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

[Marton Balint via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

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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