Image Processing Toolbox™

houghlines - Extract line segments based on Hough transform

Syntax

lines = houghlines(BW, theta, rho, peaks)
lines = houghlines(..., param1, val1, param2, val2)

Description

lines = houghlines(BW, theta, rho, peaks) extracts line segments in the image BW associated with particular bins in a Hough transform. theta and rho are vectors returned by function hough. peaks is a matrix returned by the houghpeaks function that contains the row and column coordinates of the Hough transform bins to use in searching for line segments.

The houghlines function returns lines, a structure array whose length equals the number of merged line segments found. Each element of the structure array has these fields:

lines = houghlines(..., param1, val1, param2, val2) specifies parameter/value pairs, listed in the following table. Parameter names can be abbreviated, and case does not matter.

Class Support

BW can be logical or numeric and it must be real, 2-D, and nonsparse.

Examples

Search for line segments in an image and highlight the longest segment.

I  = imread('circuit.tif');
rotI = imrotate(I,33,'crop');
BW = edge(rotI,'canny');
[H,T,R] = hough(BW);
imshow(H,[],'XData',T,'YData',R,...
            'InitialMagnification','fit');
xlabel('\theta'), ylabel('\rho');
axis on, axis normal, hold on;
P  = houghpeaks(H,5,'threshold',ceil(0.3*max(H(:))));
x = T(P(:,2)); y = R(P(:,1));
plot(x,y,'s','color','white');
% Find lines and plot them
lines = houghlines(BW,T,R,P,'FillGap',5,'MinLength',7);
figure, imshow(rotI), hold on
max_len = 0;
for k = 1:length(lines)
   xy = [lines(k).point1; lines(k).point2];
   plot(xy(:,1),xy(:,2),'LineWidth',2,'Color','green');

   % Plot beginnings and ends of lines
   plot(xy(1,1),xy(1,2),'x','LineWidth',2,'Color','yellow');
   plot(xy(2,1),xy(2,2),'x','LineWidth',2,'Color','red');

   % Determine the endpoints of the longest line segment
   len = norm(lines(k).point1 - lines(k).point2);
   if ( len > max_len)
      max_len = len;
      xy_long = xy;
   end
end

% highlight the longest line segment
plot(xy_long(:,1),xy_long(:,2),'LineWidth',2,'Color','cyan');

See also

hough, houghpeaks

hough

houghpeaks