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

[Mark Himsley]

On 16/05/2011 00:10, Michael Niedermayer wrote:
> On Mon, May 16, 2011 at 12:08:24AM +0200, Stefano Sabatini wrote:
>> 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.

Without it, I don't see how ffmpeg.c can pass the width, height, pix_fmt 
etc etc to raw decoders. There is even a comment that says that 
AVFormatParameters is "Only used in raw format right now".

>> 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?
>
> I dont think theres a single perfect awnser here
>
> demuxer private options would make it only available for one demuxer
> there can be rawvideo in many containers.

I have assumed that containers other than rawvideo can flag their 
streams as interlaced already.

> a rawvideo decoder private option might work and looks simple.
> it wont be available during find_stream_info() though but this seems
> a non issue.

Please review the attached patch (although I'm sure I'll have missed a 
version bump of something more blindingly obvious ;-)

-- 
Mark
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