Image Effect with Dark Borders

I was creating an effects library for a PhotoBooth App. I have created effects like Black/White, Vintage, Sepia, Retro etc. etc.

I wanted to create a few effects now in which I wanted to have a Dark Border at the edges which kind of form a frame for the image .. something like this -> Example Effect

How can I do this using Pixel Bender and Flash ?

The effect you are describing is called vignetting. It is basically just darkening the pixels with some weight that changes depending on distance from the center of the image. In image editing it corresponds to overlaying the image with black color and applying a circular or elliptic mask to it, for example:

_{(source: johnhpanos.com)}

You can do this by several methods depending on how you operate with image and its pixels. For example by multiplying the pixels by a weight coefficient that is smaller when closer to the center and bigger when farther away from it. The distance can be calculated from the difference between pixel coordinates.