#10987: Blend video filter *_expr options don't handle store and load functions
correctly.
-------------------------------------+-------------------------------------
Reporter: Fallan | Type:
| enhancement
Status: new | Priority: normal
Component: avutil | Version: git-
Keywords: "blend | master
filter" "store and load" st() | Blocked By:
ld() |
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
-------------------------------------+-------------------------------------
For example make a local gradient over normalized height 0.2 starting at
0.5, which will be used to transition between 2 input videos. E.G. the
inputs could be Big Buck Bunny variations, the first having been self-
modified by blend-glow and the second by blend-burn:
ffmpeg -i B3.mov -vf "split[F][G];[F][G]blend=all_mode=glow" -c:v ffv1 -an
glow.mkv
and similarly burn.mkv with blend=c0_mode=burn. Now I should be able to
do
ffmpeg -i burn.mkv -i glow.mkv -filter_complex
"[0:v][1:v]blend=all_expr=st(0,clip((Y/H-0.5)/0.2\,0.0\,1.0))\;A*ld(0)+B*(1.0-ld(0))"
-c:v libx265 -crf 16 -preset slow -an output.mkv
But the result has random snow obscuring the image. It works fine if I
omit st() and replace the 2 ld()'s with clip()'s but that takes longer to
run.
I am using ffmpeg version "2024-04-25-git-cae0f2bc55-full_build-
www.gyan.dev" and it happens in older versions too.
I suspect this is known behavior which has been described in other context
as "stored variables not having global scope" and they don't transmit
between color planes but here we have the same color plane and frame, just
different streams that are already in the same scope in avfilter. It sure
would be beneficial if this worked.
--
Ticket URL: <https://trac.ffmpeg.org/ticket/10987>
FFmpeg <https://ffmpeg.org>
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