Re: [FFmpeg-devel] [PATCH v3 1/1] avfilter/buffersink: Add video frame allocation callback

[James Almer <jamrial@gmail.com>]

On 7/22/2023 2:03 PM, John Cox wrote:
> Add a callback to enable user allocation of video frames on the final
> stage of a filter chain.
> 
> Signed-off-by: John Cox <jc@kynesim.co.uk>
> ---
>   libavfilter/buffersink.c | 21 +++++++++++++++++++++
>   libavfilter/buffersink.h | 27 +++++++++++++++++++++++++++
>   libavfilter/version.h    |  2 +-
>   3 files changed, 49 insertions(+), 1 deletion(-)
> 
> diff --git a/libavfilter/buffersink.c b/libavfilter/buffersink.c
> index 306c283f77..e99d444530 100644
> --- a/libavfilter/buffersink.c
> +++ b/libavfilter/buffersink.c
> @@ -62,6 +62,11 @@ typedef struct BufferSinkContext {
>       int sample_rates_size;
>   
>       AVFrame *peeked_frame;
> +
> +    union {
> +        AVBuffersinkAllocVideoFrameFunc video;
> +    } alloc_cb;
> +    void * alloc_v;
>   } BufferSinkContext;
>   
>   #define NB_ITEMS(list) (list ## _size / sizeof(*list))
> @@ -154,6 +159,21 @@ int attribute_align_arg av_buffersink_get_samples(AVFilterContext *ctx,
>       return get_frame_internal(ctx, frame, 0, nb_samples);
>   }
>   
> +static AVFrame *alloc_video_buffer(AVFilterLink *link, int w, int h)
> +{
> +    AVFilterContext *const ctx = link->dst;
> +    BufferSinkContext *const buf = ctx->priv;
> +    return buf->alloc_cb.video ? buf->alloc_cb.video(ctx, buf->alloc_v, w, h) :
> +                                 ff_default_get_video_buffer(link, w, h);

Does the AVBuffersinkAllocVideoFrameFunc function have access to the 
AVFilterLink ff_default_get_video_buffer() gets? I assume it'd be needed 
to get values like pixel format. If so, it should be documented how, but 
maybe it's easier to just pass link here instead of the AVFilterContext, 
and let the caller access link->dst if needed.

Also, looking at ff_default_get_video_buffer(), a hardware enabled 
filter may populate hw_frames_ctx, and that's a field only available in 
the AVFilterLink and that should not be accessed from outside lavfi, 
which includes a caller defined function used for this callback.

> +}
> +
> +void av_buffersink_set_alloc_video_frame(AVFilterContext *ctx, AVBuffersinkAllocVideoFrameFunc cb, void *v)
> +{
> +    BufferSinkContext *const buf = ctx->priv;
> +    buf->alloc_cb.video = cb;
> +    buf->alloc_v = v;
> +}
> +
>   static av_cold int common_init(AVFilterContext *ctx)
>   {
>       BufferSinkContext *buf = ctx->priv;
> @@ -381,6 +401,7 @@ static const AVFilterPad avfilter_vsink_buffer_inputs[] = {
>       {
>           .name = "default",
>           .type = AVMEDIA_TYPE_VIDEO,
> +        .get_buffer = {.video = alloc_video_buffer},
>       },
>   };
>   
> diff --git a/libavfilter/buffersink.h b/libavfilter/buffersink.h
> index 64e08de53e..2419d1bd80 100644
> --- a/libavfilter/buffersink.h
> +++ b/libavfilter/buffersink.h
> @@ -166,6 +166,33 @@ int av_buffersink_get_frame(AVFilterContext *ctx, AVFrame *frame);
>    */
>   int av_buffersink_get_samples(AVFilterContext *ctx, AVFrame *frame, int nb_samples);
>   
> +/**
> + * Callback from av_buffersink_set_alloc_video_frame to allocate
> + * a frame
> + *
> + * @param ctx pointer to a context of the abuffersink AVFilter.
> + * @param v opaque pointer passed to
> + *          av_buffersink_set_alloc_video_frame
> + * @param w width of frame to allocate
> + * @param height of frame to allocate
> + *
> + * @return
> + *         - The newly allocated frame
> + *         - NULL if error
> + */
> +typedef AVFrame *(*AVBuffersinkAllocVideoFrameFunc)(AVFilterContext * ctx, void * v, int w, int h);
> +
> +/**
> + * Set a video frame allocation method for buffersink
> + *
> + * @param ctx pointer to a context of the abuffersink AVFilter.
> + * @param cb Callback to the allocation function. If set to NULL
> + *           then the default avfilter allocation function will
> + *           be used.
> + * @param v  Opaque to pass to the allocation function
> + */
> +void av_buffersink_set_alloc_video_frame(AVFilterContext *ctx, AVBuffersinkAllocVideoFrameFunc cb, void * v);
> +
>   /**
>    * @}
>    */
> diff --git a/libavfilter/version.h b/libavfilter/version.h
> index c001693e3c..54950497be 100644
> --- a/libavfilter/version.h
> +++ b/libavfilter/version.h
> @@ -31,7 +31,7 @@
>   
>   #include "version_major.h"
>   
> -#define LIBAVFILTER_VERSION_MINOR   8
> +#define LIBAVFILTER_VERSION_MINOR   9
>   #define LIBAVFILTER_VERSION_MICRO 102
>   
>   
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