prewitt_dir — Detect edges (amplitude and direction) using the Prewitt operator.
prewitt_dir calculates an approximation of the first derivative of the image data and is used as an edge detector. The filter is based on the following filter masks:
A = 1 1 1 0 0 0 -1 -1 -1 B = 1 0 -1 1 0 -1 1 0 -1
The result image contains the maximum response of the masks A and B. The edge directions are returned in ImageEdgeDir, and are stored in 2-degree steps, i.e., an edge direction of x degrees mathematically positive sense and with respect to the horizontal axis is stored as x / 2 in the edge direction image. Furthermore, the direction of the change of intensity is taken into account. Let [Ex,Ey] denote the image gradient. Then the following edge directions are returned as r/2:
intensity increase Ex / Ey edge direction r from bottom to top 0 / + 0 from lower right to upper left - / + ]0,90[ from right to left - / 0 90 from upper right to lower left - / - ]90,180[ from top to bottom 0 / - 180 from upper left to lower right + / - ]180,270[ from left to right + / 0 270 from lower left to upper right + / + ]270,360[.
Points with edge amplitude 0 are assigned the edge direction 255 (undefined direction).
Edge amplitude (gradient magnitude) image.
Edge direction image.
read_image(Image,'fabrik') prewitt_dir(Image,PrewittA,PrewittD) threshold(PrewittA,Edges,128,255)
prewitt_dir always returns 2 (H_MSG_TRUE). If the input is empty the behaviour can be set via set_system('no_object_result',<Result>). If necessary, an exception is raised.
binomial_filter, gauss_image, sigma_image, median_image, smooth_image
hysteresis_threshold, threshold, gray_skeleton, nonmax_suppression_dir, close_edges, close_edges_length
edges_image, sobel_dir, robinson_dir, frei_dir, kirsch_dir