houghlines (Image Processing Toolbox User's Guide)

Image Processing Toolbox User's Guide

Extract line segments based on the Hough transform

Syntax

lines = houghlines(BW, theta, rho, peaks)

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:

Field
Description

point1
Two element vector [X Y] specifying the coordinates of the end-point of the line segment

point2
Two element vector [X Y] specifying the coordinates of the end-point of the line segment

theta
Angle in degrees of the Hough transform bin

rho
rho axis position of the Hough transform bin

Field	Description
`point1`	Two element vector `[X Y]` specifying the coordinates of the end-point of the line segment
`point2`	Two element vector `[X Y]` specifying the coordinates of the end-point of the line segment
`theta`	Angle in degrees of the Hough transform bin
`rho`	`rho` axis position of the Hough transform bin

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.

Parameter
Description

'FillGap'

Positive real scalar value that specifies the distance between two line segments associated with the same Hough transform bin. When the distance between the line segments is less the value specified, the houghlines function merges the line segments into a single line segment. Default: 20

'MinLength'

Positive real scalar value that specifies whether merged lines should be kept or discarded. Lines shorter than the value specified are discarded. Default: 40

Parameter	Description
`'FillGap'`	Positive real scalar value that specifies the distance between two line segments associated with the same Hough transform bin. When the distance between the line segments is less the value specified, the `houghlines` function merges the line segments into a single line segment. Default: 20
`'MinLength'`	Positive real scalar value that specifies whether merged lines should be kept or discarded. Lines shorter than the value specified are discarded. Default: 40

Class Support

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

Example

This example searches for line segments in an image and highlights 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