Algorithm Alley

Implementing Fast DCTs

By Tim Kientzle

Dr. Dobb's Journal March 1999

dct1d4PtTest(int *dctBlock) {
  static const int r2c3=554, r2s3=1337;
  int x0=dctBlock[0], x1=dctBlock[1], x2=dctBlock[2], x3=dctBlock[3];
  int x4=x0+x3; x0-=x3; x3=x1+x2; x1-=x2; /* Stage 1 */
  x2=x4+x3;  x4-=x3;  /* Stage 2 */
  x3=r2c3*(x1+x0)+512;  x0=(-r2c3+r2s3)*x0+x3;  x1=(-r2c3-r2s3)*x1+x3;
  dctBlock[0] = x2;  dctBlock[2] = x4;  /* Round and output */
  dctBlock[1] = x0>>10;  dctBlock[3] = x1>>10;
}

Example 1: Complete 4-element DCT calculation, derived from Figure 5.