One of the more subtle tricks used in compositing is the lightwrap, where you create an edge blend effect between your foreground and background elements. This gives the illusion that bright light-sources in the background are bleeding around the foreground subject. In Motion, the key to building a lightwrap is through the use of image masks.
Get the media
Download the two tutorial image files: lightwrap.zip (1.2MB). The sunset image, 'Red Sunset', is courtesy of Jeanette K. Shribbs and is provided under a Creative Commons license 
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We will be compositing a greenscreen hand shot over a Costa Rican beach scene. It will be necessary to key and color-correct the hand before we create our lightwrap.
Import and key the hand
- In the Utility Panel, click on the 'File Browser' tab.
- Navigate to the location of the downloaded greenscreen image (Hand.png) and drag it into the Canvas.
- With the hand image selected, go to the 'Add Filter' toolbar menu and choose Keying->Primatte RT.
Primatte correctly guessed the background color of our shot (green) and got us part of the way to a smooth key, but there are still some adjustments to be made. To get a better idea of the keying results, let's take a look at the alpha.
View the alpha channel
- Press Shift+A (or select 'Alpha' from the Color menu in the toolbar).
Some nasty stair-stepping is visible in the matte, as well as some loss of hair detail. We will fine-tune the Primatte filter to correct the problems.
Adjust the Primatte filter
- With the hand image selected, view the 'Filters' tab of the Inspector.
- In the Primatte RT filter settings, change the Noise Removal value to 0.10.
- Change the Matte Density value to 0.37.
- Change the Spill Supression value to 0.16.
Our matte is looking much better: the jagged areas have been smoothed and the hair detail mostly restored. Let's switch back to the color view.
View the color
- Press Shift+C (or select 'Color' from the Color menu in the toolbar).
As you can see, our changes caused a lot of green spill to appear in the hair and edge areas. Primatte's built-in spill suppressor can sometimes be a little heavy-handed, so we'll use the Spill Suppressor filter to correct our image.
Remove the spill
- With the hand image selected, go to the 'Add Filter' toolbar menu and choose Keying->Spill Suppressor.
- In the 'Filters' tab of the Inspector, set the Spill type to 'Green'.
- Set the Amount to 0.50.
The spill has been cleaned up from the edges and the hair is, once again, brown. With our foreground element completed, let's move on to our composite.