[WORK IN PROGRESS]

File: basis_spec.txt
Version 1.00

1.0 Introduction
----------------

The Basis Universal GPU texture codec supports reading and writing ".basis" files. 
The .basis 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 
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](https://github.com/BinomialLLC/basis_universal/wiki/UASTC-Texture-Specification). UASTC texture data can always be losslessly transcoded to ASTC.

2.0 High-Level File Structure
-----------------------------

A .basis file consists of multiple sections. Apart from the header, which must always
be at the start of the file, the other sections may appear in any order. 

Here's the high level organization of a typical .basis file:

* 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. 

3.0 File Enums
--------------

// basis_file_header::m_tex_type
enum basis_texture_type
{
  cBASISTexType2D = 0,     
  cBASISTexType2DArray = 1,   
  cBASISTexTypeCubemapArray = 2, 
  cBASISTexTypeVideoFrames = 3, 
  cBASISTexTypeVolume = 4,  
  cBASISTexTypeTotal
};

// basis_slice_desc::flags
enum basis_slice_desc_flags
{
  cSliceDescFlagsHasAlpha = 1,
  cSliceDescFlagsFrameIsIFrame = 2   
};

// basis_file_header::m_tex_format enum basis_tex_format
{
  cETC1S = 0,
  cUASTC4x4 = 1
};

// basis_file_header::m_flags enum basis_header_flags
{
  cBASISHeaderFlagETC1S = 1.
  cBASISHeaderFlagYFlipped = 2,
  cBASISHeaderFlagHasAlphaSlices = 4
};


4.0 File Structures
-------------------

All individual members in all file structures are byte aligned and little endian. The structs 
have no padding (i.e. they are declared with #pragma pack(1)).

4.1 "basis_file_header" structure
---------------------------------

The file header must always be at the beginning of the file.

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
};

4.1.1 Details:

* 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. 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.

4.2 "basis_slice_desc" structure
--------------------------------

struct basis_slice_desc
{
    uint24 m_image_index;  
    uint8 m_level_index;   
    uint8 m_flags;         

    uint16 m_orig_width;   
    uint16 m_orig_height;  

    uint16 m_num_blocks_x; 
    uint16 m_num_blocks_y; 

    uint32 m_file_ofs;     
    uint32 m_file_size;    

    uint16 m_slice_data_crc16; 
};

4.2.1 Details:

* m_image_index: The index of the source image provided to the encoder (will always appear in order from first to last, first image index is 0, no skipping allowed)
* m_level_index: The mipmap level index (mipmaps will always appear from largest to smallest)
* m_flags: enum basis_slice_desc_flags
* m_orig_width: The original image width (may not be a multiple of 4 pixels)
* m_orig_height: The original image height (may not be a multiple of 4 pixels)
* m_num_blocks_x: The slice's block X dimensions. Each block is 4x4 pixels. The slice's pixel resolution may or may not be a power of 2.
* m_num_blocks_y: The slice's block Y dimensions. 
* m_file_ofs: Offset from the header to the start of the slice's data
* m_file_size: The size of the compressed slice data in bytes
* m_slice_data_crc16: The CRC16 of the compressed slice data, for extra-paranoid use cases

5.0 CRC-16 Function
-------------------

.basis files use CRC-16/genibus(aka CRC-16 EPC, CRC-16 I-CODE, CRC-16 DARC) format CRC-16's. 

Here's an example function in C++:

uint16_t crc16(const void* r, size_t size, uint16_t crc)
{
  crc = ~crc;
  const uint8_t* p = static_cast<const uint8_t*>(r);
  for ( ; size; --size)
  {
    const uint16_t q = *p++ ^ (crc >> 8);
    uint16_t k = (q >> 4) ^ q;
    crc = (((crc << 8) ^ k) ^ (k << 5)) ^ (k << 12);
  }

  return static_cast<uint16_t>(~crc);
}

This function is called with 0 in the final "crc" parameter when computing CRC-16's of file data.

6.0 Compressed Huffman Tables
-----------------------------

ETC1S format .basis files rely heavily on static [canonical Huffman
prefix coding](https://en.wikipedia.org/wiki/Canonical_Huffman_code).  Multiple
Huffman tables are used by each compressed section. Huffman codes are stored in
each output byte in LSB to MSB order. (This is opposite of the JPEG format,
which stores the codes in MSB to LSB order.)

