This Disney Research study on style and abstraction in portrait sketching is fascinating:
We use a data-driven approach to study both style and abstraction in sketching of a human face portrait. We gather and analyze data from a number of artists that sketch a human face from a reference photograph. To achieve different levels of abstraction in the sketches, decreasing time limits were imposed – from 4:5 minutes to 15 seconds. We analyzed the data at two levels: strokes and geometric shape. In each, we create a model that captures both the style of the different artists and the process of abstraction. These models are then used for a portrait sketch synthesis application. Starting from a novel face photograph, we can synthesize a sketch in the various artistic styles and in different levels of abstraction.