[FFmpeg-devel] RFC: flag rawvideo input as interlaced

[Stefano Sabatini]

On date Sunday 2011-05-15 19:47:26 +0100, Mark Himsley encoded:
> Moved from FFmpeg-user:
> 
> On 14/05/2011 18:46, Stefano Sabatini wrote:
> > On date Saturday 2011-05-14 18:20:02 +0100, Mark Himsley encoded:
> >> On 14/05/2011 10:26, Stefano Sabatini wrote:
> >>> On date Thursday 2011-05-12 17:07:09 +0100, Mark Himsley encoded:
> >>> [...]
> >>>> Anyway...
> >>>>
> >>>> What I need is a way to flag rawvideo media when its used as an
> >>>> input into ffmpeg. Anyone?
> >>>
> >>> Mark, did you try with -top?
> >>
> >> Hi Stefano,
> >>
> >> Yes. Sorry, I should have said that -top was my first guess, but your
> >> showinfo filter says the rawvideo file is flagged as progressive when I
> >> have -top 1 in the input files settings.
> > 
> > Yes, check ffmpeg.c:do_video_out, top_field_first is used for setting
> > the field type *just before encoding*.
> > 
> > I wonder what could be a better approach, maybe we could implement a
> > setprops filter for changing the properties of the video in the
> > filterchain, and drop -top from ffmpeg.c.
> > 
> > What do you think of this idea?
> 
> Your 'setprops' filter idea would work, but I feel it would be better to
> be able to flag the input file correctly in the first place.
> 
> Imagine having one input file and outputting it to multiple output files
> (I'm thinking of an example where you are reading from an input pipe and
> outputting to MPEG2 and DV at the same time). You'd have to include the
> 'setprops' filter at the start of both output files filter chains. Where
> as, if you could supply the -top command against a rawvideo input you'd
> only need to supply the parameter once.
> 
> I might just be rambling inanely, but in
> libavcodec\rawdec.c:raw_decode(), interlaced_frame and top_field_first
> are set in the AVFrame 'frame' (the second parameter to raw_decode())
> variable by copying from the AVFrame contained in AVCodecContext.
> 
> It looks like the only place
> AVCodecContext->coded_frame->interlaced_frame and
> AVCodecContext->coded_frame->top_field_first are currently set is in
> libavdevice\v4l2.c:v4l2_read_packet().
> 
> So perhaps it might be good to also set those parameters in
> libavformat\rawvideocodec.c:rawvideo_read_packet().
> 
> This, at least, flags rawvideo as tff:

 
> diff --git a/libavformat/rawvideodec.c b/libavformat/rawvideodec.c
> index 127119f..9bdf950 100644
> --- a/libavformat/rawvideodec.c
> +++ b/libavformat/rawvideodec.c
> @@ -39,6 +39,12 @@ static int rawvideo_read_packet(AVFormatContext *s,
> AVPacket *pkt)
>      pkt->dts= pkt->pos / packet_size;
> 
>      pkt->stream_index = 0;
> +
> +    if (s->streams[0]->codec->coded_frame) {
> +        s->streams[0]->codec->coded_frame->interlaced_frame = 1;
> +        s->streams[0]->codec->coded_frame->top_field_first = 1;
> +    }
> +
>      if (ret < 0)
>          return ret;
>      return 0;
> 
> 

> I do not yet understand is how to get the correct value from variable
> top_field_first in ffmpeg.c into that section of code.
> ffmpeg.c:opt_input_file() looks like a good place to start.
> 
> Perhaps, if the input file format is rawvideo and top_field_first is not
> -1 a field could be set and top_field_first set to -1.
> 

> If that is a reasonable idea, AVFormatParameters would be the structure
> to extend.

AVFormatParameters is deprecated and we should get rid of it. Instead,
we could use demuxers/decoders private options when it makes
sense. I'm not yet sure where the option should be set (if in the
demuxer or in the decoder), Michael?

> Then in libavformat\rawdec:ff_raw_read_header() the extended parameters
> from AVFormatParameters could be copied into AVFormatContext...?
> 
> Sorry, my idea sounds like a lot of extending structures, so perhaps
> there is a trick I am missing.
> 
> I'd appreciate your input on this suggestion.
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