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

[Mark Himsley]

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.

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.

Thanks.

-- 
Mark