edge

Name

edge — Edge detection

Calling Sequence

E = edge(Img)

E = edge(Img, <named_args>)

E = edge(Img, method)

E = edge(Img, method, thresh)

E = edge(Img, method, thresh, direction)

E = edge(Img, method, thresh, direction, sigma)

Parameters

E: Resulting image.

Img: M x N Grayscale (intensity) image in any range.
method: may be 'sobel'(default), 'prewitt' or 'fftderiv'. Other methods will appear in the future.
thresh: sets the threshold level, from 0 to 1. Defaults to 0.5. If negative, then the output image, E, will have the un-thresholded gradient image.
direction: may be 'horizontal', 'vertical' or 'both'(default). This determines the direction to compute the image gradient.
sigma: Controls the ammount of high-frequency attenuation in some methods (only the 'fftderiv' method uses this parameter). This can be used to obtain different levels of detail and to filter out high-frequency noise. Defaults to 1.
<named_args>: This is a sequence of statements key1=value1, key2=value2,... where key1, key2,... can be any of the optional arguments above, in any order.

Description

The function edge performs edge detection on a grayscale intensity image. The user may set the method, the threshold level and the direction of the edge detection.

edge(Img): Detects edges in Img, using the sobel gradient estimator, 0.5 threshold level and in both horizontal and vertical directions.

The other parameters are optional and non-positional. That is, they may be passed to the function by their name. The following example illustrates this.

Examples

   initial_dir = PWD;
   chdir (SIPDIR + 'images');

   Img = imread('tru.jpg');
   Img = im2gray(Img);
   xbasc()
   imshow(Img);

   e = edge(Img);  // sobel, thresh = 0.5
   xbasc()
   imshow(e,2)

   e = edge(Img,'prewitt'); // thresh = 0.5
   xbasc()
   imshow(e,2)

   e = edge(Img,'fftderiv', 0.4);  // FFT gradient method; 0.4 threshold
   xbasc()
   imshow(e,[])

   // It is useful to thin the edges, eliminating redundant pixels:
   e = thin(e);
   xbasc()
   imshow(e,[])

   e = edge(Img,'fftderiv',sigma=3,thresh=-1); // thicker edges, no threshold
   xbasc()
   imshow(e,[])

   e = edge(Img,thresh=-1);
   xbasc()
   imshow(e,[])

   chdir(initial_dir);

References

"Shape Analysis and Classification", L. da F. Costa and R. M. Cesar Jr., CRC Press, section 3.3.

Authors

Ricardo Fabbri <ricardofabbri[at]users.sf.net>

Availability

The latest version of SIP can be found at

http://siptoolbox.sf.net