Fast face sketch synthesis via KD-Tree search

Abstract

Automatic face sketch synthesis has been widely applied in digital entertainment and law enforcement. Currently, most sketch synthesis algorithms focus on generating face portrait of good quality, but ignoring the time consumption. Existing methods have large time complexity due to dense computation of patch matching in the neighbor selection process. In this paper, we propose a simple yet effective fast face sketch synthesis method based on K dimensional-Tree (KD-Tree). The proposed method employs the idea of divide-and-conquer (i.e. piece-wise linear) to learn the complex nonlinear mapping between facial photos and sketches. In the training phase, all the training images are divided into regions and every region is divided into some small patches, then KD-Tree is built up among training photo patches in each region. In the test phase, the test photo is first divided into some patches as the same way in the training phase. KD-Tree search is conducted for K nearest neighbor selection by matching the test photo patches in each region against the constructed KD-Tree of training photo patches in the same region. The KD-Tree process builds index structure which greatly reduces the time consumption for neighbor selection. Compared with synthesis methods using classical greedy search strategy (i.e. KNN), the proposed method is much less time consuming but with comparable synthesis performance. Experiments on the public CUHK face sketch (CUFS) database illustrate the effectiveness of the proposed method. In addition, the proposed neighbor selection strategy can be further extended to other synthesis algorithms.