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

[Mark Himsley]

On 16/05/2011 20:55, Michael Niedermayer wrote:
> On Mon, May 16, 2011 at 05:55:13PM +0100, Mark Himsley wrote:
>> On 16/05/11 15:22, Michael Niedermayer wrote:
>>> On Mon, May 16, 2011 at 11:45:15AM +0100, Mark Himsley wrote:
>>>> 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:
>> [...]
>>>>>>
>>>>>>> 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".
>>>
>>> there are many options that are passed to the decoders, like for
>>> example idct type.
>>>
>>> Please elaborate on where the problem is
>>
>> The problem is my ignorance.
>>
>> I want to flag a rawvideo input file as interlaced and of a specific  
>> field order.
>>
>> I therefore think that the appropriate way to do that is to set the  
>> command line parameter "-top [0|1]" before an input file of format 
>> rawvideo.
>>
>> Therefore I want to get the value of the variable top_field_first from  
>> ffmpeg.c into demuxer/decoder. As Stefano pointed out, value of  
>> top_field_first is not used anywhere except for in  
>> ffmpeg.c:do_video_out(), so putting it into AVFormatParameters to be  
>> passed to the demuxer/decoder seemed the logical thing to do.
>>
>>>>>> 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.
>>>
>>> i dont think many can do that currently.
>>>
>>>
>>> also a writing into coded_frame of a decoder from a demuxer is very
>>> hasckish. there could be no decoder at all or 2 decoders.
>>> These might be unlikely enough for it to work out in practice but
>>> its not a pretty way to solve it
>>
>> Ok, I understand that. My excuse is that I was just following how  
>> libavformat/yuv4mpeg.c:yuv4_read_header() and  
>> libavdevice/v4l2.c:v4l2_read_header() both do it.
>>
>> As I said, my ignorance is preventing me from seeing a less 'hackish'  
>> solution.
> 
> Ive implemented it locally, will push in a moment.
> Please review my changes and comments welcome.

Thanks Michael. That has explained a lot more about the way ffmpeg works
too.

I have one concern, given this psudo command:

ffmpeg -top 1 -i input -vf fieldorder=bff -vcodec dvvideo -flags +ilme
output

ffmpeg.c:do_video_out() will be setting big_picture.top_field_first to
1, where as the fieldorder filter has converted the tff input file to
bff. That was why I was resetting ffmpeg.c:top_field_first to -1 in
ffmpeg.c:opt_input_file.

I can see that my solution was not correct (the set to -1 should only
happen for input video files) but I wonder what you think of my concern?

Thanks again, btw! I really do appreciate you taking the time to look at
this issue.

-- 
Mark