MATLAB: How to measure the angle of a line in the image using the Image Processing Toolbox in MATLAB

Image Processing Toolbox

I have an image with a few line objects. I would like to know if the Image Processing Toolbox has the capability to recognize these line objects from the figure and give me an estimate of their orientation.

Best Answer

To recognize line objects in your figure window, you may use the HOUGH Transform. Information about the HOUGH transform may be found at the following location:
 
 <http://www.mathworks.com/help/toolbox/images/ref/hough.html>
As a simple example, run the following code on the attached file "test.jpg":
 
% read the image into MATLAB and convert it to grayscale
I = imread('test.jpg');
Igray = rgb2gray(I);
figure, imshow(I);
% We can see that the image is noisy. We will clean it up with a few
% morphological operations
Ibw = im2bw(Igray,graythresh(Igray)); 
se = strel('line',3,90);
cleanI = imdilate(~Ibw,se);
figure, imshow(cleanI);
% Perform a Hough Transform on the image 
% The Hough Transform identifies lines in an image 
[H,theta,rho] = hough(cleanI);
peaks  = houghpeaks(H,10);
lines = houghlines(Ibw,theta,rho,peaks);
figure, imshow(cleanI)
% Highlight (by changing color) the lines found by MATLAB
hold on
for k = 1:numel(lines)
    x1 = lines(k).point1(1);
    y1 = lines(k).point1(2);
    x2 = lines(k).point2(1);
    y2 = lines(k).point2(2);
    plot([x1 x2],[y1 y2],'Color','g','LineWidth', 2)
end
hold off
% Identify the angles of the lines. 
% The following command shows the angle of the 2nd line found by the Hough
% Transform
lines(1).theta
lines(2).theta
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