https://bugs.kde.org/show_bug.cgi?id=476659
Bug ID: 476659
Summary: Rendering a Reversed Clip is Extremely Slow
Classification: Applications
Product: kdenlive
Version: 22.08.2
Platform: Microsoft Windows
OS: Microsoft Windows
Status: REPORTED
Severity: normal
Priority: NOR
Component: Video Display & Export
Assignee: j...@kdenlive.org
Reporter: bugs....@boffinbrain.com
Target Milestone: ---
SUMMARY
When rendering a video that has a reversed clip, the reversed portion of the
video takes an extremely long time. I understand that most codecs are designed
under the assumption that videos are played in a forward direction and thus
i-frames depend on the preceding frames, thus a lot of seeking is necessary if
frames are requested in reverse order. However, in my testing I found that
rendering regular video could be around 30fps while the reversed section would
be approximately 0.5fps. Surely there are some optimizations we can make to
this process, and perhaps they can be applied to within the editor too for
faster playback.
STEPS TO REPRODUCE
1. Add a clip to the timeline (recommended at least 5 seconds long) that's an
MP4 video or similar.
2. Make a copy of it, place it after the first clip and reverse the clip.
3. Start rendering the project.
OBSERVED RESULT
Rendering the first half of the video is fast, while the second half is
*extremely* slow.
EXPECTED RESULT
Speed for rendering the second half should ideally be within the same order of
magnitude as the first half.
SOFTWARE/OS VERSIONS
Tested on Kdenlive 22.08.1 and 22.08.2
Windows 10
KDE Frameworks Version 5.110.0
Qt Version 5.15.10 (built against 5.15.10)
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