[FFmpeg-devel] [PATCH] lavc/h274: transpose IDCT

[Niklas Haas <ffmpeg@haasn.xyz>]

From: Niklas Haas <git@haasn.dev>

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.
---
 libavcodec/h274.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/libavcodec/h274.c b/libavcodec/h274.c
index a5caf09564d..5709200322e 100644
--- a/libavcodec/h274.c
+++ b/libavcodec/h274.c
@@ -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);
         }
-- 
2.42.0

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