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

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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>
(cherry picked from commit 2a8c6d30b9)
This commit is contained in:
lusoris
2026-10-09 18:19:54 +02:00
committed by ffmpeg-devel
parent 669f5ab083
commit 6dfaba896f
1 changed files with 6 additions and 6 deletions
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12
libavcodec/av1dec.c
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@@ -1087,12 +1087,12 @@ 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;
}