Still haven’t quite got the hang of the MagickWand/MagickCore stuff so I’ve been fiddling on the command line so I can get the best sequence of operators together. I’ve found one. A few actually. If I smooth the image (morphology open -> close) it obliterates numbers writen by hand but preserves the printed ones. Which is amusing. This however is great, it’s a skeletonisation, repeated thinning until no more can be done:
Morphological Skeletonisation
If you look closely you can still make it all out. 😛