[FFmpeg-trac] #11438(avfilter:new): vf_crop filter breaks setting variables like x=x+1

[FFmpeg]

#11438: vf_crop filter breaks setting variables like x=x+1
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             Reporter:  Alan Amaral  |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  avfilter     |                  Version:  6.1.1
             Keywords:  vf_crop      |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 Summary of the bug:
 If you create a filter graph with a crop filter, and zmq enabled, you
 can't set any of the values, x, y as recursive expressions, like x=x+1.
 In the overlay filter you can set x=x+1 and for each frame the value of x
 is increased by 1, and automatically pans the video, for example.  This
 does not work with the crop filter.  Setting x=10 works, as does setting
 x=10+10 but setting x=x+10 doesn't work and breaks things.  In fact, as
 far as I can tell setting x or y to any expression involving x or y, like
 x=x or y=y, which should leave the positions unchanged, doesn't give the
 expected results.  Setting x=x or y=y seems to set the x,y position to the
 center of the input area.  This looks intentional given this code from
 vf_crop.c:

 {"x", "set the x crop area expression", OFFSET(x_expr),
 AV_OPT_TYPE_STRING, {.str = "(in_w-out_w)/2"}, 0, 0, TFLAGS },
 {"y", "set the y crop area expression", OFFSET(y_expr),
 AV_OPT_TYPE_STRING, {.str = "(in_h-out_h)/2"}, 0, 0, TFLAGS },

 but seems to make absolutely no sense, given that other filters (such as
 vf_overlay) allow recursive variable expressions, like x=x+1, which cause
 the x value to increase by 1 for each frame, and x=x causes the motion to
 stop.


 How to reproduce:
 Build an ffmpeg command with a crop filter and try to control the x and y
 position using zmqsend.
 Patches should be submitted to the ffmpeg-devel mailing list and not this
 bug tracker.
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Ticket URL: <https://trac.ffmpeg.org/ticket/11438>
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