[FFmpeg-trac] #7242(avfilter:new): zoompan filter variables 'in' and 'on' start at 2, 1 instead of at 0

FFmpeg trac at avcodec.org
Sun Jun 3 11:19:31 EEST 2018

#7242: zoompan filter variables 'in' and 'on' start at 2, 1 instead of at 0
             Reporter:  jdlh        |                    Owner:
                 Type:  defect      |                   Status:  new
             Priority:  normal      |                Component:  avfilter
              Version:  git-master  |               Resolution:
             Keywords:  zoompan     |               Blocked By:
             Blocking:              |  Reproduced by developer:  0
Analyzed by developer:  0           |

Comment (by jdlh):

 Replying to [comment:2 Gyan]:
 > ... the assignment logic of **in** and **on** is correct. They are meant
 to be cardinal values, measuring the amount fed in and fed. The **n** in
 crop, drawtext amd some other filters are ordinal values and represent the
 index which starts from 0.

 Oh, ffmpeg project....

 Where is this cardinal vs ordinal intention recorded? The documentation is
 vague. There's no comment in the code saying that these are to be
 cardinal. I don't see a compelling reason for this filter to be
 inconsistent, in using cardinal numbers where other filters use ordinal

 That said, the ordinal vs cardinal distinction could be viewed as an
 interface commitment with users of the filter. Changing starting numbers
 from 1 to 0 could disrupt some users of the filter. I don't suppose we
 have much of a way of knowing how bad the disruption would be.

 And, cardinal = ordinal + 1, so anyone who wants an ordinal value can
 easily calculate it from a cardinal value.

 Certainly, it would be very desirable to state the ordinal or cardinal
 choice clearly in both the code and the docs.

Ticket URL: <https://trac.ffmpeg.org/ticket/7242#comment:4>
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