Currently I'm working on converting 30+ artworks to a colouring book, extracting the lines as best as I can and vectorizing them. Both my gf and I work as graphic designers and neither of us have come up with a solution that gives satisfying results. Of course having to clean-up manually is unavoidable, which is what we have done so far, but perhaps there are some techniques that we overlooked, some time savers. I don't need each and every line, but it does need to be quite detailed also because the artist likes to decide on what to leave out.
And here is an example of one I did before:
Techniques I used: increase contrast and reduce colours in various ways: convert to CMYK then take out CMY, index colours, posterize, adjust levels de-saturate, duplicate as a colour dodge layer (fairly effective) and gimp cartoon effect was also pretty good. Nevertheless after extracting the outline a lot of handywork remained, the example took me 6 hours before I could trace a satisfyingly smooth vector. The artist is very specific about the way his lines should look. He wants to preserve his trademark brush strokes. Unfortunately he doesn't exclusively use black for the outlines, they can be dark green, brown etcetera. and his paintings are not as 'clean' as the colouring book outlines need to be.
If there is any technique the saves me an hour of work, times 30 that's a lot of hours saved. Any suggestions please drop them in the comments below, I'll give it all a try.
(I cant drop anything high resolution here but I do have high-res tiffs myself)
As requested, another more high resolution example to play with. (PNG went over the 2MB max, I hope this is good enough for you?)