Edge detection of Reflectance Transformation Imaging for colorless traces

Reflectance Transformation Imaging (RTI) is a computational photography technology that can enhance the legibility and the fine details of the sample. Colorless trace is a special kind of trace with shallow depth and its grayscale is similar to the background. It is difficult to perceive details of these traces by traditional observation and detection methods. In this paper, we propose some improved detection methods which combine RTI with the Canny algorithm to get more details of colorless trace. The threshold is one of the key factors which affect the performance of the Canny algorithm so we put forward a threshold selection method based on the gradient histogram of the image. Three detection methods that combine RTI with the improved Canny algorithm are proposed in this paper. The experimental results show that these detection methods can extract weak and colorless traces effectively, and would have great potential application in criminal investigation.