Huffman coding in .basis is compatible with the canonical Huffman methods used
by Deflate encoders/decoders. Section 3.2.2 of [Deflate - RFC
1951](https://tools.ietf.org/html/rfc1951), which describes how to compute the
value of each Huffman code given an array of symbol codelengths. This document
assumes familiarity with how Huffman coding works in Deflate.

First, some enums:

enum
{
    // Max supported Huffman code size is 16-bits
    cHuffmanMaxSupportedCodeSize = 16, 

    // The maximum number of symbols  is 2^14
    cHuffmanMaxSymsLog2 = 14, 
    cHuffmanMaxSyms = 1 << cHuffmanMaxSymsLog2,

    // Small zero runs may range from 3-10 entries
    cHuffmanSmallZeroRunSizeMin = 3, 
    cHuffmanSmallZeroRunSizeMax = 10, 
    cHuffmanSmallZeroRunExtraBits = 3,

    // Big zero runs may range from 11-138 entries
    cHuffmanBigZeroRunSizeMin = 11, 
    cHuffmanBigZeroRunSizeMax = 138, 
    cHuffmanBigZeroRunExtraBits = 7,

    // Small non-zero runs may range from 3-6 entries
    cHuffmanSmallRepeatSizeMin = 3, 
    cHuffmanSmallRepeatSizeMax = 6, 
    cHuffmanSmallRepeatExtraBits = 2,

    // Big non-zero run may range from 7-134 entries
    cHuffmanBigRepeatSizeMin = 7, 
    cHuffmanBigRepeatSizeMax = 134, 
    cHuffmanBigRepeatExtraBits = 7,

    // There are a maximum of 21 symbols in a compressed Huffman code length table.
    cHuffmanTotalCodelengthCodes = 21, 
    
    // Symbols [0,16] indicate code sizes. Other symbols indicate zero runs or repeats:
    cHuffmanSmallZeroRunCode = 17, 
    cHuffmanBigZeroRunCode = 18, 
    cHuffmanSmallRepeatCode = 19, 
    cHuffmanBigRepeatCode = 20
};

A .basis Huffman table consists of 1 to cHuffmanMaxSyms symbols. Each compressed
Huffman table is described by an array of symbol code lengths in bits.

The table's symbol code lengths are themselves RLE+Huffman coded, just like
Deflate. (Note this can be confusing to developers unfamiliar with Deflate.)
Each table begins with a small fixed header:

    14 bits: total_used_syms [1, cHuffmanMaxSyms]
    5 bits: num_codelength_codes [1, cHuffmanTotalCodelengthCodes]
    
Next, the code lengths for the small Huffman table which is used to send the compressed codelengths (and RLE/repeat codes) are sent uncompressed but in a reordered manner:
    
    3*num_codelength_codes bits: Code size of each Huffman symbol for the compressed Huffman codelength table.
    
    These code lengths are sent in this order (to help reduce the number that must be sent):
    
    { 
        cHuffmanSmallZeroRunCode, cHuffmanBigZeroRunCode, cHuffmanSmallRepeatCode, cHuffmanBigRepeatCode, 
        0, 8, 7, 9, 6, 0xA, 5, 0xB, 4, 0xC, 3, 0xD, 2, 0xE, 1, 0xF, 0x10 
    };
            
A canonical Huffman decoding table (of up to 21 symbols) should be built from
these code lengths. Immediately following this data are the Huffman symbols
(sometimes intermixed with raw bits) which describe how to unpack the
codelengths of each symbol in the Huffman table:

    - Symbols [0,16] indicate a specific symbol code length in bits.
    
    - Symbol cHuffmanSmallZeroRunCode (17) indicates a short run of symbols with 0 bit code lengths.
      cHuffmanSmallZeroRunExtraBits (3) bits are sent after this symbol, which indicates the run's size after adding the minimum size (cHuffmanSmallZeroRunSizeMin).
      
