houghlines

Extract line segments based on Hough transform

Syntax

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

lines = houghlines(___,Name,Value)

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 return value lines contains information about the extracted line segments.

lines = houghlines(___,Name,Value) uses name-value pair arguments to control various aspects of the line extraction.

Examples

Find Line Segments and Highlight longest segment

Read image into workspace.

I  = imread('circuit.tif');

Rotate the image.

rotI = imrotate(I,33,'crop');

Create a binary image.

BW = edge(rotI,'canny');

Create the Hough transform using the binary image.

[H,T,R] = hough(BW);
imshow(H,[],'XData',T,'YData',R,...
            'InitialMagnification','fit');
xlabel('\theta'), ylabel('\rho');
axis on, axis normal, hold on;

Find peaks in the Hough transform of the image.

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 by coloring it cyan.

plot(xy_long(:,1),xy_long(:,2),'LineWidth',2,'Color','cyan');

Input Arguments

BW — Binary image

2-D logical matrix | 2-D numeric matrix

Binary image, specified as a 2-D logical matrix or 2-D numeric matrix. For numeric input, any nonzero pixels are considered to be 1 (true).

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | logical

theta — Line rotation angle

numeric matrix

Line rotation angle, in degrees, specified as a numeric matrix. The angle is measured between the x-axis and the rho vector.

Data Types: double

rho — Distance from origin to line

numeric matrix

Distance from the coordinate origin, specified as a numeric matrix. The coordinate origin is the top-left corner of the image (0,0).

Data Types: double

peaks — Row and column coordinates of Hough transform bins

numeric matrix

Row and column coordinates of Hough transform bins, specified as a numeric matrix.

Data Types: double

Name-Value Arguments

Specify optional comma-separated pairs of Name,Value arguments. Name is the argument name and Value is the corresponding value. Name must appear inside quotes. You can specify several name and value pair arguments in any order as Name1,Value1,...,NameN,ValueN.

Example: lines = houghlines(BW,T,R,P,'FillGap',5,'MinLength',7);

FillGap — Distance between two line segments associated with the same Hough transform bin

20 (default) | positive number

Distance between two line segments associated with the same Hough transform bin, specified as a positive number. When the distance between the line segments is less than the value specified, the houghlines function merges the line segments into a single line segment.

Data Types: double

MinLength — Minimum line length

40 (default) | positive number

Minimum line length, specified as a positive number. houghlines discards lines that are shorter than the value specified.

Data Types: double

Output Arguments

lines — Detected lines

structure array

Detected lines, returned as 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

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• houghlines supports the generation of C code (requires MATLAB^® Coder™). For more information, see Code Generation for Image Processing.

• The optional parameter names 'FillGap' and 'MinLength' must be compile-time constants. Their associated values need not be compile-time constants.

GPU Code Generation

Generate CUDA® code for NVIDIA® GPUs using GPU Coder™.

Usage notes and limitations:

• The optional parameter names 'FillGap' and 'MinLength' must be compile-time constants. Their associated values need not be compile-time constants.