[FFmpeg-devel] [PATCH 1/2] avfilter: change ff_inlink_make_frame_writable() to take AVFrame* argument

[Marton Balint]

On Mon, 30 Jan 2017, Rostislav Pehlivanov wrote:

> On 30 January 2017 at 00:09, Marton Balint <cus at passwd.hu> wrote:
>
>>
>> On Sat, 28 Jan 2017, Nicolas George wrote:
>>
>> Le nonidi 9 pluviôse, an CCXXV, Muhammad Faiz a écrit :
>>>
>>>> so the behavior will be similar to
>>>> av_frame_make_writable().
>>>>
>>>
>>> The point was to move away from that. Who copies a struct when you can
>>> move a pointer?
>>>
>>>
>> By the way, why av_frame_make_writable copies the struct?
>>
>> As far as I see it can be implemented just like this:
>>
>> int av_frame_make_writable(AVFrame *frame)
>> {
>>     int ret;
>>     int i;
>>
>>     if (!frame->buf[0])
>>         return AVERROR(EINVAL);
>>
>>     for (i = 0; i < FF_ARRAY_ELEMS(frame->buf); i++) {
>>         if (frame->buf[i]) {
>>             ret = av_buffer_make_writable(&frame->buf[i]);
>>             if (ret < 0)
>>                 return ret;
>>             frame->data[i] = frame->buf[i]->data;
>>         }
>>     }
>>     for (i = 0; i < frame->nb_extended_buf; i++) {
>>         ret = av_buffer_make_writable(&frame->extended_buf[i]);
>>         if (ret < 0)
>>             return ret;
>>         frame->extended_data[i] = frame->extended_buf[i]->data;
>>     }
>>
>>     return 0;
>> }
>>
>> It even passes fate. What do I miss?
>>
>> Don't get me wrong, I know that this approach cannot be implemented
>> directly into the filtering case, because of the custom get buffer callback
>> and the frame pool, but for the generic frame function, is there any
>> downside doing this?
>>
>> Thanks,
>> Marton
>>
> The memory referenced by the AVFrame can be e.g. in hardware or used by
> something else. It makes no sense to try to write to it, you'd just mess up
> whatever else the user is using the data for.
> So you allocate new buffers, make a copy of the data and unref the ref you
> have.

I don't follow. The code above is also doing that, making writable all 
the AVBufferRef-s, therefore allocating a new buffer for them, copying 
their content to the newly allocated buffer, and unrefing the old buffer.

Regards,
Marton