h263_vld_c.c

h263,jpeg,mpeg2编解码核心程序(TI DSP C64xx)

/* ======================================================================== */
/*  TEXAS INSTRUMENTS, INC.                                                 */
/*                                                                          */
/*  NAME                                                                    */
/*      h263_vld                                                            */
/*                                                                          */
/*  USAGE                                                                   */
/*      This routine is C-callable and can be called as:                    */
/*                                                                          */
/*          int  h263_vld                                                   */
/*          (                                                               */
/*              unsigned int            *restrict *restrict bufPtr,         */
/*              int                     *restrict bitPtr,                   */
/*              const unsigned short    *restrict tab,                      */
/*              const unsigned char     *restrict len,                      */
/*              short                   Q,                                  */
/*              int                     dq,                                 */
/*              const unsigned char     *restrict zz,                       */
/*              short                   *restrict idct,                     */
/*              int                     inter                               */
/*          )                                                               */
/*                                                                          */
/*                                                                          */
/*          bufPtr =  pointer to VLD buffer word pointer.                   */
/*          bitPtr =  pointer to the number of valid bits.n VLD_buffer      */
/*          tab    =  pointer to VLC Coeff/Run table.                       */
/*          len    =  pointer to VLC Length table.                          */
/*          Q      =  Quantization value.                                   */
/*          dq     =  Quant/Mismatch control value.                         */
/*          zz     =  Ptr to zig-zag scan table.                            */
/*          idct   =  8x8 IDCT Coeff block. (Output)                        */
/*          inter  =  inter/intra 1/0                                       */
/*                                                                          */
/*      The function places decoded data in the idct[] array.  Also,        */
/*      it returns a two-bit error code as its return value:  Bit 0         */
/*      of the return value denotes whether the zig-zag array was           */
/*      overrun, and bit 1 denotes whether an invalid VLC symbol was        */
/*      encountered.  A return value of 0 denotes no error.                 */
/*                                                                          */
/*  DESCRIPTION                                                             */
/*      This code performs the variable length decode of a H.263 bit        */
/*      stream using the Table 16 and Table 17 of the H.263 standard.       */
/*      The function accepts a packed Variable length encoded buffer        */
/*      and returns the idct array for one block.  This function also       */
/*      returns state variables that allow chaining for calling it          */
/*      multiple times to perform VLD for a macro-block.  The function      */
/*      returns 0 if a valid VLD decode of a block was performed and a      */
/*      1 if the block had an error.                                        */
/*                                                                          */
/*      The following is the implementation:                                */
/*                                                                          */
/*      This routine performs VLD on the AC terms in a Huffman coded 8x8    */
/*      block for H.263 decode.  Inverse Quantization, Run-Length Decode    */
/*      and Zig-Zag reordering are performed on each coefficient before     */
/*      it is written to the IDCT array.  The IDCT array is presumed to     */
/*      prezeroed by the calling function.                                  */
/*                                                                          */
/*      Error detection is performed by detecting invalid Huffman codes     */
/*      and for run-values which send the run-length decode outside the     */
/*      current 8x8 block.  No test is made, however, to determine if the   */
/*      bitstream input underflows.  The user is advised to place a         */
/*      sentinel bitstream at the end of the VLD buffer to prevent buffer   */
/*      underflow in the case of an error.                                  */
/*                                                                          */
/*      The bitstream is processed in word-sized chunks, using word-wide    */
/*      loads from the bitstream.  Therefore, it is necessary to store the  */
/*      bitstream in "word order".   For example, consider the following    */
/*      bitstream:  (leftmost bits are earliest in time.)                   */
/*                                                                          */
/*         +--------------------------------+----------------------         */
/*         |ABCDEFGHIJKLMNOPQRSTUVWXYZabcdef|ghijklmnopqrstuvwxyz . . .     */
/*         +--------------------------------+----------------------         */
/*                      Word 0                          Word 1              */
/*                                                                          */
/*      The VLD routine will read the bitstream a word at a time, as        */
/*      labeled above.  On a little endian CPU, this implies that the       */
/*      bit-stream will be stored as follows in memory:                     */
/*                                                                          */
/*        +--------+--------+--------+--------+-----                        */
/*        |YZabcdef|QRSTUVWX|IJKLMNOP|ABCDEFGH| . . .                       */
/*        +--------+--------+--------+--------+-----                        */
/*        | Byte 0   Byte 1   Byte 2   Byte 3 |                             */
/*        |<------------ Word 0 ------------->|                             */
/*                                                                          */
/*      The VLC codes fall into several different classes.  The VLD         */
/*      attempts to isolate what VLC code it has by looking at various      */
/*      ranges of bits.  The following diagram illustrates the various      */
/*      regions of the bitstream that are extracted into different          */
/*      variables.                                                          */
/*                                                                          */
/*               3 3 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1                */
/*               1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0                */
