Rotomasking in Adobe After Effects 4

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Recently I found myself with a flattened .mov file of some planet rotation and no access to my PaperVision3D class files to re-publish the movie. With a sudden need to add motion tracking and particle effects to the movie, I turn to AE4 and a technique called rotomasking.

There’s no real wiki for Rotomasking. It is something some people call tediously masking a bunch of frames – if not all the frames in your source, which is sometimes necessary for rotoscoping or motion tracking. You probably have heard of junk masking which is loosely masking out unwanted areas of video usually when there are issues with keying.

Rotomasking in Adobe After Effects 4
I’m not really going to a step-by-step tutorial on this one. Instead I will let the video do the talking but will point out some key frames as to what is going on in the video.

In my case I have some revolving planets that go behind and in front of one another. I want to add some effects and motion tracking to the Sun and I want to add some effects and motion tracking to Mercury when it goes in front of the Sun. In order to do this with a flattened movie, I will need to duplicate the video onto itself, and mask Mercury in front of the Sun out frame by frame so that I can add effects behind – and on top of it.

I will not cover the motion tracking or particle effects on the Sun here, but instead just the rotomasking of Mercury that passes in front of the sun. This is key to isolating video.

Other Practicle Uses:

Off camera prep work:

  1. I have created a new composition by dragging the .mov into the composition icon under the Project window. I duplicate the .mov into a new layer and split the video where Mecury comes in front of the Sun.
  2. Using the pen tool I directly path onto the video on the top layer (Mercury). I start with rough masks in general key frames and then tweak smaller areas as needed.
  3. I activate the keyframe clock on the Mask Path Layer so that any changes to the path become Keyframes.

Move between keyframes with the arrow tools next to the keyframe indicator to the left of the clock. Make sure not to deselect your video source when masking with the pen tool.

Highlights of the Video Demo

  1. Key frames are shown showing each change in the mask.
  2. Edit a point by using the pen tool or by holding the command key at any time
  3. Be carefull not to deselect your mask while working.
  4. Hit ` (tilde) key to hide all other windows.
  5. Use command – option – shift to zoom from any tool, just like Adobe Illustrator.
  6. Hold spacebar down to move the window around while another tool is selected.
  7. Hold command key down to edit individual points or to pull handles to smooth out arcs.

Adding Effects

Once you have your movies split and Mercury is on top of the Sun, you can motion track effects on the appropriate layers. Duplicate the layers if you want to add additional effects to the planets as well.

After Effects CS4 is a powerful tool to have in your back pocket as an animator. For a Flash animator that likes to mess around in Papervision3D for animation purposes, AE can add some powerful effects in a hurry. It is well worth it to take the time to get to know this application and some of the tricks of the trade that come with it.

Hope this helps. Feedback and comments are appreciated. See you next time.

Posted on June 22, 2010 at 5:33 pm by Runtime · Permalink
In: After Effects, CS4, animation · Tagged with: , , , ,

One Response

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  1. Written by vink
    on July 6, 2010 at 11:28 pm

    That actually made it sound easy. still not ready for AE though…

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