[vlc-devel] [PATCH 1/9] video_format: add the number of fields per picture in the video format

[Steve Lhomme]

On 2020-11-16 15:38, Romain Vimont wrote:
> On Mon, Nov 16, 2020 at 01:44:12PM +0100, Steve Lhomme wrote:
>> In general it's 2 fields. If the number of fields changes in the source, the
>> (deinterlacing) filters should be updated.
>>
>> By default the video format is progressive and the i_num_fields can be ignored.
>> It should be set to 2 for progressive to be consistent with how the value was
>> set so far.
>>
>> This field is more stable than the picture i_nb_fields that also included
>> repeated fields and pictures. It only contains the information about the
>> number of fields in each actual picture.
>> ---
>>   include/vlc_es.h                      | 3 +++
>>   modules/visualization/visual/visual.c | 1 +
>>   2 files changed, 4 insertions(+)
>>
>> diff --git a/include/vlc_es.h b/include/vlc_es.h
>> index 404a937d833..459d16b3c37 100644
>> --- a/include/vlc_es.h
>> +++ b/include/vlc_es.h
>> @@ -376,6 +376,8 @@ struct video_format_t
>>       video_multiview_mode_t multiview_mode;        /** Multiview mode, 2D, 3D */
>>       bool b_multiview_right_eye_first;   /** Multiview left or right eye first*/
>>   
>> +    unsigned int    i_num_fields;          /**< number of fields per picture */
> 
> The number of fields may vary per picture:
> https://code.videolan.org/videolan/vlc/-/blob/a708c988091d6bc770212f5deeaba45e2395f4b2/modules/video_filter/deinterlace/deinterlace.c#L223-224

This is not what it means. In a picture buffer you never have 3 fields 
(3 heights of a field). You either have 1, 2, 2 plus the first one is 
supposed to be repeated (telecine). That's what the "avcodec: repeat the 
pictures marked as repeating" patch assumes.

In telecine the field first is supposed to flip with every picture that 
has the repeat flag because of that. One picture has TopBottom(+repeat 
Top) and the next picture will have BottomTop. That's why the field 
order is per picture and not in the video format.

(I have yet to find a sample where this is properly coded)

The field I propose above is the number of field in the picture buffer. 
The picture_t can still have more. But that's not going to change the 
deinterlacing mode like this video format field.

> So I think it does not make sense to expose it in video_format_t, and
> the deinterlace/splitfields information should be encoded differently.
> 
> Regards
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