Reference: pmEdgeDetection
pmEdgeDetection(pm), where pm is a pixmap, returns the pixmap containing the grey-scaled edges of pm. pmEdgeDetection(pm, "horizontal"), where pm is a pixmap, returns the pixmap containing the grey-scaled edges of pm. Only horizontal edges are detected. pmEdgeDetection(pm, "vertical"), where pm is a pixmap, returns the pixmap containing the grey-scaled edges of pm. Only vertical edges are detected.