There are 3 basic methods for creating outlines around alpha channels and other hi contrast images:
A Well-planned Gradient Map
Simulating 2D Metaball Blobbies with Photoshop
The Exclusion Blend Mode
Exclusion Photoshop Blend Mode Killer App
The Multiply Blend Mode
(illustrated in detail below)
Sometimes it's handy to turn black and white images into smooth outlines. Here's a little recipe for doing just that.
Introduce some grey into the black and white image. One easy way to do this is to blur the image a bit. The bigger the blur, the broader the outline will be.
Make a copy of the original image and invert it.
When the two images are multiplied, wherever either image is black the resulting product is black. Wherever either image is grey, the other image is by necessity also grey, and the product of the multiplication is a dark grey.
If black is represented by an intensity of zero, and white is represented by an intensity of one, then the middle of a blur in a black and white image is a .5 grey.
The product of a .5 grey with itself is a .25 grey. To make a .25 grey look white, you need to brighten it by 400%
Here's the "outline" recipe in action. Black and white 3D turbulence wipes this movie set from "normal" to "icy." All of the gray areas in the wipe have been extracted using the recipe described above and turned electric blue.