Additional paper
I am not sure if you have seen [KidPen: a stroke-based method for kid-style sketches synthesis from photos](https://dl.acm.org/citation.cfm?id=3145749.3149434), but I think it takes a good approach to the sketch generation problem with impressive results in terms of shape abstraction. The one caveat is that it relies on human interaction for the segmentation portion of the task, but it is still a good read. 