src/audio/idct.cc


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// This file is part of the 64k demo project.
// It implements the 512-point Inverse Discrete Cosine Transform.
// Used for real-time synthesis of audio from spectral data.

#include "dct.h"
#include <math.h>

void idct_512(const float *input, float *output) {
  const float PI = 3.14159265358979323846f;
  for (int n = 0; n < DCT_SIZE; ++n) {
    float sum = input[0] / 2.0f;
    for (int k = 1; k < DCT_SIZE; ++k) {
      sum +=
          input[k] * cosf(PI / (float)DCT_SIZE * (float)k * ((float)n + 0.5f));
    }
    output[n] = sum * (2.0f / (float)DCT_SIZE);
  }
}