Home / Articles / Adobe After Effects / Using the Roto Brush tool in Adobe After Effects CS5

Using the Roto Brush tool in Adobe After Effects CS5

Article Description

This excerpt from Adobe After Effects CS5 Classroom in a Book shows you how to use the new Roto Brush tool in After Effects CS5, which is much faster than conventional rotoscoping, and for movies with complex backgrounds, much easier than keying.

Fine-tuning the matte

Roto Brush does a pretty good job, but there may be stray bits of background in the matte, or foreground areas that didn’t make it into the matte. You’ll clean those up, and then refine the edge.

Touching up the matte

Depending on your clip, there may be some areas of the foreground subject that the Roto Brush tool cannot separate from the background. You can touch up your matte by painting directly on the alpha channel.

  1. Select the Brush tool (brush.jpg) in the Tools panel.
  2. In the Paint panel, select the following settings:
    • Opacity: 100%
    • Channels: Alpha
    • Duration: Single Frame
  3. In the Brushes panel, select a brush size appropriate for the area you’re touching up. For a defined edge, a Hardness value of 100% is usually best.
  4. Step through each frame of the animation, and remove any areas of the background that remain. If you need to add to the foreground, switch the foreground and background colors. Painting in black adds to the matte; painting in white removes from it.

Refining the matte

When you use the Roto Brush tool, After Effects applies the Roto Brush effect to the layer. You can modify the effect using settings in the Effect Controls panel. You’ll use those settings to further refine the edge of the matte.

As you’ve moved through the clip, you’ve probably noticed that the segmentation boundary seems a bit jittery. You’ll use the Reduce Chatter setting to make it smoother.

  1. Click the Effect Controls tab, if the Effect Controls panel isn’t active.
  2. Click the box next to Refine Matte to select it.
  3. Increase the Reduce Chatter amount to 75%.

    The Reduce Chatter value determines how much influence the current frame has when performing a weighted average across adjacent frames.

  4. Increase the Smooth value to 3.0.
  5. Increase the Feather amount to 40% and the Choke amount to 30%.

    Feathering the edge of a matte helps to blend it with the rest of the composition.

  6. Save your work so far.
5. Creating a transition from the full clip to the foreground | Next Section Previous Section