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# 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._
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