[x264-devel] [PATCH] fix bug of weighted bipred according to Rec. ITU-T H.264 (10/2016) 8.4.3

[BugMaster]

On Fri, 15 Jan 2021 17:20:22 +0800, Lingjiang Fang wrote:
> From: Lingjiang Fang <vacingfang at tencent.com>

> ---
>  common/macroblock.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)

> diff --git a/common/macroblock.c b/common/macroblock.c
> index 9a3c7132..638a5c42 100644
> --- a/common/macroblock.c
> +++ b/common/macroblock.c
> @@ -1890,13 +1890,17 @@ void x264_macroblock_bipred_init( x264_t *h )
>                  int poc0 = l0->i_poc +
> mbfield*l0->i_delta_poc[field^(i_ref0&1)];
>                  for( int i_ref1 = 0; i_ref1 <
> (h->i_ref[1]<<mbfield); i_ref1++ )
>                  {
> +                    int use_default = 0;
>                      int dist_scale_factor;
>                      x264_frame_t *l1 = h->fref[1][i_ref1>>mbfield];
>                      int cur_poc = h->fdec->i_poc +
> mbfield*h->fdec->i_delta_poc[field];
>                      int poc1 = l1->i_poc +
> mbfield*l1->i_delta_poc[field^(i_ref1&1)];
>                      int td = x264_clip3( poc1 - poc0, -128, 127 );
>                      if( td == 0 /* || pic0 is a long-term ref */ )
> +                    {
>                          dist_scale_factor = 256;
> +                        use_default = 1;
> +                    }
>                      else
>                      {
>                          int tb = x264_clip3( cur_poc - poc0, -128, 127 );
> @@ -1907,7 +1911,7 @@ void x264_macroblock_bipred_init( x264_t *h )
>                     
> h->mb.dist_scale_factor_buf[mbfield][field][i_ref0][i_ref1] = dist_scale_factor;

>                      dist_scale_factor >>= 2;
> -                    if( h->param.analyse.b_weighted_bipred
> +                    if( !use_default &&
> h->param.analyse.b_weighted_bipred
>                            && dist_scale_factor >= -64
>                            && dist_scale_factor <= 128 )
>                      {
> --
> 2.29.2

Hi.

I understand which part of H.264 spec you tried to fix. But was you
able to really trigger this bug (code path) in x264? I in my quick
test I couldn't because x264 does not use long-term refs and I don't
know when poc0 and poc1 would be equal for B-frame (which frame-types
pattern will cause this?).