hough - Hough transform for line detection

Calling Sequence

[ht, rho_range] = hough(imbin)

Input Parameters

• imbin: binary image array (foreground equals 1)

Output Parameters

• ht: Hough transform accumulation matrix. Rho varies along rows; theta varies along columns. Theta range is 0-179 degrees w/ 180 samples.
• rho_range: Vector containing the range of radius values. Equals -rmax:rmax, where rmax is the maximum radius possibly found in the input image.

Description

Function hough calculates the hough transform of a binary image. The coordinate system is centered in the image, and the Y axis points downwards. Theta grows from X axis to Y axis. Negative rhos point to the upper half of the image.

Examples

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

   // ======= Example 1

   im = imread('star.bmp');
   im = bwborder(im);
   xbasc()
   imshow(im,2);

   h = hough(im);
   xbasc()
   imshow(h,[]);  // theta varies horizontally from 0 to 180

   ht = 1*(h>= 40); // threshold the hough space
   lim = ihough(ht,size(im)); // draw the detected lines
   xbasc()
   imshow(lim + 2*im +1, hotcolormap(4)) // detected lines shown in yellow


   // ======= Example 2: how to obtain the parameters
   //
   // creating a empty picture with a line at y = -90
   e = zeros(200,200);
   e(10,:) = 1;

   // (remember that the Y axis points downwards and is centered in the
   // middle of the image)

   // getting its hough transform, and finding the points 
   // corresponding to y=10
   [h, rrange] = hough(e);
   [r,c] = find(h == max(h))

   // Gets the parameters of the line
   theta = c - 1       // 90 degrees
   rho   = rrange(r)   // -90 rho (upper half of image)

   // thx to Herve Lombaert for inspiring example #2 !

   chdir(initial_dir);
   

Authors

Ricardo Fabbri <ricardofabbri (AT) users DOT sf DOT net>

Bibliography

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

"Practical Computer Vision using C", J. R. Parker, Wiley.

Availability

http://siptoolbox.sourceforge.net

See Also

ihough,  radon,  drawline,  edge,