astcenc

NAME

astcenc - compress or decompress images using the ASTC format

SYNOPSIS

[-cl|-cs|-ch|-cH] <in> <out> <blocksize> <preset> [options] [-dl|-ds|-dh|-dH] <in> <out> <blocksize> <preset> [options] [-tl|-ts|-th|-tH] <in> <out> <blocksize> <preset> [options]

DESCRIPTION

astcenc compresses image files into the Adaptive Scalable Texture Compression (ASTC) image format, a lossy compression format design for use in real-time graphics applications. It is a fully featured compressor implementation, supporting all of the compression profiles and block sizes specified by the ASTC format:

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All color profiles (LDR linear, LDR sRGB, and HDR)

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All 2D block sizes (4x4 through to 12x12)

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All 3D block sizes (3x3x3 through to 6x6x6)

The compressor provides a number of pre-determined quality presets, which allow users to tradeoff compressed image quality against compression performance. For advanced users the compressor provides many additional control options. astcenc can also be used to decompress ASTC compressed images, and perform compression image quality analysis.

COMPRESSION

To compress an image using the ASTC format you must specify the color profile, the input file name, the output file name, the target block size, and the quality preset. The color profile is specified using the -cl (LDR linear), -cs (LDR sRGB), -ch (HDR RGB, LDR A), or -cH(HDR RGBA) encoder options. Note that not all hardware implementations of ASTC support the HDR profile. The input file path must match a valid file format for compression, and the output file format must be a valid output for compression. See the FILE FORMATS section for the list of supported formats. The block size must be a valid ASTC block size. Every block compresses into 128 bits of compressed output, so the block size determines the compressed data bitrate. Supported 2D block sizes are:

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4x4: 8.00 bpp

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5x4: 6.40 bpp

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5x5: 5.12 bpp

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6x5: 4.27 bpp

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6x6: 3.56 bpp

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8x5: 3.20 bpp

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8x6: 2.67 bpp

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10x5: 2.56 bpp

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10x6: 2.13 bpp

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8x8: 2.00 bpp

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10x8: 1.60 bpp

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10x10: 1.28 bpp

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12x10: 1.07 bpp

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12x12: 0.89 bpp

Supported 3D block sizes are:

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3x3x3: 4.74 bpp

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4x3x3: 3.56 bpp

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4x4x3: 2.67 bpp

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4x4x4: 2.00 bpp

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5x4x4: 1.60 bpp

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5x5x4: 1.28 bpp

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5x5x5: 1.02 bpp

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6x5x5: 0.85 bpp

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6x6x5: 0.71 bpp

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6x6x6: 0.59 bpp

The quality preset configures the quality-performance tradeoff for the compressor; more complete searches of the search space improve image quality at the expense of compression time. The available presets are:

-fastest

 

-fast

 

-medium

 

-thorough

 

-exhaustive

Using the -fastest setting throws away a lot of image quality compared. It is useful for quickly roughing-out new content, but we recommend using higher quality settings for production builds. Using the -exhaustive setting significantly increases compression time, but typically only gives minor quality improvements over using -thorough. There are a number of additional compressor options which are useful to consider for common usage, based on the type of image data being compressed.

-mask

The input texture is a mask texture with unrelated data stored in the various color channels, so enable error heuristics that aim to improve perceptual quality by minimizing the effect of error cross-talk across the color channels.

-normal

The input texture is a three channel linear LDR normal map storing unit length normals as (R=X, G=Y, B=Z). The output will be a two channel X+Y normal map stored as (RGB=X, A=Y), optimized for angular error instead of simple PSNR. The Z component can be recovered programmatically in shader code by using the equation:

nml.xy = texture(...).ga; // Load in [0,1]

nml.xy = nml.xy * 2.0 - 1.0; // Unpack to [-1,1]

nml.z = sqrt(1 - dot(nml.xy, nml.xy)); // Compute Z

-perceptual

The codec should optimize perceptual error, instead of direct RMS error. This aims to improves perceived image quality, but typically lowers the measured PSNR score. Perceptual methods are currently only available for normal maps.

-array <size>

Loads an array of <size> 2D image slices to use as a 3D image. The input filename given is used is decorated with the postfix "_<slice>" to find the file to load. For example, an input named "input.png" would load as input_0.png, input_1.png, etc.

-pp-normalize

Run a preprocess over the image that forces normal vectors to be unit length. Preprocessing applies before any codec encoding swizzle, so normal data must be in the RGB channels in the source image.

