[FFmpeg-devel] [PATCH] avfilter: add backgroundkey video filter

[Paul B Mahol]

On 11/2/22, Andreas Rheinhardt <andreas.rheinhardt at outlook.com> wrote:
> Paul B Mahol:
>> +static int filter_frame(AVFilterLink *link, AVFrame *frame)
>> +{
>> +    AVFilterContext *avctx = link->dst;
>> +    BackgroundkeyContext *s = avctx->priv;
>> +    int64_t sum = 0;
>> +    int ret;
>> +
>> +    if (!s->background) {
>> +        s->background = av_frame_clone(frame);
>> +        if (!s->background)
>> +            return AVERROR(ENOMEM);
>> +        av_frame_make_writable(s->background);
>
> You are never writing to the background frame, so there is no point in
> making it writable; what you actually want to achieve here is making
> frame writable again and to achieve this you should make frame, not
> background writable (and of course you should check said call).
>

This is invalid, input pad receives always writable frames as there is flag.

> (Actually, you never
>
>> +    }
>> +
>> +    if (ret = ff_filter_execute(avctx, s->do_slice, frame, NULL,
>> +                                FFMIN(frame->height, s->nb_threads)))
>> +        return ret;
>> +
>> +    for (int n = 0; n < s->nb_threads; n++)
>> +        sum += s->sums[n];
>> +    if (s->max_sum * s->threshold < sum) {
>> +        av_frame_free(&s->background);
>> +        s->background = av_frame_clone(frame);
>> +        if (!s->background)
>> +            return AVERROR(ENOMEM);
>> +        av_frame_make_writable(s->background);
>
> Given that you never write to background, there is no need to make it
> writable. This time, there is also no need to make frame writable (the
> next filter in the chain may not need a writable frame anyway), so just
> remove this.
> And the av_frame_free+av_frame_clone can become an
> av_frame_unref+av_frame_ref (this will necessitate modifying the check
> above to not only check for to existence of s->background).
>
>> +    }
>> +
>> +    return ff_filter_frame(avctx->outputs[0], frame);
>> +}
>
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