72 lines
1.7 KiB
Markdown
72 lines
1.7 KiB
Markdown
# The .astc File Format
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The default file format for compressed textures generated by `astcenc`, as well
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as from many other ASTC compressors, is the `.astc` format. This is a very
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simple format consisting of a small header followed immediately by the binary
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payload for a single image surface.
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Header
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======
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The header is a fixed 16 byte structure, defined as storing only bytes to avoid
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any endianness issues or incur any padding overhead.
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```
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struct astc_header
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{
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uint8_t magic[4];
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uint8_t block_x;
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uint8_t block_y;
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uint8_t block_z;
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uint8_t dim_x[3];
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uint8_t dim_y[3];
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uint8_t dim_z[3];
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};
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```
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Magic number
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------------
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The 4 byte magic number at the start of the file acts as a format identifier.
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```
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magic[0] = 0x13;
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magic[1] = 0xAB;
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magic[2] = 0xA1;
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magic[3] = 0x5C;
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```
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Block size
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----------
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The `block_*` fields store the ASTC block dimensions in texels. For 2D images
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the Z dimension must be set to 1.
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Image dimensions
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----------------
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The `dim_*` fields store the image dimensions in texels. For 2D images the
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Z dimension must be set to 1.
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Note that the image is not required to be an exact multiple of the compressed
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block size; the compressed data may include padding that is discarded during
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decompression.
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Each dimension is a 24 bit unsigned value that is reconstructed from the stored
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byte values as:
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```
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decoded_dim = dim[0] + (dim[1] << 8) + (dim[2] << 16);
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```
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Binary payload
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==============
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The binary payload is a byte stream that immediately follows the header. It
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contains 16 bytes per compressed block. The number of compressed blocks is
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determined from the header information.
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- - -
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_Copyright © 2020-2022, Arm Limited and contributors. All rights reserved._
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