<p><b>a</b> Examples of full color natural images extracted from the database <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0069154#pone.0069154-Olmos1" target="_blank">[23]</a>, available at: <a href="http://tabby.vision.mcgill.ca/html/browsedownload.html" target="_blank">http://tabby.vision.mcgill.ca/html/browsedownload.html</a>. <b>b</b> Digitized versions of images in (a). <b>c</b> Sketches obtained from the images in (b), by using the optimal pattern set of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0069154#pone-0069154-g003" target="_blank">fig. 3b</a>.</p
