#9781: zscale adds distortion when resampling video data
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Reporter: Llyw | Type: defect
Status: new | Priority: normal
Component: avfilter | Version: git-
Keywords: zscale | master
distortion | Blocked By:
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
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I noticed that the zscale filter can introduce distortion (by either
cutting or crushing of the image, I have not been able to pinpoint the
exact type of distortion yet) when resampling video input using
[https://github.com/FFmpeg/FFmpeg/commit/30e2bb0f64 FFmpeg@30e2bb0f64],
but not when using FFmpeg 5.0.1:
Take, for example, the 720p VP9 WebM version of Blender Foundation's
Elephants Dream at
https://upload.wikimedia.org/wikipedia/commons/transcoded/2/28/Elephants_Dream_%282006%29_1080p24.webm/Elephants_Dream_%282006%29_1080p24.webm.720p.vp9.webm.
At roughly 7 minutes, 2 seconds a stills image of this video looks like
this in VLC 3.0.17.4: (See first attachment)
Then, resample to 480x272 pixels with a display aspect ratio of 16:9 and a
sample aspect ratio of 136:135:
{{{
ffmpeg -i 'Elephants_Dream_(2006)_1080p24.webm.720p.vp9.webm' -vf
'zscale=w=480:h=272:f=lanczos,setdar=dar=16/9' -pix_fmt yuv420p
Output_Distorted.mp4
}}}
The corresponding stills image will look like this when resizing the video
port to show a 720p resolution: (See second attachment)
I understand that there is some movement in this scene but if you look at
the background you should clearly see the distortion I was talking about.
Compare this with the same operation done using FFmpeg's `scale` filter:
{{{
ffmpeg -i 'Elephants_Dream_(2006)_1080p24.webm.720p.vp9.webm' -vf
'scale=w=480:h=272:flags=lanczos+accurate_rnd,setdar=dar=16/9' -pix_fmt
yuv420p Output_Undistorted.mp4
}}}
Here, the corresponding stills image looks like this and does not show
significant distortion when comparing with the stills image of the input
video (some minor mismatch is obviously expected due to the strong
resampling and movement in the scene): (See third attachment)
I am assuming that this might have something to do with the recent-ish
efforts to parallelize the `zscale` filter and would appreciate it, if
this behavior is investigated.
--
Ticket URL: <https://trac.ffmpeg.org/ticket/9781>
FFmpeg <https://ffmpeg.org>
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