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