I have an image with an alpha channel that I am using for a texture. Right now I just use its alpha channel (that's the only thing checked in the influence panel).
I'd like to give it a "halo"/border. Consider the following image where I take the outline of japan (from http://en.wikipedia.org/wiki/Template:Japan_football_clubs_map_2011), and give it a black border (this uses an intermediate image from the compositor of a separate scene, very awkward):
My first instinct is to create a second plane underneath with a second version of the texture where I dilate (add pixels around the edge, ⟏ for those of you familiar with Image Algebra) the alpha channel for a dark material. Unfortunately, I can't seem to find a node which performs dilation in the texture Node Editor.
I welcome suggestions of how to achieve this effect using blender. (I am using BI, but if you want to answer with a screenshot of a node setup for Cycles I won't complain).