[FFmpeg-devel] lavfi noise generator

[Vitor Sessak]

Stefano Sabatini wrote:
> On date Monday 2008-12-29 16:27:04 +0100, Vitor Sessak encoded:
>> Stefano Sabatini wrote:
>>> On date Monday 2008-12-29 12:06:20 +0100, Vitor Sessak encoded:
> [...]
>>>> Indeed, we are lacking good examples of filters in the SoC tree. But  
>>>> exactly for this reason, I think vf_noise.c should fill one slice at 
>>>> a time to give the good example. After this is done, IMO it is 
>>>> welcome to soc svn, at least to serve as a template.
>>> Hi Vitor, vsrc_noise.c is a *source* rather than a filter, so I don't
>>> think it is possible to use the the draw_slice() API.
>> Indeed, it should not be possible, at least not with the current svn  
>> code. See my attached patch.
>>
>>> What I'm currently doing is:
>>>
>>> static int request_frame(AVFilterLink *link)
>>> {
>>>     NoiseContext *ctx = link->src->priv;
>>>     AVFilterPicRef *picref = avfilter_get_video_buffer(link, AV_PERM_WRITE);
>>>
>>>     fill_picture(ctx, picref);
>>>     picref->pts = av_rescale_q(ctx->pts++, (AVRational){ ctx->frame_rate.den, ctx->frame_rate.num }, AV_TIME_BASE_Q);
>>>
>>>     avfilter_start_frame(link, avfilter_ref_pic(picref, ~0));
>>>     avfilter_draw_slice(link, 0, picref->h);
>>>     avfilter_end_frame(link);
>>>
>>>     avfilter_unref_pic(picref);
>>>
>>>     return 0;
>>> }
>> Could something like the following work?
>>
>> #define SLICE_SIZE 32
>>
>> static int request_frame(AVFilterLink *link)
>> {
>>     NoiseContext *ctx = link->src->priv;
>>     AVFilterPicRef *picref = avfilter_get_video_buffer(link,  
>> AV_PERM_WRITE);
>>     int h;
>>
>>     picref->pts = av_rescale_q(ctx->pts++, (AVRational) {  
>> ctx->frame_rate.den, ctx->frame_rate.num }, AV_TIME_BASE_Q);
>>
>>     avfilter_start_frame(link, avfilter_ref_pic(picref, ~0));
>>     for(h=0; h < ctx->h; h += SLICE_SIZE) {
>>        fill_picture(ctx, picref, h, FFMIN(h+SLICE_SIZE, ctx->h));
>>        avfilter_draw_slice(link, h, FFMIN(h+SLICE_SIZE, ctx->h));
>>     }
>>     avfilter_end_frame(link);
>>
>>     avfilter_unref_pic(picref);
>>
>>     return 0;
>> }
> 
> It should work, the only thing I don't like is that in this way the
> code *needs* to know about the picture structure (it needs to know how
> to access the slice), while before I was simply filling the whole
> buffer, and this consideration leads to this (maybe silly) question:
> what's the advantage of per-slice filling in this case?

Imagine a filter chain like

noise -> slow_filter -> scale -> another_slow_filter -> 
overlay_with_something -> output

The noise filter will pass slices to slow_filter that will pass slices 
to the next filters and etc. Not that one could not explicitly add a 
slicify filter for that, but it should be faster thanks to a better use 
of the data cache.

As a rule of thumb, if a filter don't need to work with whole frames, it 
should work with slices. At least, that how I understand lavfi is 
expected to work. But in this case I agree it is more up to the 
framework to deal with it...

-Vitor