[FFmpeg-devel] Re: [PATCH v8] avfilter/vf_blackframe: add slice threading

[Raja Rathour via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

Hi Marton,

Thank you for the feedback.

As you suggested, I have replaced the heap allocation (av_calloc) with a
single stdatomic (atomic_uint) counter. This simplifies the logic and
removes the memory overhead.

I have also opened the formal Pull Request on code.ffmpeg.org as requested:
https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21408

Summary of changes in v9:
- Switched to stdatomic for the global pixel counter.
- Enabled AVFILTER_FLAG_SLICE_THREADS.
- Cleaned up the commit history to include only this feature.
- Verified that all FATE tests are passing on the PR.

Best regards,
Raja Rathour

On Mon, Jan 5, 2026 at 3:46 AM Marton Balint via ffmpeg-devel <
ffmpeg-devel@ffmpeg.org> wrote:

>
>
> On Thu, 1 Jan 2026, Raja Rathour via ffmpeg-devel wrote:
>
> > v8: Restored original struct comments (unrelated to threads) and updated
> height comment to Doxygen style (///<) as requested.
> >
> > Thank you for the detailed feedback on the patch structure and project
> style.
> >
> > Signed-off-by: Raja Rathour <imraja729@gmail.com>
> > ---
> > libavfilter/vf_blackframe.c | 69 ++++++++++++++++++++++++++++++++-----
> > 1 file changed, 60 insertions(+), 9 deletions(-)
> >
> > diff --git a/libavfilter/vf_blackframe.c b/libavfilter/vf_blackframe.c
> > index f0aa53e133..5b198e6594 100644
> > --- a/libavfilter/vf_blackframe.c
> > +++ b/libavfilter/vf_blackframe.c
> > @@ -32,6 +32,7 @@
> >
> > #include "libavutil/internal.h"
> > #include "libavutil/opt.h"
> > +#include "libavutil/mem.h"
> > #include "avfilter.h"
> > #include "filters.h"
> > #include "video.h"
> > @@ -45,6 +46,15 @@ typedef struct BlackFrameContext {
> >     unsigned int last_keyframe; ///< frame number of the last received
> key-frame
> > } BlackFrameContext;
> >
> > +typedef struct ThreadData {
> > +    const uint8_t *data;
> > +    int linesize;
> > +    int bthresh;
> > +    int width;
> > +    int height;           ///< height for cleaner slice math
> > +    unsigned int *counts;
>
> You can use a single stdatomic atomic int for this and avoid the
> allocation.
>
> Also I suggest you open a pull request at code.ffmpeg.org.
>
> Regards,
> Marton
>
> > +} ThreadData;
> > +
> > static const enum AVPixelFormat pix_fmts[] = {
> >     AV_PIX_FMT_YUV410P, AV_PIX_FMT_YUV420P, AV_PIX_FMT_GRAY8,
> AV_PIX_FMT_NV12,
> >     AV_PIX_FMT_NV21, AV_PIX_FMT_YUV444P, AV_PIX_FMT_YUV422P,
> AV_PIX_FMT_YUV411P,
> > @@ -55,22 +65,60 @@ static const enum AVPixelFormat pix_fmts[] = {
> >     snprintf(buf, sizeof(buf), format, value);  \
> >     av_dict_set(metadata, key, buf, 0)
> >
> > +static int blackframe_slice(AVFilterContext *ctx, void *arg, int jobnr,
> int nb_jobs)
> > +{
> > +    ThreadData *td = arg;
> > +    int slice_start = (td->height * jobnr) / nb_jobs;
> > +    int slice_end   = (td->height * (jobnr+1)) / nb_jobs;
> > +    int x, y;
> > +    const uint8_t *p;
> > +    unsigned int black_pixels_count = 0;
> > +
> > +    p = td->data + slice_start * td->linesize;
> > +
> > +    for (y = slice_start; y < slice_end; y++) {
> > +        for (x = 0; x < td->width; x++)
> > +            black_pixels_count += p[x] < td->bthresh;
> > +        p += td->linesize;
> > +    }
> > +
> > +    td->counts[jobnr] = black_pixels_count;
> > +    return 0;
> > +}
> > +
> > static int filter_frame(AVFilterLink *inlink, AVFrame *frame)
> > {
> >     AVFilterContext *ctx = inlink->dst;
> >     BlackFrameContext *s = ctx->priv;
> > -    int x, i;
> >     int pblack = 0;
> > -    uint8_t *p = frame->data[0];
> >     AVDictionary **metadata;
> >     char buf[32];
> > +    ThreadData td;
> > +    int nb_threads, nb_jobs, i;
> > +    unsigned int *nb_black_pixels_per_slice;
> > +
> > +    nb_threads = ff_filter_get_nb_threads(ctx);
> > +
> > +    nb_black_pixels_per_slice = av_calloc(nb_threads,
> sizeof(*nb_black_pixels_per_slice));
> > +    if (!nb_black_pixels_per_slice)
> > +        return AVERROR(ENOMEM);
> > +
> > +    td.data = frame->data[0];
> > +    td.linesize = frame->linesize[0];
> > +    td.width = inlink->w;
> > +    td.height = inlink->h;
> > +    td.bthresh = s->bthresh;
> > +    td.counts = nb_black_pixels_per_slice;
> > +
> > +    nb_jobs = FFMIN(td.height, nb_threads);
> > +
> > +    ff_filter_execute(ctx, blackframe_slice, &td, NULL, nb_jobs);
> >
> > -    for (i = 0; i < frame->height; i++) {
> > -        for (x = 0; x < inlink->w; x++)
> > -            s->nblack += p[x] < s->bthresh;
> > -        p += frame->linesize[0];
> > +    s->nblack = 0;
> > +    for (i = 0; i < nb_jobs; i++) {
> > +        s->nblack += nb_black_pixels_per_slice[i];
> >     }
> > -
> > +
> >     if (frame->flags & AV_FRAME_FLAG_KEY)
> >         s->last_keyframe = s->frame;
> >
> > @@ -89,6 +137,9 @@ static int filter_frame(AVFilterLink *inlink, AVFrame
> *frame)
> >
> >     s->frame++;
> >     s->nblack = 0;
> > +
> > +    av_free(nb_black_pixels_per_slice);
> > +
> >     return ff_filter_frame(inlink->dst->outputs[0], frame);
> > }
> >
> > @@ -118,9 +169,9 @@ const FFFilter ff_vf_blackframe = {
> >     .p.name        = "blackframe",
> >     .p.description = NULL_IF_CONFIG_SMALL("Detect frames that are
> (almost) black."),
> >     .p.priv_class  = &blackframe_class,
> > -    .p.flags       = AVFILTER_FLAG_METADATA_ONLY,
> > +    .p.flags       = AVFILTER_FLAG_METADATA_ONLY |
> AVFILTER_FLAG_SLICE_THREADS,
> >     .priv_size     = sizeof(BlackFrameContext),
> >     FILTER_INPUTS(avfilter_vf_blackframe_inputs),
> >     FILTER_OUTPUTS(ff_video_default_filterpad),
> >     FILTER_PIXFMTS_ARRAY(pix_fmts),
> > -};
> > +};
> > \ No newline at end of file
> > --
> > 2.48.1
> >
> > _______________________________________________
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