Re: [FFmpeg-devel] [PATCH 4/4] avfilter/vf_zscale: realign output buffer if needed

[James Almer <jamrial@gmail.com>]

On 3/14/2022 6:06 PM, Marton Balint wrote:
> Output buffer alignment might be different to ZIMG_ALIGNMENT or it may not be
> aligned at all if a downstream filter (e.g. vf_pad) intentionally misaligns it.

av_frame_get_buffer() align parameter is used to align linesizes, not 
buffers. av_malloc() has a hardcoded alignment defined at compile time 
based on configure settings.

If what you need is aligned data pointers, you should either use the 
trick i used in the libdav1d wrapper, or change av_malloc() in order to 
use av_cpu_max_align().

> 
> Or maybe we should unconditionally always allocate output with
> av_frame_get_buffer() instead of ff_get_video_buffer()?
> 
> Signed-off-by: Marton Balint <cus@passwd.hu>
> ---
>   libavfilter/vf_zscale.c | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/libavfilter/vf_zscale.c b/libavfilter/vf_zscale.c
> index ceefc95224..2061e38bcc 100644
> --- a/libavfilter/vf_zscale.c
> +++ b/libavfilter/vf_zscale.c
> @@ -632,7 +632,7 @@ static int graphs_build(AVFrame *in, AVFrame *out, const AVPixFmtDescriptor *des
>       return 0;
>   }
>   
> -static int realign_frame(const AVPixFmtDescriptor *desc, AVFrame **frame)
> +static int realign_frame(const AVPixFmtDescriptor *desc, AVFrame **frame, int needs_copy)
>   {
>       AVFrame *aligned = NULL;
>       int ret = 0, plane, planes;
> @@ -654,10 +654,10 @@ static int realign_frame(const AVPixFmtDescriptor *desc, AVFrame **frame)
>               if ((ret = av_frame_get_buffer(aligned, ZIMG_ALIGNMENT)) < 0)
>                   goto fail;
>   
> -            if ((ret = av_frame_copy(aligned, *frame)) < 0)
> +            if (needs_copy && (ret = av_frame_copy(aligned, *frame)) < 0)
>                   goto fail;
>   
> -            if ((ret = av_frame_copy_props(aligned, *frame)) < 0)
> +            if (needs_copy && (ret = av_frame_copy_props(aligned, *frame)) < 0)
>                   goto fail;
>   
>               av_frame_free(frame);
> @@ -786,9 +786,12 @@ static int filter_frame(AVFilterLink *link, AVFrame *in)
>               goto fail;
>           }
>   
> +        if ((ret = realign_frame(odesc, &out, 0)) < 0)
> +            goto fail;
> +
>           av_frame_copy_props(out, in);
>   
> -        if ((ret = realign_frame(desc, &in)) < 0)
> +        if ((ret = realign_frame(desc, &in, 1)) < 0)
>               goto fail;
>   
>           snprintf(buf, sizeof(buf)-1, "%d", outlink->w);
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