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       yadif=[mode[:field_dominance]]
              Yet another deinterlacing filter
                 <mode>
                      0: Output 1 frame for each frame.
                      1: Output 1 frame for each field.
                      2: Like 0 but skips spatial interlacing check.
                      3: Like 1 but skips spatial interlacing check.

So yes you look right.

> 2. The mplayer code has a correct_pts variable that seems to allow
> doubling of frame rate. Here is it safe to assume this is always
> allowed/true?

I suppose so, that will also depend on the mode, from what I can
understand you'll need to duplicate the frame rate only with mode 1/3,
this will be done manipulating the output pts.

> 3. If I add a 'fields' member to AVFilterPicRef struct, how do I
> make sure it will be set to the right value (i.e. TOP_FIRST etc)?

It is a problem of the calling application to set this, for example an
application requests a picref with avfilter_get_video_buffer() and
sets the field accordingly. In the case of ffmpeg.c/ffplay.c this
depends on the options set in the codec context or by the user
(e.g. -top_field 1).
 
> Thanks

Regards.