text_src/14.03__vp8-bitstream__implementation-of-wht-inversion.txt

#### 14.3 Implementation of the WHT Inversion                {#h-14-03}


As previously discussed (see Chapters 2 and 13), for macroblocks encoded using prediction modes other than `B_PRED` and `SPLITMV`, the DC values derived from the DCT transform on the 16 Y blocks are collected to construct a 25th block of a macroblock (16 Y, 4 U, 4 V constitute the 24 blocks). This 25th block is transformed using a Walsh-Hadamard transform (WHT). 

The inputs to the inverse WHT (that is, the dequantized coefficients), the intermediate "horizontally detransformed" signal, and the completely detransformed residue signal are all stored as arrays of 16-bit signed integers. 

Following the tradition of specifying bitstream format using the decoding process, we specify the inverse WHT in the decoding process using the following C style source code:  


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
void vp8_short_inv_walsh4x4_c(short *input, short *output)
{
  int i;
  int a1, b1, c1, d1;
  int a2, b2, c2, d2;
  short *ip = input;
  short *op = output;
  int temp1, temp2;

  for(i=0;i<4;i++)
  {
    a1 = ip[0] + ip[12];
    b1 = ip[4] + ip[8];
    c1 = ip[4] - ip[8];
    d1 = ip[0] - ip[12];

    op[0] = a1 + b1;
    op[4] = c1 + d1;
    op[8] = a1 - b1;
    op[12]= d1 - c1;
    ip++;
    op++;
  }
  ip = output;
  op = output;
  for(i=0;i<4;i++)
  {
    a1 = ip[0] + ip[3];
    b1 = ip[1] + ip[2];
    c1 = ip[1] - ip[2];
    d1 = ip[0] - ip[3];

    a2 = a1 + b1;
    b2 = c1 + d1;
    c2 = a1 - b1;
    d2 = d1 - c1;

    op[0] = (a2+3)>>3;
    op[1] = (b2+3)>>3;
    op[2] = (c2+3)>>3;
    op[3] = (d2+3)>>3;

    ip+=4;
    op+=4;
  }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}


In the case that there is only one non-zero DC value in input, the inverse transform can be simplified to the following: 


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
void vp8_short_inv_walsh4x4_1_c(short *input, short *output)
{
  int i;
  int a1;
  short *op=output;


  
  a1 = ((input[0] + 3)>>3);

  for(i=0;i<4;i++)
  {
    op[0] = a1;
    op[1] = a1;
    op[2] = a1;
    op[3] = a1;
    op+=4;
  }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}


It should be noted, a conforming decoder should implement the inverse transform using exactly the same rounding to achieve bit-wise matching output to the output of the process specified by the above "C" source code. 

The reference decoder WHT inversion may be found in the file `idct_add.c`.