[FFmpeg-devel] [PATCH 1/4] lavf: add probe device API

[Michael Niedermayer]

On Sun, Jan 26, 2014 at 07:40:56PM +0100, Nicolas George wrote:
> Le sextidi 6 pluviôse, an CCXXII, Lukasz Marek a écrit :
> > >From c1e66d75f698fbd301743cd0664733a5d48f03e8 Mon Sep 17 00:00:00 2001
> > From: Lukasz Marek <lukasz.m.luki at gmail.com>
> > Date: Sat, 25 Jan 2014 22:46:03 +0100
> > Subject: [PATCH] lavd: add probe device API
> > 
> > ---
> >  libavformat/avformat.h | 103 +++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 103 insertions(+)
> > 
> > diff --git a/libavformat/avformat.h b/libavformat/avformat.h
> > index a495ee0..0ff4560 100644
> > --- a/libavformat/avformat.h
> > +++ b/libavformat/avformat.h
> > @@ -323,6 +323,101 @@ typedef struct AVFrac {
> >      int64_t val, num, den;
> >  } AVFrac;
> >  
> 
> > +//TODO: Move this stuff to libavdevice.
> 
> I wonder. Being able to query the list of codecs supported by RTP, for
> example, would be nice.

also its a bit redudant with query_codec()


> 
> > +
> > +/**
> > + * TODO: desc
> > + */
> > +typedef struct AVDeviceFormat
> > +{
> > +    enum AVCodecID codec;  /**< codec */
> > +    int *formats;          /**< list of formats supported with codec.
> > +                                AVPixelFormat/AVSampleFormat terminated with -1 */
> > +} AVDeviceFomat;
> 
> First, I suggest, here and everywhere, to replace "terminated with X" lists
> with a count indicator: "int *formats; unsigned nb_formats;". I believe this
> is more practical for everyone. Also, some list are terminated by -1, some
> by 0, some by NULL, that takes effort to remember.
> 

> Second, I am not sure whether codec/format is the only pair that needs to be
> linked. That is the most obvious one, but I can easily imagine a camera with
> limited bandwidth supporting 50 FPS in MJPEG mode but only 30 FPS in
> RAWVIDEO mode.

+1

also see  AVClass.query_ranges() and av_opt_query_ranges()
its more generic and simpler


> 
> Basically, we have the full Cartesian product:
> CODECS × PIXEL_FORMAT × RESOLUTIONS × FRAME_RATES
> CODECS × SAMPLE_FORMAT × CHANNEL_COUNTS × SAMPLE_RATES
> and we need to be able to express a part of the set.
> 
> The obvious simple idea is to consider that the subset is itself a Cartesian
> product:
> 
> SUPPORTED_FORMATS × SUPPORTED_RESOLUTIONS × SUPPORTED_FRAME_RATES \subset
> PIXEL_FORMAT × RESOLUTIONS × FRAME_RATES
> 
> Except for CODECS and FORMAT, because they always are strongly linked.
> 
> I can suggest two solutions, one simple and one powerful.
> 
> The simple one: each device can return a small list of AVDeviceCapabilities.
> Each device expands its list the way it is most convenient. For example:
> 
> [
>   { codec = MJPEG, format = YUV420, list of supported resolutions and rates },
>   { codec = MJPEG, format = YUV422, list of supported resolutions and rates },
>   { codec = RAWVIDEO, format = RGB, list of supported resolutions and rates },
> ]
> 
> The powerful one: the AVDeviceCapabilities can leave fields unset and point
> to a list of AVDeviceCapabilities to define them. Something like that:
> 
> [
>   { codec = MJPEG, pointer to sublist by format for MJPEG },
>   { codec = RAWVIDEO, idem },
> ]
> 
> sublist = [
>   { format = YUV420, pointer to sublist of frame sizes and rates },
>   { format = YUV422, pointer to sublist of frame sizes and rates },
> ]

even more complex and still a subset of the existing
AVOptionRange(s) API


[...]

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