/* Inverse 2-D Discrete Cosine Transform. */
void IDCT(const FBlock * input, PBlock * output)
{
int Y[64];
int k, l;
/* Pass 1: process rows. */
for (k = 0; k < 8; k++) {
/* Prescale k-th row: */
for (l = 0; l < 8; l++)
Y(k, l) = SCALE(input->block[k][l], S_BITS);
/* 1-D IDCT on k-th row: */
idct_1d(&Y(k, 0));
/* Result Y is scaled up by factor sqrt(8)*2^S_BITS. */
}
/* Pass 2: process columns. */
for (l = 0; l < 8; l++) {
int Yc[8];
for (k = 0; k < 8; k++)
Yc[k] = Y(k, l);
/* 1-D IDCT on l-th column: */
idct_1d(Yc);
/* Result is once more scaled up by a factor sqrt(8). */
for (k = 0; k < 8; k++) {
int r = 128 + DESCALE(Yc[k], S_BITS + 3); /* includes level shift */
/* Clip to 8 bits unsigned: */
r = r > 0 ? (r < 255 ? r : 255) : 0;
X(k, l) = r;
}
}
}
static void idct(float *dst, const float *src)
{
float tmp[N*N];
idct_1d(tmp, src, 1, N, 1, N);
idct_1d(dst, tmp, N, 1, N, 1);
}