Sketch synthesis is trying to simulate the hand drawing style of artist on generating new images automatically. Existing methods like PhotoShop produce sketch effect according to image edges, which is suitable for normal images. However these methods commonly fail to capture important facial details, which would be sensitive to human beings. So the generated sketches are more like the input images rather than artist work. To solve this problem a training dataset is involved which contains photo-sketch pairs. The output sketch is reconstructed from training sketches through mapping functions learned from input and training photos.
Reference: Yibing Song, Linchao Bao, Qingxiong Yang, Ming-Hsuan Yang,
Real-Time Exemplar-Based Face Sketch Synthesis,
European Conference on Computer Vision (ECCV) 2014.