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# Counter-rotating circles

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 Counter-rotating circles on Sep 8, 2010 at 5:56:05 pm

Hi all,

My first question as I learn to use this software.

GOAL: To superimpose two counter-rotating semi-circles of the same diameter on a video.

PROBLEM: I want the rotation to be 'infinite'.

Yes, that's not clear, but it summarizes what I want to do. Here's the long version:

I'm trying to create an effect of optical expansion/contraction. For that, imagine having to semi-circumferences (unfilled circles) that are dashed (to allow perceiving their rotation). Now imagine them forming a perfect circle, but independent to rotate clockwise (CW) or counter-clockwise(CCW). Now, spin one CCW, the other CW. Now, the problem is that on one end they would diverge away from each other, and on the other they would overlap each other. This is not desired. The idea is that their continuous rotation is restricted to a single half of the circle. That is, you can see it as a 'magic' dashed circle, which in one half exhibits an infinite motion CW and the other half CCW.

APPROACH: So I have to this on top of a video so here is where I am:

1. Split the video into two halves in AE (2 layers).
2. Create two dashed circle (images) in illustrator.
3. Order the layers:
- Video (bottom-half)
- Dashed circle #1 (rotates CW)
- Video (top-half)
- Dashed circle #2 (rotates CCW)
4. Create a mask for Dashed circle #2

The layer ordering makes the circle #1 do right thing. The problem is that for circle #2, the mask (step 4.) rotates with the layer it's in creating an undesired effect.

Any solution to this problem would be appreciated.

-AV

 Re: Counter-rotating circleson Sep 8, 2010 at 6:40:11 pm

Create 1 dashed line circle shape in Illustrator- bring in AE and animate a 360 degree rotation (render out without last frame for a seamless loop) . Render that out with alpha. Bring in the rendered clip and check the loop option on import (give it a really high number). you have now a rotating circle that loops for the length of your project. Bring in comp this footage and draw a simple rectangular mask to reveal only half of circle. Duplicate layer. Invert mask. Change speed to -100%. You have the second half of the circle rotating in the other direction.

Tudor "Ted" Jelescu
Senior Compositor/VFX Artist
Bucharest, Romania

 Re: Counter-rotating circleson Sep 9, 2010 at 8:35:40 pm

Awesome! Thanks for the idea, it worked great.

-AV

-AV