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

[James Almer <jamrial@gmail.com>]

On 3/16/2022 5:52 PM, James Almer wrote:
> 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()?

Also, ff_get_video_buffer() can be made to accept an align parameter, 
much like av_frame_get_buffer().
The latter (and av_frame_make_writable) should not be used in lavfi 
since you're losing the benefits from the AVFilterLink buffer pool.

>>
>> 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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