    - Symbol cHuffmanBigZeroRunCode (18) indicates a long run of symbols with 0 bit code lengths. 
      cHuffmanBigZeroRunExtraBits (7) bits are sent after this symbol, which indicates the run's size after adding the minimum size (cHuffmanBigZeroRunSizeMin)

    - Symbol cHuffmanSmallRepeatCode (19) indicates a short run of symbols that repeat the previous symbol's code length.
      cHuffmanSmallRepeatExtraBits (2) bits are sent after this symbol, which indicates the number of times to repeat the previous symbol's code length, 
      after adding the minimum size (cHuffmanSmallRepeatSizeMin).
      Cannot be the first symbol, and the previous symbol cannot have a code length of 0.
      
    - Symbol cHuffmanBigRepeatCode (20) indicates a short run of symbols that repeat the previous symbol's code length.
      cHuffmanBigRepeatExtraBits (7) bits are sent after this symbol, which indicates the number of times to repeat the previous symbol's code length,
      after adding the minimum size (cHuffmanBigRepeatSizeMin).
      Cannot be the first symbol, and the previous symbol cannot have a code length of 0.
      
There should be exactly total_used_syms code lengths stored in the compressed Huffman table. If not the stream is either corrupted or invalid.

After all the symbol codelengths are uncompressed, the symbol codes can be computed and the canonical Huffman decoding tables can be built.

7.0 ETC1S Endpoint Codebooks
----------------------------

The endpoint codebook section starts at file offset
basis_file_header::m_endpoint_cb_file_ofs and is m_endpoint_cb_file_size bytes
long. The endpoint codebook will have basis_file_header::m_total_endpoints total
entries.

At the beginning of the compressed endpoint codebook section are four compressed
Huffman tables, stored using the procedure outlined in section 6.0. The Huffman tables
appear in this order:

    1. color5_delta_model0
    2. color5_delta_model1
    3. color5_delta_model2
    4. inten_delta_model

Following the data for these Huffman tables is a single 1-bit code which
indicates if the color endpoint codebook is grayscale or not. 

Immediately following this code is the compressed color endpoint codebook data. 
A simple form of DPCM coding is used to send the ETC1S intensity table indices and
color values. Here is the procedure to decode the endpoint codebook:

    const int COLOR5_PAL0_PREV_HI = 9, COLOR5_PAL0_DELTA_LO = -9, COLOR5_PAL0_DELTA_HI = 31;
    const int COLOR5_PAL1_PREV_HI = 21, COLOR5_PAL1_DELTA_LO = -21, COLOR5_PAL1_DELTA_HI = 21;
    const int COLOR5_PAL2_PREV_HI = 31, COLOR5_PAL2_DELTA_LO = -31, COLOR5_PAL2_DELTA_HI = 9;

    // Assume previous endpoint color is (16, 16, 16), and the previous intensity is 0.
    color32 prev_color5(16, 16, 16, 0);
    uint32_t prev_inten = 0;

    // For each endpoint codebook entry
    for (uint32_t i = 0; i < num_endpoints; i++)
    {
        // Decode the intensity delta Huffman code
        uint32_t inten_delta = sym_codec.decode_huffman(inten_delta_model);
        m_endpoints[i].m_inten5 = static_cast<uint8_t>((inten_delta + prev_inten) & 7);
        prev_inten = m_endpoints[i].m_inten5;

        // Now decode the endpoint entry's color or intensity value
        for (uint32_t c = 0; c < (endpoints_are_grayscale ? 1U : 3U); c++)
        {
            // The Huffman table we used to decode the delta depends on the previous color's value
            int delta;
            if (prev_color5[c] <= basist::COLOR5_PAL0_PREV_HI)
                delta = sym_codec.decode_huffman(color5_delta_model0);
            else if (prev_color5[c] <= basist::COLOR5_PAL1_PREV_HI)
                delta = sym_codec.decode_huffman(color5_delta_model1);
            else
                delta = sym_codec.decode_huffman(color5_delta_model2);

            // Apply the delta
            int v = (prev_color5[c] + delta) & 31;

            m_endpoints[i].m_color5[c] = static_cast<uint8_t>(v);

            prev_color5[c] = static_cast<uint8_t>(v);
        }

        // If the endpoints are grayscale, set G and B to match R.
        if (endpoints_are_grayscale)
        {
            m_endpoints[i].m_color5[1] = m_endpoints[i].m_color5[0];
            m_endpoints[i].m_color5[2] = m_endpoints[i].m_color5[0];
        }
    }

The rest of the section's data (if any) can be ignored.

8.0 ETC1S Selector Codebooks
----------------------------