.basis file format specification

[This is a work in progress. It will be copied & pasted into the Basis Universal wiki.]

The Basis Universal GPU texture codec supports reading and writing ".basis" files. Currently the file format supports ETC1S or UASTC 4x4 texture data:

• ETC1S is a simplified subset of ETC1.

The mode is always differential (diff bit=1), the Rd, Gd, and Bd color deltas are always (0,0,0), and the flip bit is always set. ETC1S texture data is fully 100% compliant with all existing software and hardware ETC1 decoders. Existing encoders can be easily modified to limit their output to ETC1S.

• UASTC 4x4 is a 19 mode subset of the ASTC texture format. Its specification is here. UASTC texture data can always be losslessly transcoded to ASTC.

At a high level, a typical .basis file consists of multiple sections:

• The file header
• Optional ETC1S compressed endpoint/selector codebooks
• Optional ETC1S Huffman table information
• A required "slice" description array describing the resolutions and file offset/compressed sizes of each texture slice present in the file
• 1 or more slices containing ETC1S or UASTC compressed texture data. 
• For future expansion, the format supports an "extended" header which may be located anywhere in the file. This section contains .PNG-like chunked data. 

Apart from the header, which must always be present at the start of the file, the other sections can appear in any order.

Enums

.basis file enums:
enum basis_texture_type
{
  cBASISTexType2D = 0,
  cBASISTexType2DArray = 1,
  cBASISTexTypeCubemapArray = 2,
  cBASISTexTypeVideoFrames = 3,
  cBASISTexTypeVolume = 4,
  cBASISTexTypeTotal

};

enum basis_slice_desc_flags
{
  cSliceDescFlagsHasAlpha = 1,
  cSliceDescFlagsFrameIsIFrame = 2

};

enum basis_tex_format
{
  cETC1S = 0,
  cUASTC4x4 = 1

};

enum basis_header_flags
{
  cBASISHeaderFlagETC1S = 1.
  cBASISHeaderFlagYFlipped = 2,
  cBASISHeaderFlagHasAlphaSlices = 4

};

File Header

The file header must always be at the beginning of the file. The individual values are byte aligned and always little endian.

struct basis_file_header
{
  uint16      m_sig;              // 2 byte file signature
  uint16      m_ver;              // File version
  uint16      m_header_size;      // Header size in bytes, sizeof(basis_file_header) or 0x4D
  uint16      m_header_crc16;     // CRC16/genibus of the remaining header data

  uint32      m_data_size;        // The total size of all data after the header
  uint16      m_data_crc16;       // The CRC16 of all data after the header

  uint24      m_total_slices;     // The number of compressed slices 
  uint24      m_total_images;     // The total # of images
         
  byte        m_tex_format;       // enum basis_tex_format
  uint16      m_flags;            // enum basis_header_flags
  byte        m_tex_type;         // enum basis_texture_type
  uint24      m_us_per_frame;     // Video: microseconds per frame

  uint32      m_reserved;         // For future use
  uint32      m_userdata0;        // For client use
  uint32      m_userdata1;        // For client use

  uint16      m_total_endpoints;          // ETC1S: The number of endpoints in the endpoint codebook 
  uint32      m_endpoint_cb_file_ofs;     // ETC1S: The compressed endpoint codebook's file offset relative to the header
  uint24      m_endpoint_cb_file_size;    // ETC1S: The compressed endpoint codebook's size in bytes

  uint16      m_total_selectors;          // ETC1S: The number of selectors in the selector codebook 
  uint32      m_selector_cb_file_ofs;     // ETC1S: The compressed selector codebook's file offset relative to the header
  uint24      m_selector_cb_file_size;    // ETC1S: The compressed selector codebook's size in bytes

  uint32      m_tables_file_ofs;          // ETC1S: The file offset of the compressed Huffman codelength tables.
  uint32      m_tables_file_size;         // ETC1S: The file size in bytes of the compressed Huffman codelength tables.

  uint32      m_slice_desc_file_ofs;      // The file offset to the slice description array, usually follows the header

  uint32      m_extended_file_ofs;        // The file offset of the "extended" header and compressed data, for future use
  uint32      m_extended_file_size;       // The file size in bytes of the "extended" header and compressed data, for future use
};

• m_sig is always 'B' * 256 + 's', or 0x4273.
• m_ver is currently always 0x10.
• m_header_size is sizeof(basis_file_header). It's always 0x4D.
• m_header_crc16 is the CRC-16 of the remaining header data. The CRC-16 parameters are "CRC-16/genibus"(aka CRC-16 EPC, CRC-16 I-CODE, CRC-16 DARC). See the "CRC-16" section for more information.
• m_data_size, m_data_crc16: The size of all data following the header, and its CRC-16.
• m_total_slices: The total number of slices, from [1,2^24-1]
• m_total_images: The total number of images (where one image can contain multiple mipmap levels, and each mipmap level is a different slice).
• m_tex_format: basis_tex_format. Either cETC1S (0), or cUASTC4x4 (1).
• m_flags: A combination of flags from the basis_header_flags enum.
• m_tex_type: The texture type, from enum basis_texture_type
• m_us_per_frame: Microseconds per frame, only valid for cBASISTexTypeVideoFrames texture types.
• m_total_endpoints, m_endpoint_cb_file_ofs, m_endpoint_cb_file_size: Information about the compressed ETC1S endpoint codebook: The total # of entries, the offset to the compressed data, and the compressed data's size.
• m_total_selectors, m_selector_cb_file_ofs, m_selector_cb_file_size: Information about the compressed ETC1S selector codebook: The total # of entries, the offset to the compressed data, and the compressed data's size.
• m_tables_file_ofs, m_tables_file_size: The file offset and size of the compressed Huffman tables for ETC1S format files. 
• m_slice_desc_file_ofs: 
  The file offset to the array of slice description structures. There will be m_total_slices structures at this file offset.
• m_extended_file_ofs, m_extended_file_size: The "extended" header, for future expansion. Currently unused.