For my part, the two key issues are whether the camera is moving (tripod? handheld? locked down?) and how busy the space behind the windsurfer is. If there’s a lot of camera movement, or if there are a bunch of other surfers behind the to-be-slowed-down surfer, then the shot will be more complicated to pull off….
The basic process:
1. Rotoscope the windsurfer. If s/he’s against a plain blue sky, then the roto can be rough. If there are detailed things that intersect with the edges of the windsurfer, then you’ll need a much tighter matte.
2. Slow down the windsurfer. You’d probably want to use the Timewarp effect (included with AE) or Twixtor (a third-party plugin). You may need/want to render out the rotoscoped surfer before doing this, or at least precomp it so the rotoscoping is “applied” before the time manipulation filters go into effect.
3. Remove the “real-time” windsurfer from the background. MochaPro (different from MochaAE) can do this almost automatically in certain shots. The clone-stamp tool in AE might help. It all depends on what the footage is doing, but the idea is that you probably don’t want to see the real-time surfer behind the slowed-down surfer.
Note that if your camera isn’t locked down, then you’ll need to do a couple extra steps. First, you need to stabilize the camera movement on the slowed-down surfer, so it seems like it was a static shot. Second, you’d need to reintroduce the real-time camera movement to the slowed-down surfer. This is tricky, but frankly it would do a ton to help sell the shot, make it convincing, etc. Good luck!
Ben Unguren
Motion Graphics & Editing
http://www.mostlydocumentary.com
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