Variable Length Decoding

Video data in the bit stream is encoded with Variable Length Code (VLC) tables so that the shortest codes correspond to the most frequent values and the longer codes correspond to the less frequent values. Tables for each video standard contain specific possible codes and their values.

VLC decoding functions can work with two types of tables:

• Tables, in which one code corresponds to one value (see Table “One-to-One Code/Value Correspondence” for an example)
• Tables, in which one code corresponds to two values (see Table “One-to-Two Code/Value Correspondence” for an example).

Video data VLC-decoding starts with memory allocation and table initialization. Then the decoding proper should be made and the allocated memory must be released.

For using ippiVCHuffmanInitAlloc_32s and ippiVCHuffmanInitAllocRL_32s functions, the source table should have the following structure:

static Ipp32s Table[]=
{
max_bits,                         // The maximum length of code
total_subt,                       // The total number of all subtables
sub_sz1,                          // The sizes of subtables. Their sum must 
sub_sz2,                          // be equal to the maximum length of codes
...,                                // max_bits and their number should be
                                  // equal to total_subt.
N1,                               // The number of 1-bit codes
Code1, Value11, [Value21]          // 
Code2, Value12, [Value22]          // The 1-bit codes 
...,                                //
N2,                               // The number of 2-bit codes
Code1, Value11, [Value21]          //
Code2, Value12, [Value22]          // The 2-bit codes
...,                                //
Nm,                               // The number of max_bits, -bit codes
Code1, Value11, [Value21]         //
Code2, Value12, [Value22]         // max_bits codes
...,                               //
-1                               // The significant value to indicate the
                                 // end of table
};

Notes:

• The values Value2_i are used for the second type table (one-to-two code/value correspondence) and must be in the range [-32768, 32767] or [0, 65535].
• The values Value1_i must be in the range:
  [-8388608, 8388607] or [0, 16777215] in the case of first type table (one-to-one correspondence) [-128, 127] or [0, 255] in the case of second type table (one-to-two correspondence).

• Using Subtables
  

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