Last year, Edgar Simo-Serra, Satoshi Iizuka, Hiroshi Ishikawa conducted a research at the Waseda University (a Japanese private research university in Shinjuku, Tokyo) about training a computer to turn pencil sketches into cleaned line-art. The researcher used the hi-resolution scan pencil sketches you can find in the sources of the Pepper&Carrot website (published under Creative Commons Attribution) to conduct tests and show visual results of their experiments. You can find many example illustrations of "before" and "after" in the paper. I was flattered that my artworks were used as examples in this well-made study; Creative Commons licenses FTW! A good reason to share that document with the Pepper&Carrot community!
You can read the full paper as a PDF at this address: https://arxiv.org/pdf/1703.08966.pdf