git.sesse.net Git - fjl/blob - idct.c

   1 #include <math.h>
   2
   3 #include "idct.h"
   4
   5 void idct_reference(const int16_t* input, const uint32_t* quant_table, uint8_t* output)
   6 {
   7         double temp[DCTSIZE2];
   8
   9         for (unsigned y = 0; y < 8; ++y) {
  10                 for (unsigned x = 0; x < 8; ++x) {
  11                         double acc = 0.0;
  12                         for (unsigned u = 0; u < 8; ++u) {
  13                                 for (unsigned v = 0; v < 8; ++v) {
  14                                         double c_u = (u == 0) ? 1/sqrt(2.0) : 1.0;
  15                                         double c_v = (v == 0) ? 1/sqrt(2.0) : 1.0;
  16                                         acc += c_u * c_v
  17                                                 * input[u * DCTSIZE + v] * quant_table[u * DCTSIZE + v]
  18                                                 * cos((2 * x + 1) * v * M_PI / 16.0)
  19                                                 * cos((2 * y + 1) * u * M_PI / 16.0);
  20                                 }
  21                         }
  22                         temp[y * DCTSIZE + x] = 0.25 * acc;
  23                 }
  24         }
  25
  26         for (unsigned y = 0; y < 8; ++y) {
  27                 for (unsigned x = 0; x < 8; ++x) {
  28                         double val = temp[y * DCTSIZE + x];
  29                         if (val < 0.0) {
  30                                 output[y * DCTSIZE + x] = 0;
  31                         } else if (val >= 255.0) {
  32                                 output[y * DCTSIZE + x] = 255;
  33                         } else {
  34                                 output[y * DCTSIZE + x] = (uint8_t)(val + 0.5);
  35                         }
  36                 }
  37         }
  38 }
  39