Re: [FFmpeg-devel] [PATCH] lavfi/vf_libplacebo: simplify SAR normalization

[Niklas Haas <ffmpeg@haasn.xyz>]

On Tue, 20 Jun 2023 17:12:48 +0200 Niklas Haas <ffmpeg@haasn.xyz> wrote:
> From: Niklas Haas <git@haasn.dev>
> 
> The old logic was trying to be excessively clever in "deducing" that the
> user wanted to stretch/scale the image when ow/oh differed from iw/ih
> aspect ratio. But this is almost surely unintended except in
> pathological cases, and in those cases users should simply disable
> normalize_sar and do all the stretching/scaling logic themselves. This
> is especially important in multi-input mode, where the canvas may be
> vastly different from the input dimensions of any stream. Also, passing
> through input 0 SAR in multi-input mode is arbitrary and nearly useless,
> so again force output SAR to 1:1 here.
> ---
>  libavfilter/vf_libplacebo.c | 19 ++++++-------------
>  1 file changed, 6 insertions(+), 13 deletions(-)
> 
> diff --git a/libavfilter/vf_libplacebo.c b/libavfilter/vf_libplacebo.c
> index 0f7c6481925..e58183a5caa 100644
> --- a/libavfilter/vf_libplacebo.c
> +++ b/libavfilter/vf_libplacebo.c
> @@ -160,7 +160,6 @@ typedef struct LibplaceboContext {
>      // Parsed expressions for input/output crop
>      AVExpr *crop_x_pexpr, *crop_y_pexpr, *crop_w_pexpr, *crop_h_pexpr;
>      AVExpr *pos_x_pexpr, *pos_y_pexpr, *pos_w_pexpr, *pos_h_pexpr;
> -    AVRational target_sar;
>      float pad_crop_ratio;
>      float corner_rounding;
>      int force_original_aspect_ratio;
> @@ -795,9 +794,9 @@ static void update_crops(AVFilterContext *ctx, LibplaceboInput *in,
>              target->crop.y0 = av_expr_eval(s->pos_y_pexpr, s->var_values, NULL);
>              target->crop.x1 = target->crop.x0 + s->var_values[VAR_POS_W];
>              target->crop.y1 = target->crop.y0 + s->var_values[VAR_POS_H];
> -
> -            if (s->target_sar.num) {
> -                float aspect = pl_rect2df_aspect(&target->crop) * av_q2d(s->target_sar);
> +            if (s->normalize_sar) {
> +                float aspect = pl_rect2df_aspect(&image->crop);
> +                aspect *= av_q2d(in->link->sample_aspect_ratio);
>                  pl_rect2df_aspect_set(&target->crop, aspect, s->pad_crop_ratio);
>              }
>          }
> @@ -1188,7 +1187,6 @@ static int libplacebo_config_output(AVFilterLink *outlink)
>      const AVPixFmtDescriptor *out_desc = av_pix_fmt_desc_get(outlink->format);
>      AVHWFramesContext *hwfc;
>      AVVulkanFramesContext *vkfc;
> -    AVRational scale_sar;
>  
>      /* Frame dimensions */
>      RET(ff_scale_eval_dimensions(s, s->w_expr, s->h_expr, inlink, outlink,
> @@ -1198,20 +1196,15 @@ static int libplacebo_config_output(AVFilterLink *outlink)
>                                 s->force_original_aspect_ratio,
>                                 s->force_divisible_by);
>  
> -    scale_sar = (AVRational){outlink->h * inlink->w, outlink->w * inlink->h};
> -    if (inlink->sample_aspect_ratio.num)
> -        scale_sar = av_mul_q(scale_sar, inlink->sample_aspect_ratio);
> -
> -    if (s->normalize_sar) {
> -        /* Apply all SAR during scaling, so we don't need to set the out SAR */
> +    if (s->normalize_sar || s->nb_inputs > 1) {
> +        /* SAR is normalized, or we have multiple inputs, set out to 1:1 */
>          outlink->sample_aspect_ratio = (AVRational){ 1, 1 };
> -        s->target_sar = scale_sar;
>      } else {
>          /* This is consistent with other scale_* filters, which only
>           * set the outlink SAR to be equal to the scale SAR iff the input SAR
>           * was set to something nonzero */
>          if (inlink->sample_aspect_ratio.num)
> -            outlink->sample_aspect_ratio = scale_sar;
> +            outlink->sample_aspect_ratio = inlink->sample_aspect_ratio;
>      }
>  
>      /* Frame rate */
> -- 
> 2.41.0
> 

Merged as 5fdb12d6a06.
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