Commit 2a8c6d30b9 for ffmpeg

commit 2a8c6d30b934f9bd8429f96f605e5fc7c5751679
Author: lusoris <lusoris@proton.me>
Date:   Fri Oct 9 18:19:54 2026 +0200

    avcodec/av1dec: remove the coding bias from exported chroma grain params

    export_film_grain() copied cb_mult, cb_luma_mult and cb_offset (and the
    Cr equivalents) straight from the bitstream, where they are coded with
    a bias of 128, 128 and 256. AVFilmGrainAOMParams holds them unbiased:
    the uv_offset documentation gives a range of [-256, 255], the AFGS1
    parser in aom_film_grain.c subtracts the bias, the film grain
    synthesizer uses the values as signed multipliers and offsets, and
    libdav1d exports them unbiased. The AR coefficients a few lines above
    are already unbiased the same way.

    With -export_side_data film_grain and a hwaccel, the native decoder
    therefore exported values 128 or 256 higher than libdav1d for the
    same stream, and any consumer applying the exported grain scaled
    chroma noise wrong.

    Signed-off-by: lusoris <lusoris@proton.me>

diff --git a/libavcodec/av1dec.c b/libavcodec/av1dec.c
index 5f06dd1467..0fa6e9245b 100644
--- a/libavcodec/av1dec.c
+++ b/libavcodec/av1dec.c
@@ -1129,12 +1129,12 @@ static int export_film_grain(AVCodecContext *avctx, AVFrame *frame)
         aom->ar_coeffs_uv[1][i] = film_grain->ar_coeffs_cr_plus_128[i] - 128;
     }

-    aom->uv_mult[0] = film_grain->cb_mult;
-    aom->uv_mult[1] = film_grain->cr_mult;
-    aom->uv_mult_luma[0] = film_grain->cb_luma_mult;
-    aom->uv_mult_luma[1] = film_grain->cr_luma_mult;
-    aom->uv_offset[0] = film_grain->cb_offset;
-    aom->uv_offset[1] = film_grain->cr_offset;
+    aom->uv_mult[0] = film_grain->cb_mult - 128;
+    aom->uv_mult[1] = film_grain->cr_mult - 128;
+    aom->uv_mult_luma[0] = film_grain->cb_luma_mult - 128;
+    aom->uv_mult_luma[1] = film_grain->cr_luma_mult - 128;
+    aom->uv_offset[0] = film_grain->cb_offset - 256;
+    aom->uv_offset[1] = film_grain->cr_offset - 256;

     return 0;
 }