[FFmpeg-trac] #5745(avfilter:new): The ‘fieldmatch’ filter sometimes fails to mark combed frames as interlaced.

[FFmpeg]

#5745: The ‘fieldmatch’ filter sometimes fails to mark combed frames as
interlaced.
-------------------------------------+-------------------------------------
             Reporter:               |                    Owner:
  alexander_behringer                |                   Status:  new
                 Type:  defect       |                Component:  avfilter
             Priority:  normal       |               Resolution:
              Version:  git-master   |               Blocked By:
             Keywords:  fieldmatch   |  Reproduced by developer:  0
             Blocking:               |
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------

Comment (by alexander_behringer):

 After I came across this issue, I experimented a bit and read the filter
 source code (`libavfilter/vf_fieldmatch.c`) and found the described
 problem of the missing combing score calculation there.

 The problem is independent of any heuristics of the field matcher (with
 the exception of the scene change detection, which is intentionally not
 triggered by my test video) since it is about the comb detection at the
 end of the filter (near the end of the `filter_frame` function). Its an
 program logic issue, not a random or heuristics problem.

 My example is constructed to trigger and thus show and help debugging the
 problem. But if this is not enough, you can use any truly interlaced video
 as input. With truly interlaced I mean a video whose fields are not
 constructed from progressive material (via telecine for example). If you
 work with (de-)interlacing filters, you should have some test video
 yourself.

 With such a video the following will produce combed frames, although it
 should not:

 {{{
 ffplay some_truly_interlaced_source -vf
 'fieldmatch,w3fdif=deint=interlaced'
 }}}

 But the following will produce deinterlaced content:

 {{{
 ffplay some_truly_interlaced_source -vf
 'fieldmatch=combdbg=pcn,w3fdif=deint=interlaced'
 }}}

 Since I have nearly no experience in programming in C or working with the
 FFmpeg libaries, I will refrain from proposing a patch. I just wanted to
 document the existence of the problem.

--
Ticket URL: <https://trac.ffmpeg.org/ticket/5745#comment:3>
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