houghpeaks

Identify peaks in Hough transform

Syntax

peaks = houghpeaks(H,numpeaks)

peaks = houghpeaks(H,numpeaks,Name,Value)

Description

peaks = houghpeaks(H,numpeaks) locates peaks in the Hough transform matrix, H, generated by the hough function. numpeaks specifies the maximum number of peaks to identify. The function returns peaks a matrix that holds the row and column coordinates of the peaks.

peaks = houghpeaks(H,numpeaks,Name,Value) controls aspects of the operation using name-value pair arguments.

Examples

Locate and Display Peaks in Hough Transform of Rotated Image

Read image into workspace.

I  = imread('circuit.tif');

Create binary image.

BW = edge(imrotate(I,50,'crop'),'canny');

Create Hough transform of image.

[H,T,R] = hough(BW);

Find peaks in the Hough transform of the image and plot them.

P  = houghpeaks(H,2);
imshow(H,[],'XData',T,'YData',R,'InitialMagnification','fit');
xlabel('\theta'), ylabel('\rho');
axis on, axis normal, hold on;
plot(T(P(:,2)),R(P(:,1)),'s','color','white');

Input Arguments

H — Hough transform matrix

numeric array

Hough transform matrix, specified as a numeric array. The rows and columns correspond to rho and theta values. Use the hough function to create a Hough transform matrix.

Data Types: double

numpeaks — Maximum number of peaks to identify

1 (default) | positive integer

Maximum number of peaks to identify, specified as a positive integer.

Data Types: double

Name-Value Pair 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: P = houghpeaks(H,2,'Threshold',15);

'Threshold' — Minimum value to be considered a peak

0.5*max(H(:)) (default) | nonnegative number

Minimum value to be considered a peak, specified as a nonnegative number.

Data Types: double

'NHoodSize' — Size of suppression neighborhood

2-element vector of positive odd integers

Size of the suppression neighborhood, specified as a 2-element vector of positive odd integers. The suppression neighborhood is the neighborhood around each peak that is set to zero after the peak is identified. The default value of NHoodSize is the smallest odd values greater than or equal to size(H)/50. The dimensions of NHoodSize must be smaller than the size of the Hough transform matrix, H.

Data Types: double

'Theta' — Hough transform theta values

-90:89 (default) | numeric vector

Hough transform theta values, specified as a numeric vector returned by the hough function. Each element of the vector specifies the theta value for the corresponding column of the output matrix H. houghpeaks uses the theta values specified for peak suppression. Use the hough function to create a Hough transform matrix.

Note

If you specify the 'Theta' parameter as input to the hough function, you must specify the theta parameter with the houghpeaks function. Use the theta output value from the hough function as the theta input value for houghpeaks. Otherwise, peak suppression can result in unexpected results.

Data Types: double

Output Arguments

peaks — Row and column coordinates of found peaks

Q-by-2 matrix

Row and column coordinates of found peaks, returned as a Q-by-2 matrix. The value Q can range from 0 to numpeaks.

Extended Capabilities

C/C++ Code Generation

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

Usage notes and limitations:

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

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