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Pete Harris Update Encoding.md a8f9dd7 Jul 5, 2019
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Effective ASTC Encoding

Most texture compression schemes encode a single color format at single bitrate, so there are relatively few configuration options available to content creators beyond selecting which compressed format to use.

ASTC on the other hand is an extremely flexible container format which can compress multiple color formats at multiple bit rates. Inevitably this flexibility gives rise to questions about how to best use ASTC to encode a specific color format, or what the equivalent settings are to get a close match to another compression format.

This page aims to give some guidelines, but note that they are only guidelines and are not exhaustive so please deviate from them as needed.

Traditional format reference

The most commonly used non-ASTC compressed formats, their color format, and their compressed bitrate are shown in the table below.

Name Color Format Bits/Pixel Notes
BC1 RGB+A 4 RGB565 + 1-bit A
BC3 RGB+A 8 BC1 RGB + BC4 A
BC3nm G+R 8 BC1 G + BC4 R
BC4 R 4 L8
BC5 R+G 8 BC1 R + BC1 G
BC6 RGB (HDR) 8
BC7 RGB / RGBA 8
EAC_R11 R 4 R11
EAC_RG11 RG 8 RG11
ETC1 RGB 4 RGB565
ETC2 RGB+A 4 RGB565 + 1-bit A
ETC2+EAC RGB+A 8 RGB565 + EAC A
PVRTC RGBA 2 or 4

Note: BC2 (RGB+A) is not included in the table because it's rarely used in practice due to poor quality alpha encoding; BC3 is nearly always used instead.

Note Color representations shown with a + symbol indicate non-correlated compression groups; e.g. an RGB + A format compresses RGB and A independently and does not assume the two signals are correlated. This can be a strength (it improves quality when compressing non-correlated signals), but also a weakness (it reduces quality when compressing correlated signals).

ASTC Format Mapping

The main question which arises with the mapping of another format on to ASTC is how to handle cases where the input isn't a 4 channel RGBA input. ASTC is a container format which always decompresses in to a 4 channel RGBA result. However, the internal compressed representation is very flexible and can store 1-4 channels as needed on a per-block basis.

To get the best quality for a given bitrate, or the lowest bitrate for a given quality, it is important that as few channels as possible are stored in the internal representation to avoid wasting coding space.

Specific optimizations in the ASTC coding scheme exist for:

  • Encoding the RGB channels as a single luminance channel, so only a single value needs to be stored in the coding instead of three.
  • Encoding the A channel as a constant 1.0 value, so the coding doesn't actually need to store a per-pixel alpha value at all.

... so mapping your inputs given to the compressor to hit these paths is really important if you want to get the best output quality for your chosen bitrate.

Encoding 1-4 channel data

The table below shows the recommended channel usage for data with different numbers of color channels present in the data.

The coding swizzle should be applied when compressing an image. This can be handled by the compressor when reading an uncompressed input image by specifying the swizzle using the -esw command line option.

The sampling swizzle is what your should use in your shader programs to read the data from the compressed texture, assuming no additional API-level channel swizzling is specified by the application.

Input Channels ASTC Endpoint Coding Swizzle Sampling Swizzle
1 L + 1 rrr1 .g 1
2 L + A rrrg .ga 1
3 RGB + 1 rgb1 .rgb
4 RGB + A rgba .rgba

1: Sampling from g is preferred to sampling from r because it allows a single shader to be compatible with ASTC, BC1, or ETC formats. BC1 and ETC1 store color endpoints as RGB565 data, so the g channel will have higher precision. For ASTC it doesn't actually make any difference; the same single channel luminance will be returned for all three of the .rgb channels.

Equivalence with other formats

Based on these channel encoding requirements we can now derive the the ASTC coding equivalents for most of the other texture compression formats in common use today.

Formant ASTC Coding Swizzle ASTC Sampling Swizzle Notes
BC1 rgba 1 .rgba
BC3 rgba .rgba
BC3nm gggr .ag
BC4 rrr1 .r1
BC5 rrrg .ra 2
BC6 rgb1 .rgb HDR profile only
BC7 rgba .rgba
EAC_R11 rrr1 .r
EAC_RG11 rrrg .ra 2
ETC1 rgba .rgba
ETC2 rgba 1 .rgb
ETC2+EAC rgba .rgba
ETC2+EAC rgba .rgba

1: ASTC has no equivalent of the 1-bit punch-through alpha encoding supported by BC1 or ETC2; if alpha is present it will be a full alpha channel.

2: ASTC relies on using the L+A color endpoint type for coding efficiency for two channel data. It therefore has no direct equivalent of a two-plane format sampled though the .rg channels such as BC5 or EAC_RG11. This can be emulated by setting texture channel swizzles in the runtime API - e.g. via glTexParameteri() for OpenGL ES - although it has been noted that API controlled swizzles are not available in WebGL.

Other Considerations

This section outlines some of the other things to consider when encoding textures using ASTC.

Encoding non-correlated channels

Most other texture compression formats have a static channel assignment in terms of the expected data correlation. For example, ETC2+EAC assumes that RGB are always correlated and that alpha is non-correlated. ASTC can automatically encode data as either fully correlated across all 4 channels, or with any one channel assigned to a separate non-correlated partition to the other three.

The non-correlated channel can be changed on a block-by-block basis, so the compressor can dynamically adjust the coding based on the data present in the image. This means that there is no need for non-correlated data to be stored in a specific channel in the input image.

It is however worth noting that the alpha channel is treated differently to the RGB color channels in some circumstances:

  • When coding for sRGB the alpha channel will always be stored in linear space.
  • When coding for HDR the alpha channel can optionally be kept as LDR data.

Encoding normal maps

The best way to store normal maps using ASTC is similar to the scheme used by BC5; store the X and Y components of a unit-length tangent space normal. The Z component of the normal can be reconstructed in shader code based on the knowledge that the vector is unit length.

To encode this we therefore want to store two input channels and should therefore use the rrrg coding swizzle, and the .ga sampling swizzle. The OpenGL ES shader code for reconstruction of the Z value is:

vec3 normal;
normal.xy = texture(...).ga;
normal.z = sqrt(1 - dot(normal.xy, normal.xy));

In addition to this it is useful to use the -normal_psnr command line option, which switches the compressor to optimize for angular error in the resulting vector rather than for absolute color error in the data.

Encoding sRGB data

The ASTC LDR profile can compress sRGB encoded color, which is a more efficient use of bits than storing linear encoded color because the gamma corrected value distribution more closely matches human perception of luminance.

For color data it is nearly always a perceptual quality win to use sRGB input source textures that are then compressed using the ASTC sRGB compression mode (compress using the -cs command line option rather than the -c command line option). Note that sRGB gamma correction is only applied to the RGB channels during decode; the alpha channel is always treated as linear encoded data.

Important: The uncompressed input texture provided on the command line must be stored in the sRGB color space for -cs to function correctly.

Encoding HDR data

HDR data can be encoded just like LDR data, but with some caveats around handling the alpha channel.

For many use cases the alpha channel is an actual alpha opacity channel and is therefore used for storing an LDR value between 0 and 1. For these cases use the -hdr compressor option which will treat the RGB channels as HDR, but the A channel as LDR.

For other use cases the alpha channel is simply a fourth data channel which is also storing an HDR value. For these cases use the -hdra compressor option which will treat all channels as HDR data.

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