Hi guys, I heard from Bob today that he had integrated my "potrace" into inkscape. That is really great! I checked out the inkscape-devel archives and I noticed that Bob was tinkering with brightness settings to prepare an image for black-and-white tracing.
Did you look at the "mkbitmap" program, which comes with the potrace distribution? It is a pretty good pre-processor for potrace for certain types of images, like cartoons or photographs of handwriting. For some examples, see
http://potrace.sourceforge.net/mkbitmap.html
It does not do too great for Bob's venus example, I have to say. The problem (which is shared by the "canny" edge detector) seems to be that both programs look only at edges in the contrast data, and ignores edges in the hue and saturation. Therefore, there are lots of missing edges in the left part of the image.
-- Peter