Thread Counting in X-Rays of Plain-Weave Painting Canvas

Abstract

In this work we present some results on the development and test of software for thread counting in X-rays images of plain-weave canvas. We have reviewed the state of the art in this area to select good and simple approaches to be implemented. Hence, we propose the fast Fourier transform (FFT) as the core of our software. We show how with little modifications of the software the frequency of line patterns in simple twills and zig-zag twills can be also analyzed. The algorithm looks for most probable and mean values within a predefined range of frequency values. The method provides as output a couple of images with the local frequency in every point of the selected area. It also includes the estimated degree deviation from the vertical and horizontal for vertical and horizontal lines (threads in simple-weave) for every point, as images. The histograms of the frequencies found in the X-ray are also displayed. The software was designed following the requirements of the Museo Nacional del Prado Conservation and Research department. It was tested with X-rays authored by 17th century painters Miguel de Pret, Claudio de Lorena, Van Dyck and Rubens. In the analysis of a single image, the results of the software have proved useful in the automatic frequency estimation, detection of cusping and analysis of cut edges or areas. In the two images comparison, the method has been already used to draw conclusions on the canvas coming from the same roll, based on the thread counting. Furthermore, we propose the analysis of the histogram as a useful feature to help in the comparison.