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.

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];
};

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;

The block_* fields store the ASTC block dimensions in texels. For 2D images the Z dimension must be set to 1.

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);

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.

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