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 programmatically based on the header information.