/NEON). This can be a significant optimization for paths that are load unit limited.
  • Optimization: Decompressed image block writes in the decompressor now use a vectorized approach to writing each row of texels in the block, including to ability to exploit masked stores if the target supports them.
  • Optimization: Weight scrambling has been moved into the physical layer; the rest of the codec now uses linear order weights.
  • Optimization: Weight packing has been moved into the physical layer; the rest of the codec now uses unpacked weights in the 0-64 range.
  • Optimization: Consistently vectorize the creation of unquantized weight grids when they are needed.
  • Optimization: Remove redundant per-decimation mode copies of endpoint and weight structures, which were really read-only duplicates.
  • Optimization: Early-out the same endpoint mode color calculation if it cannot be applied.
  • Optimization: Numerous type size reductions applied to arrays to reduce both context working buffer size usage and stack usage.

Key for charts:

• Color = block size (see legend).
• Letter = image format (N = normal map, G = grayscale, L = LDR, H = HDR).

Relative performance vs 3.7 release:

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