[FFmpeg-soc] Trying to write a new filter but unclear on callback order

[Brandon Mintern]

On Sat, Mar 27, 2010 at 9:27 AM, Stefano Sabatini
<stefano.sabatini-lala at poste.it> wrote:
> On date Saturday 2010-03-27 07:46:26 -0400, Brandon Mintern encoded:
>> I'm trying to build a filter called "clone" (as discussed with Vitor
>> on #ffmpeg-devel). I was modeling my filter after fifo, but apparently
>> I'm wrong about the callback process (sidenote: a more detailed
>> explanation in the wiki documentation would be very helpful!). I think
>> I'm reasonably close with the code I have, but I can't finish it until
>> I better understand the callback order in the filter chain. I would
>> appreciate it if someone could point me in the write direction.
>
> Quick reply as I don't have much time at this moment: enable the DEBUG
> symbol in avfilter.c and use -logleve debug, that should show you when
> every callback is called and understand how the filterchain processing
> stuff works.

Thank you for this tip. I made a lot of changes after checking out
vf_fps.c, tried your DEBUG tip, and the first function I see being
called is request_frame. This seemed incorrect (shouldn't at least
init() be called first?), so I added av_log(..., AV_LOG_DEBUG, ...)
messages to the beginning and end of all my functions. It turns out
that for some reason, init really isn't being called, which helps
explain the current Segmentation fault that I'm running into.

I think that for now I'll move onto something easier (vf_fade.c), but
if someone wouldn't mind taking a look at what I've done I would
really appreciate it. I really can't figure out what's wrong now. The
following patch:

http://bmintern.homeunix.com/~brandon/vf_clone.patch

is the result of running "svn diff" against the latest libavfilter
(revision 5726). I guess if I don't hear anything soon, I'll write a
simpler "vf_frame.c" which will simply output a single given frame a
specified number of times (rather than a sequence of frames) and see
if maybe I can get that working.

> And thanks for the contribution. BTW if you're a student and
> interested we have a libavfilter GSoC task just waiting for you, or
> for any interested student.

Unfortunately I'm no longer a student. Otherwise I would be *very* interested.

Thanks,
Brandon