Just test for either attribute number, or RGB differences...

If Pixel A is significantly different in RGB value from Pixel B, then you have an edge. It doesn't really matter which is "foreground" and which is "background", only that the edge be rendered in a neutral shade.

I'm not sure how this code will be applied, however. If it's meant to be applied to many images, rather than a specific image you're familiar with, then it becomes tricky. Also, it depends on whether the colors are relatively uniform, or varied as in a bitmap. All of my antialasing experience has been "hands on". I've never written code to accomplish it automatically.

-Bob