how to make slow motion only in a part of video

If you planned it out ahead of time (greenscreens, shot correctly) you could create 2 layers of the video, time remap it and just mask the area you want in slow motion.

If it's not shot well though, you will have very visible seams between the the stuff moving at regular speed and the slow motion, it will look like 2 different shots composited on top of each other.

Johnny Cuevas, Editor
Thinkck.com

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