[FFmpeg-devel] [PATCH] libavcodec/libx264.c: Fix chromaoffset of libx264 doesn't work

[Jan Ekström]

On Tue, Jul 28, 2020 at 3:30 PM Takio Yamaoka <y.takio at gmail.com> wrote:
>
> An initial value of `AVCodecContext::chromaoffset` is zero,
> then it causes to block `-chromaoffset` setting as result.
> In addition, even though a negative number of `chromaoffset`
> is meaningful, `X264Context::chroma_offset` is initialized
> with `-1` as no setting and ignored if it is negative number.
>
> To fix above, it changes ...
> - a value of `X264Context::chroma_offset` to 0 as no setting
>     - due to x264's default value
> - conditional statement to import `-chromaoffset`
>
> Signed-off-by: Takio Yamaoka <y.takio at gmail.com>
> ---
>  libavcodec/libx264.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/libavcodec/libx264.c b/libavcodec/libx264.c
> index 7bbeab7d4c..347d29df27 100644
> --- a/libavcodec/libx264.c
> +++ b/libavcodec/libx264.c
> @@ -681,11 +681,11 @@ static av_cold int X264_init(AVCodecContext *avctx)
>
>  #if FF_API_PRIVATE_OPT
>  FF_DISABLE_DEPRECATION_WARNINGS
> -    if (avctx->chromaoffset >= 0)
> +    if (avctx->chromaoffset)
>          x4->chroma_offset = avctx->chromaoffset;
>  FF_ENABLE_DEPRECATION_WARNINGS
>  #endif
> -    if (x4->chroma_offset >= 0)
> +    if (x4->chroma_offset)
>          x4->params.analyse.i_chroma_qp_offset = x4->chroma_offset;
>
>      if (avctx->gop_size >= 0)
> @@ -1140,7 +1140,7 @@ static const AVOption options[] = {
>      { "vlc",              NULL, 0, AV_OPT_TYPE_CONST, { .i64 = 0 },  INT_MIN, INT_MAX, VE, "coder" },
>      { "ac",               NULL, 0, AV_OPT_TYPE_CONST, { .i64 = 1 },  INT_MIN, INT_MAX, VE, "coder" },
>      { "b_strategy",   "Strategy to choose between I/P/B-frames",          OFFSET(b_frame_strategy), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 2, VE },
> -    { "chromaoffset", "QP difference between chroma and luma",            OFFSET(chroma_offset), AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX, VE },
> +    { "chromaoffset", "QP difference between chroma and luma",            OFFSET(chroma_offset), AV_OPT_TYPE_INT, { .i64 = 0 }, INT_MIN, INT_MAX, VE },
>      { "sc_threshold", "Scene change threshold",                           OFFSET(scenechange_threshold), AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX, VE },
>      { "noise_reduction", "Noise reduction",                               OFFSET(noise_reduction), AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX, VE },
>

General change looks alright to me, after checking the following points.
- AVCodecContext's chromaoffset indeed defaults to 0 through the
DEFAULT define (#define DEFAULT 0 //should be NAN but it does not work
as it is not a constant in glibc as required by ANSI/ISO C)
- x264 default is 0 (thus we thankfully haven't overwritten this
before, and the patch doesn't lead to a change in that behavior,
either)
- value in x264 can be between -12 and 12, thus both positive and
negative values indeed can be set
(x264_clip3(h->param.analyse.i_chroma_qp_offset, -12, 12) can be found
in x264's code)

Best regards,
Jan