-pp-premultiply

Run a preprocess over the image that scales RGB components in the image by the alpha value. Preprocessing applies before any codec encoding swizzle, so color data must be in the RGB channels in the source image.

COMPRESSION TIPS & TRICKS

ASTC is a block-based format that can be prone to block artifacts. If block artifacts are a problem when compressing a given texture, adding some or all of following command-line options may help:

-b 1.8

 

-v 2 1 1 0 25 0.1

 

-va 1 1 0 25

 

-dblimit 60

The -b option is a general-purpose block-artifact reduction option. The -v and -va option settings will concentrate effort where smooth regions lie next to regions with high detail, which are particularly prone to block artifacts. Increasing the -dblimit option is sometimes also needed to force the compressor to keep searching for a better encoding, which can be needed in images with smooth gradients. If a texture exhibits severe block artifacts in only some of the color channels, which is a common problem for mask textures, then using the -cw option to raise the weighting of the affected color channel(s) may help. For example, if the green color channel is particularly badly encoded then try -cw 1 6 1 1.

ADVANCED COMPRESSION

Error weighting options

These options provide low-level control of the codec error metric computation, used to determine what good compression looks like.

-v <radius> <power> <base> <mean> <stdev> <mix>

Compute the per-texel relative error weighting for the RGB color channels as follows:

weight = 1 / (<base> + <mean> * <mean> + <stdev> * <stdev>)

The <radius> argument specifies the texel radius of the neighborhood over which the average and standard deviation are computed.

The <mix> parameter is used to control the degree of mixing of the average and stddev error components across the color channels. Setting this parameter to 0 causes the computation to be done completely separately for each color channel; setting it to 1 causes the results from the RGB channels to be combined and applied to all three together. Intermediate values between these two extremes do a linear mix of the two error values. The <power> argument is a power used to raise the values of the input texels before computing average and standard deviation; e.g. a power of 0.5 causes the codec to take the square root of every input texel value.

-va <power> <base> <mean> <stdev>

Compute the per-texel relative error weighting for the alpha channel, when used in conjunction with -v. See documentation for -v for parameter documentation.

-a <radius>

For textures with alpha channel, scale per-texel weights by the alpha value. The alpha value chosen for scaling of any particular texel is taken as an average across a neighborhood of the texel defined by the <radius> argument. Setting <radius> to 0 causes only the texel's own alpha to be used.

-cw <red> <green> <blue> <alpha>

Assign an additional weight scaling to each color channel, allowing the channels to be treated differently in terms of error significance. Set values above 1 to increase a channel's significance, and values below 1 to decrease it. Set to 0 to exclude a channel from error computation completely.

-b <weight>

Assign an additional weight scaling for texels at compression block edges and corners. Setting this to a value above 1 increases the significance of texels closer to the edges of a block, and can help to reduce block artifacts.

-mpsnr <low> <high>

Set the low and high f-stop values for the mPSNR error metric. The mPSNR error metric only applies to HDR textures.

Performance-quality tradeoff options

These options provide low-level control of the codec heuristics that drive the performance-quality trade off.

-partitionlimit <number>

Test only <number> block partitions. Higher numbers give better quality, however large values give diminishing returns especially for smaller block sizes. Preset defaults are:

-fastest : 2

 

-fast : 4

 

-medium : 25

 

-thorough : 100

 

-exhaustive : 1024

-blockmodelimit <number>

Test only block modes below the <number> usage centile in an empirically determined distribution of block mode frequency. This option is ineffective for 3D textures. Preset defaults are:

-fastest : 25

 

-fast : 50

 

-medium : 75

 

-thorough : 95

 

-exhaustive : 100

-refinementlimit <value>

Iterate only <value> refinement iterations on colors and weights. Minimum value is 1. Preset defaults are:

-fastest : 1

 

-fast : 1

 

-medium : 2

 

-thorough : 4

 

-exhaustive : 4

-candidatelimit <value>

Trial only <value> candidate encodings for each block mode:

-fastest : 1

 

-fast : 2

 

-medium : 2

 

-thorough : 3

 

-exhaustive : 4

-dblimit <number>

Stop compression work on a block as soon as the PSNR of the block, measured in dB, exceeds <number>. This option is ineffective for HDR textures. Preset defaults, where N is the number of texels in a block, are:

-fastest : MAX(53-19*log10(N), 70-35*log10(N))

 

