# houghlines

Extract line segments based on Hough transform

## Syntax

lines = houghlines(BW,theta,rho,peaks)
lines = houghlines(___,Name,Value)

## Description

example

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.

example

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

## Examples

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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

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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

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

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

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

Data Types: double

### Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Before R2021a, use commas to separate each name and value, and enclose Name in quotes.

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

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

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

Data Types: double

## Output Arguments

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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

## Version History

Introduced before R2006a