[FFmpeg-devel] [GSoC] Qualification Task

Stefano Sabatini stefano.sabatini-lala at poste.it
Thu Mar 24 18:33:11 CET 2011

On date Thursday 2011-03-24 10:35:53 +0200, Mina Nagy Zaki encoded:
> On Wednesday 23 March 2011 19:03:28 Stefano Sabatini wrote:
> > On date Wednesday 2011-03-23 06:25:15 +0200, Mina Nagy Zaki encoded:
> > Check avfilter_request_frame(), it calls the request_frame callback if
> > defined on the source pad, otherwise calls avfilter_request_frame() on
> > the first input link of the source filter.
> > 
> > In other words we have a mixed pull/push model, you pull frames, and
> > as a consequence the filterchain pushes frames on the
> > filterchain. There is nothing wrong in implementing a frame_request
> > callback in the output pads.
> Ok, I'm still a bit confused. So first request_frame is called on my output 
> pad, at which point I need to pass on the request, and the chain of events 
> eventually leads to filter_samples being called on my input pad _OR_ I get EOF 
> on my request_frame.


> So essentially that means I do filtering in filter_samples 
> and draining in request_frames (where I know it's EOF), and when I'm done with 
> draining I need to actually send down the samples through 
> avfilter_filter_samples to the rest of the chain, right?


In other words suppose that a filter receives a request_frame() (for
example, aout pulling filtered samples).

If you don't need to buffer samples for filtering, you can simply
handle the incoming samples in filter_samples() and send them
immediately to the output filter(s).

If you need to buffer samples, then you'll implement a request_frame()
callback, which will buffer samples by calling avfilter_request_frame
until you filled your buffer and can proceed with the actual
> I haven't actually found a filter that employs both draw_slice/filter_samples 
> and request_frame callbacks in such a manner. 

IIRC fifo works this way.

> > This is essentially what is done in the case of video, check for
> > example how it is done in fifo (and of course you need to implement it
> > in a source). I don't know if this is a mess, but don't worry, we'll
> > fix it ;-).
> Yes, vf_fifo does pass on frames through avfilter_draw_slice from inside its 
> request_frame callback, and actually breaks the draw_slice function stack
> using an empty callback. 

Uhm, what do you exctly mean with "actually breaks the draw_slice function stack"?

> But then what's wrong with a pure pull model at least for audio (because 
> video, it seems, requires the start_frame/end_frame calls)? That is, I get 
> data on request_frame and I process it and return? I could actually do that 
> and use a null filter_frames callback to simply break the function stack.

Anyway I suppose you can also implement a pure pull model.  You simply
call filter_samples on the source, which will create the samples and
send them to the next filter, up to the last filter in the
filterchain. But note that in ffplay/ffmpeg you actually need to pull
frames from the sink, because this is how the applications work (that
is, they call avfilter_frame when they need data to inject to the
output file / to SDL).

What's exactly the problem that you're addressing?

> [snip: patch comments]
> I've applied all requested changes, added copyright notice, check
> for 44.1kHz and docs.
> -- 
> Mina

> diff --git a/doc/filters.texi b/doc/filters.texi
> index 32f68f7..3b6bf4e 100644
> --- a/doc/filters.texi
> +++ b/doc/filters.texi

Looks fine, I'll push it to the public branch I'm going to setup for
audio filters, nice job! :)

PS For posting patches the recommended way is attaching the patches
obtained with:
git format-patch REV.
FFmpeg = Forgiving and Funny Minimalistic Puristic Earthshaking Guru

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