-fast : MAX(63-19*log10(N), 85-35*log10(N))

 

-medium : MAX(70-19*log10(N), 95-35*log10(N))

 

-thorough : MAX(77-19*log10(N), 105-35*log10(N))

 

-exhaustive : 999

-partitionearlylimit <factor>

Stop compression work on a block after only testing blocks with up to two partions and one plane of weights, unless the two partition error term is lower than the error term from encoding with one partition by more than the specified factor. This option is ineffective for normal maps. Preset defaults are:

-fastest : 1.0

 

-fast : 1.0

 

-medium : 1.2

 

-thorough : 2.5

 

-exhaustive : 1000.0

-planecorlimit <factor>

Stop compression after testing only one planes of weights, unless the minimum color correlation factor between any pair of color channels is below this factor. This option is ineffective for normal maps. Preset defaults are:

-fastest : 0.50

 

-fast : 0.50

 

-medium : 0.75

 

-thorough : 0.95

 

-exhaustive : 0.99

Other options

-esw <swizzle>

Swizzle the color components before compression. The swizzle is specified using a 4-character string, which defines the output format ordering. The characters may be taken from the set [rgba01], selecting either input color channels or a literal zero or one. For example to swap the RG channels, and replace alpha with 1, the swizzle 'grb1' should be used.

The input swizzle takes place before any compression, and all error weighting applied using the -cw option is applied to the post-swizzle channel ordering.

By default all 4 post-swizzle channels are included in the error metrics during compression. When using -esw to map two channel data to the L+A endpoint (e.g. -esw rrrg) the luminance data stored in the rgb channels will be weighted three times more strongly than the alpha channel. This can be corrected using the -cw option to zero the weights of unused channels; e.g. using -cw 1 0 0 1.

-dsw <swizzle>

Swizzle the color components after decompression. The swizzle is specified using the same method as the -esw option, with support for an additional "z" character. This is used to specify that the compressed data stores an X+Y normal map, and that the Z output channel should be reconstructed from the two channels stored in the data. For the typical ASTC normal encoding, which uses an 'rrrg' compression swizzle, you should specify an 'raz1' swizzle for decompression.

-yflip

Flip the image in the vertical axis prior to compression and after decompression. Note that using this option in a test mode (-t*) will have no effect as the image will be flipped twice.

-j <threads>

Explicitly specify the number of compression/decompression theads to use in the codec. If not specified, the codec will use one thread per CPU detected in the system.

-silent

Suppresses all non-essential diagnostic output from the codec. Error messages will always be printed, as will mandatory outputs for the selected operation mode. For example, the test mode will always output image quality metrics and compression time but will suppress all other output.

DECOMPRESSION

To decompress an image stored in the ASTC format you must specify the color profile, the input file name, and the output file name. The color profile is specified using the -dl (LDR linear), -ds (LDR sRGB), -dh (HDR RGB, LDR A), or -dH(HDR RGBA) decoder options. The input file path must match a valid file format for decompression, and the output file format must be a valid output for a decompressed image. Note that not all output formats that the coompression path can produce are supported for decompression. See the FILE FORMATS section for the list of supported formats. The -dsw options documented in ADVANCED COMPRESSION option documentation are relevant to decompression.

TEST

To perform a compression test which round-trips a single image through compression and decompression and stores the decompressed result back to file, you must specify same settings as COMPRESSION other than swapping the color profile to select test mode. Note that the compressed intermediate data is discarded in this mode. The color profile is specified using the -tl (LDR linear), -ts (LDR sRGB), -th (HDR RGB, LDR A), or -tH (HDR RGBA) encoder options. This operation mode will print error metrics suitable for either LDR and HDR images, allowing some assessment of the compression image quality.

COMPRESSION FILE FORMATS

The following formats are supported as compression inputs: For the KTX and DDS formats only a subset of the features of the formats are supported:

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Texture topology must be 2D, 2D-array, 3D, or cube-map. Note that 2D-array textures are treated as 3D block input.

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Texel format must be R, RG, RGB, BGR, RGBA, BGRA, L, or LA.

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Only the first mipmap in the file will be read.

The following formats are supported as compression outputs:

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ASTC (*.astc)

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Khronos Texture KTX (*.ktx)

DECOMPRESSION FILE FORMATS

The following formats are supported as decompression inputs:

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ASTC (*.astc)

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Khronos Texture KTX (*.ktx)

The following formats are supported as decompression outputs: