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I can think of two ways of doing it. Someone else may be able to think of other ways.
- Import the .mp4 video into krita as an animation and then make an animated layer (or layers) over it.
Depending on the frame size and length, this can use lots of RAM so if possible you may want to cut the video up and import a bit at a time then splice it all together in a video editor.
There’s a plugin for this called Animator Video Reference and it can be downloaded from:
Instructions/explanations and a url to a YouTube video are included in the download.
- You can make an animation in krita (quite a long one if you have enough RAM) then render it out to a set of .png frame images (with transparency) and then import them as a track on your live action video for overlay in a video editor.
This has the disadvantage that you can’t see the live action video as you’re making the animation in krita.