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lavc/h274: transpose IDCT 

This is mathematically equivalent to what we were doing before, but
gives subtly different results due to rounding (rows first vs columns
first). Doing it this way makes our film grain database generation match
reference implementation and now produces bit-exact outputs in my
testing.

Rename the transposed variables to be a bit less confusing.
This commit is contained in:
Niklas Haas 2023-09-28 22:14:40 +02:00
parent 2b9c6c70e6
commit 22530ad1ce
1 changed files with 10 additions and 10 deletions
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20
libavcodec/h274.c
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@ -59,13 +59,13 @@ static void init_slice_c(int8_t out[64][64], uint8_t h, uint8_t v,
//
// Note: To make the subsequent matrix multiplication cache friendlier, we
// store each *column* of the starting image in a *row* of `out`
for (int y = 0; y <= freq_v; y++) {
for (int x = 0; x <= freq_h; x += 4) {
for (int l = 0; l <= freq_v; l++) {
for (int k = 0; k <= freq_h; k += 4) {
uint16_t offset = seed % 2048;
out[x + 0][y] = Gaussian_LUT[offset + 0];
out[x + 1][y] = Gaussian_LUT[offset + 1];
out[x + 2][y] = Gaussian_LUT[offset + 2];
out[x + 3][y] = Gaussian_LUT[offset + 3];
out[l][k + 0] = Gaussian_LUT[offset + 0];
out[l][k + 1] = Gaussian_LUT[offset + 1];
out[l][k + 2] = Gaussian_LUT[offset + 2];
out[l][k + 3] = Gaussian_LUT[offset + 3];
prng_shift(&seed);
}
}
@ -74,9 +74,9 @@ static void init_slice_c(int8_t out[64][64], uint8_t h, uint8_t v,
// 64x64 inverse integer transform
for (int y = 0; y < 64; y++) {
for (int x = 0; x <= freq_h; x++) {
for (int x = 0; x <= freq_v; x++) {
int32_t sum = 0;
for (int p = 0; p <= freq_v; p++)
for (int p = 0; p <= freq_h; p++)
sum += R64T[y][p] * out[x][p];
tmp[y][x] = (sum + 128) >> 8;
}
@ -85,8 +85,8 @@ static void init_slice_c(int8_t out[64][64], uint8_t h, uint8_t v,
for (int y = 0; y < 64; y++) {
for (int x = 0; x < 64; x++) {
int32_t sum = 0;
for (int p = 0; p <= freq_h; p++)
sum += tmp[y][p] * R64T[x][p]; // R64T^T = R64
for (int p = 0; p <= freq_v; p++)
sum += tmp[x][p] * R64T[y][p]; // R64T^T = R64
// Renormalize and clip to [-127, 127]
out[y][x] = av_clip((sum + 128) >> 8, -127, 127);
}