frei_dir

frei_dir ( Image : ImageEdgeAmp, ImageEdgeDir : : )

Detect edges (amplitude and direction) using the Frei-Chen operator.

frei_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       sqrt(2)        1
         0          0           0
        -1      -sqrt(2)       -1

  B =
         1          0          -1
       sqrt(2)      0       -sqrt(2) 
         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 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).

Parameters

`Image` (input_object)	(multichannel-)image(-array) `->` object : byte / int2 / uint2
Input image.

`ImageEdgeAmp` (output_object)	(multichannel-)image(-array) `->` object : byte / int2 / uint2
Edge amplitude (gradient magnitude) image.

`ImageEdgeDir` (output_object)	(multichannel-)image(-array) `->` object : direction
Edge direction image.

Example

read_image(Image,'fabrik') 
frei_dir(Image,Frei_dirA,Frei_dirD) 
threshold(Frei_dirA,Res,128,255).

Result

frei_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 handling is raised.

Parallelization Information

frei_dir is reentrant and automatically parallelized (on tuple level, channel level, domain level).

Possible Predecessors

gauss_image, sigma_image, median_image, smooth_image

Possible Successors

hysteresis_threshold, threshold, gray_skeleton, nonmax_suppression_dir, close_edges, close_edges_length

Alternatives

edges_image, sobel_dir, robinson_dir, prewitt_dir, kirsch_dir

See also

bandpass_image, laplace_of_gauss

Module

Image filters