/*              +-------------------------------------------------          */
/*              |A B C D E F G H I J K L M N O P Q R S T U V W . . .        */
/*              +-------------------------------------------------          */
/*              |<--f5--->|   |   | |   |                                   */
/*              |<----f7----->|   | |   |                                   */
/*              |<------f9------->| |   |                                   */
/*              |<-------idx2------>|   |                                   */
/*              |<---------idx1-------->|                                   */
/*                                                                          */
/*                                                                          */
/*      The Len/Run/Last lookup for VLD then proceeds as follows:           */
/*                                                                          */
/*       -- If 'f5' is less than 4, then we have a code in the range        */
/*          00000xxxxx to 00011xxxxx.                                       */
/*                                                                          */
/*          -- For 00000xxxxxxx codes, lookup tab[idx1].  Since f5          */
/*             is zero (and therefore bits A...E are zero), idx1 has        */
/*             7 significant bits.  Therefore, the lookup occurs over       */
/*             over the range [0,127].                                      */
/*                                                                          */
/*          -- For 00001xxxxx thru 00011xxxxx codes, lookup                 */
/*             tab[64 + idx0].  For this range of codes, idx0 >= 64,        */
/*             and has 7 significant bits.  Therefore, the lookup           */
/*             occurs over the range [128,191].                             */
/*                                                                          */
/*       -- If 'f5' is not less than 4, then we need to look at the         */
/*          first 7 bits to determine what code we have.                    */
/*                                                                          */
/*           -- For 001xxxx through 1xxxxxx codes, lookup                   */
/*              tab[176 + f7].  We know that f7 >= 32, so therefore,        */
/*              the lookup will occur over the range [198,303].             */
/*                                                                          */
/*       -- If 'f7' equals 3, we have an escape code of the form            */
/*          0000011Lrrrrrrllllllll, where 'L' is the LAST flag,             */
/*          'rrrrrr' is the Run, and 'llllllll' is the signed level.        */
/*                                                                          */
/*                                                                          */
/*      The length of the VLC code is determined with one pair of           */
/*      lookups.                                                            */
/*                                                                          */
/*       -- If 'f5' is less than 4, then we have a code in the range        */
/*          00000xxxxx to 00011xxxxx.  For this case, we look up the        */
/*          length at len[f9].  This lookup occurs over the range           */
/*          [0,63].                                                         */
/*                                                                          */
/*       -- If 'f5' is >= 4, we use the first 5 bits to look in the         */
/*          length table.  We look at len[60 + f5].  Since f5 >= 4,         */
/*          this lookup occurs over the range [64, 91].                     */
/*                                                                          */
/*      The actual code table contains 304 entires to get multiple of 8.    */
/*      Each code word is stored in the table in the following format:      */
/*                                                                          */
/*          Last(1/0)  RUNVALUE   LEVEL                                     */
/*          14         13....8    7....0                                    */
/*                                                                          */
/*      The code word used in this context refers to the run, level         */
/*      associated with each Huffman code word that is decoded and not      */
/*      to the Huffman code word itself.                                    */
/*                                                                          */
/*      Each code word thus has a pad of 2 bits and is stored as a half     */
/*      word                                                                */
/*                                                                          */
/*      Example:                                                            */
/*                                                                          */
/*      1) Code: 0000 0000 110s Run: 0   Level: 11                          */
/*                                                                          */
/*      Clearly for this code f5 the first 5 bits from the left are zero.   */
/*      first 12 bits from the left are extracted in idx1. If the upper     */
/*      5 bits are zero then idx1 can have a maximum value of upto 127.     */
/*      In this case this value will be either 13 or 14 depending on the    */
/*      sign bit.                                                           */
/*      0         000000 001011  ------------->   0x0b                      */
/*                                                                          */
/*                                                                          */
/*      The length table is a set of values constructed as follows. The     */
/*      code words are taken and divided into two categories for this       */
/*      purpose:                                                            */
/*               Class 1:  Codes 00000xxxxx...00011xxxxx                    */
/*               Class 2:  Codes 001xxxxx...1xxxxx                          */
/*      Therfore it can be seen that under class 2 there can be a set of    */
/*      32 codes and under class 1 there are 60 enties that the given       */
/*      code table maps into. Therfore there are a total of 32 entries.     */
/*      This can be seen by way of example:                                 */
/*                                                                          */
/*      Code at Index: 50 of the Table 16 in H.263 spec                     */
/*                                                                          */
/*      Run: 19 Level: 1 Code: 0000 1110 1s and falls under Class 1         */
/*      When the 9 left most bits are examined this gives us 29.  The       */
/*      29th entry in the length table gives us the length of the code      */
/*      word as 10.                                                         */
/*                                                                          */
/*                                                                          */
/*      Similarly a code at Index 40 of the Table 16 in H.263 spec          */