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isolated slow motion effect

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john skye
isolated slow motion effect
on Sep 29, 2011 at 11:02:26 am

I am trying to create an effect where part of the action in a shot goes to slow motion, whilst the rest/background continues at normal speed. I have a shot of a windsurfer jumping, with lots of other sailors in the background. I would like to isolate the jumper, slow it down, and have the rest of the world sailing normally in the background.

I am sure its covered here somewhere, but I am not too clued up on all the gargon, so not even sure what to search for. If anyone can let me know what the effect would be called, or point me in a good direction it would be very much appreciated.

thanks in advance

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ben g unguren
Re: isolated slow motion effect
on Sep 29, 2011 at 2:03:42 pm

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

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john skye
Re: isolated slow motion effect
on Sep 29, 2011 at 4:34:44 pm

Thanks so much. That is perfect. Not sure I will be able to pull it off, but at least i know the process!

The shot is a tripod shot and I tired not to pan at all, so the background is a constant.

If I can do it, I will post a link to show you the outcome.

Thanks a lot


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john skye
Re: isolated slow motion effect
on Oct 19, 2011 at 9:25:46 pm

Thanks for the reply.

I tried it and had reasonable success. Rotoscoping the windsurfer was relatively easy, but the problem came when trying to take the real time windsurfer out. I used the clone tool, but as it was water and waves it was constantly changing and therefore made it very difficult to look right. I also made the mistake of not stabilising the shot first. It was a fixed camera on a tripod, but the subtle shakey movement was enough to mess a few things up.

Overall however the technique was perfect, but the initial shot was not the right one to use. I will try again form the beginning and aim to shoot the action lower and hopefully get the windsurfer above the water and make it easier to erase them from the shot.

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