[FFmpeg-trac] #11610(swscale:new): Split automatically when input needs a consumed output (was: scale with reference prevents video to be scaled with a second scale filter instance)

[FFmpeg]

#11610: Split automatically when input needs a consumed output
-------------------------------------+-------------------------------------
             Reporter:  Alexander    |                    Owner:  (none)
                 Type:  enhancement  |                   Status:  new
             Priority:  normal       |                Component:  swscale
              Version:  7.1          |               Resolution:
             Keywords:  scale        |               Blocked By:
  scale2ref                          |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
Changes (by Alexander):

 * summary:
     scale with reference prevents video to be scaled with a second scale
     filter instance
     => Split automatically when input needs a consumed output
 * type:  defect => enhancement


Old description:

> == Summary of the bug:
>
> A user needs to overlay a logo which is scaled using a reference video as
> input. The reference video should be also scaled in the same command.
>
> **Result:** Scale filter fails to scale a video that is used as input for
> the reference.
>
> **Tests:** Logo scaled without reference video does not prevent video to
> be scaled (the video is scaled, logo scaled, logo overlayed).
>
> == How to reproduce:
> {{{
> % ffmpeg -y -t 10 -i 'input.mp4' -loop 1 -t 13 -i logo.png
> -filter_complex "\
> \
>  [0:v]scale=-2:720[prepaired]; \
>  [1:v][prepaired]scale=-1:rw/8[logo_scaled]; \
>  [prepaired][logo_scaled]overlay=10:10" -c:v libx264 -preset fast
> output.mp4
>
> ffmpeg version N-119657-gf1b3e51950 [2025-05-22] [7.1.git]
>
> built with gcc 12 (Debian 12.2.0-14+deb12u1)
> }}}
> Patches should be submitted to the ffmpeg-devel mailing list and not this
> bug tracker.

New description:

 == Summary on UX:

 A user needs to overlay a logo which is scaled using a reference video as
 input. The reference video should be also scaled in the same command.

 Scale filter fails to scale a video that is used as input for the
 reference. No error showed, so the user does not have a clue for
 debugging.

 == How to reproduce:
 {{{
  [0:v]scale=-2:720[prepaired]; \
  [1:v][prepaired]scale=-1:rw/8[logo_scaled]; \
  [prepaired][logo_scaled]overlay=10:10

 ffmpeg version N-119657-gf1b3e51950 [2025-05-22] [7.1.git]

 built with gcc 12 (Debian 12.2.0-14+deb12u1)
 }}}

 The working example:
 {{{
  [0:v]scale=-2:720,split=2[base][ref]; \
  [1:v][ref]scale=-1:rw/8[logo_scaled]; \
  [base][logo_scaled]overlay=10:10"
 }}}

 While showing an error would be a great help for a user, splitting the
 output automatically could highly improve user experience.

--
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Ticket URL: <https://trac.ffmpeg.org/ticket/11610#comment:3>
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