[FFmpeg-devel] [PATCH] avfilter/vf_overlay: fix alpha blending when main source has an alpha channel

[Paul B Mahol]

On 7/15/17, Peter Grosse <pegro at friiks.de> wrote:
> Use alpha value from alpha channel instead of selected component channel.
>
> Signed-off-by: Peter Grosse <pegro at friiks.de>
> ---
>  libavfilter/vf_overlay.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/libavfilter/vf_overlay.c b/libavfilter/vf_overlay.c
> index 5c9f590957..cde09f5fb3 100644
> --- a/libavfilter/vf_overlay.c
> +++ b/libavfilter/vf_overlay.c
> @@ -508,7 +508,7 @@ static av_always_inline void blend_plane(AVFilterContext
> *ctx,
>      int dst_hp = AV_CEIL_RSHIFT(dst_h, vsub);
>      int yp = y>>vsub;
>      int xp = x>>hsub;
> -    uint8_t *s, *sp, *d, *dp, *a, *ap;
> +    uint8_t *s, *sp, *d, *dp, *a, *ap, *da, *dap;
>      int jmax, j, k, kmax;
>
>      j = FFMAX(-yp, 0);
> @@ -517,12 +517,16 @@ static av_always_inline void
> blend_plane(AVFilterContext *ctx,
>                        + (yp+j)    * dst->linesize[dst_plane]
>                        + dst_offset;
>      ap = src->data[3] + (j<<vsub) * src->linesize[3];
> +    if (main_has_alpha)
> +        dap = dst->data[3] + (j<<vsub) * dst->linesize[3];
>
>      for (jmax = FFMIN(-yp + dst_hp, src_hp); j < jmax; j++) {
>          k = FFMAX(-xp, 0);
>          d = dp + (xp+k) * dst_step;
>          s = sp + k;
>          a = ap + (k<<hsub);
> +        if (main_has_alpha)
> +            da = dap + (k<<hsub);
>
>          for (kmax = FFMIN(-xp + dst_wp, src_wp); k < kmax; k++) {
>              int alpha_v, alpha_h, alpha;
> @@ -545,16 +549,16 @@ static av_always_inline void
> blend_plane(AVFilterContext *ctx,
>                  // average alpha for color components, improve quality
>                  uint8_t alpha_d;
>                  if (hsub && vsub && j+1 < src_hp && k+1 < src_wp) {
> -                    alpha_d = (d[0] + d[src->linesize[3]] +
> -                               d[1] + d[src->linesize[3]+1]) >> 2;
> +                    alpha_d = (da[0] + da[src->linesize[3]] +
> +                               da[1] + da[src->linesize[3]+1]) >> 2;
>                  } else if (hsub || vsub) {
>                      alpha_h = hsub && k+1 < src_wp ?
> -                        (d[0] + d[1]) >> 1 : d[0];
> +                        (da[0] + da[1]) >> 1 : da[0];
>                      alpha_v = vsub && j+1 < src_hp ?
> -                        (d[0] + d[src->linesize[3]]) >> 1 : d[0];
> +                        (da[0] + da[src->linesize[3]]) >> 1 : da[0];
>                      alpha_d = (alpha_v + alpha_h) >> 1;
>                  } else
> -                    alpha_d = d[0];
> +                    alpha_d = da[0];
>                  alpha = UNPREMULTIPLY_ALPHA(alpha, alpha_d);
>              }
>              *d = FAST_DIV255(*d * (255 - alpha) + *s * alpha);
> --
> 2.13.0
>
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>

how can one reproduce this bug?