How to detect lines in certain angle range using Hough transform

Question

I am trying to find all straight lines (vertical or diagonal) in an image using Matlab. The problem is that it include any line even horizontal lines. this is my code. How can I detect all straight lines in the image below (they can be between 45 to 115 degrees)?

function [linesnum, avg]= hh(inp_file,tresh)

I  = imread(inp_file);

BW = edge(I,'canny');
% [H,T,R] = hough(BW,'Theta', 45:0.5:90); % it has no efect

[H,T,R] = hough(BW);
P  = houghpeaks(H,300,'threshold',ceil(0.3*max(H(:))));   

lines = houghlines(BW,T,R,P,'FillGap',10,'MinLength',30);

if (do_plot)
    figure, imshow(I), hold on
    x = T(P(:,2));
    y = R(P(:,1));
    plot(x,y,'s','color','black');
end
max_len = 0;
linesnum = 0;
sumLen = 0;

for k = 1:length(lines)
    xy = [lines(k).point1; lines(k).point2];
    vert = xy(1,1)==xy(2,1);
    if (do_plot)
        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');
    end

    len = norm(lines(k).point1 - lines(k).point2);
    sumLen = sumLen + len;

    linesnum = linesnum +1;

end   
avg = sumLen / linesnum;
end

This is a sample image:

There is a similar question for grid detection but it seems in that question they rely on the holes of the grid to detect the grid pattern and its orientation, while my input is different.

Your question is related to the marked duplicate. Have a look at Amro's answer for insight. — rayryeng, Aug 10 '16 at 19:29
@rayryeng it seems in that question they rely on the holes of the grid to detect the grid pattern and its orientation, while my input is different — Ahmad, Aug 11 '16 at 04:38
You'd need to do an edge detection first which is what you're doing already. I'd still try what they're doing. If it really doesn't work, I'll consider reopening. — rayryeng, Aug 11 '16 at 04:48
Did you have a look at the Answer by Amro in that other question? — Eiko, Aug 11 '16 at 07:26
have you tried it without the theta limitation in the hough command? — marco wassmer, Aug 11 '16 at 08:18
@marcowassmer yes, but actually I need to limit the angle of the lines, how can I do that? — Ahmad, Aug 11 '16 at 15:39

score 0 · Answer 1 · answered Aug 24 '16 at 13:09

After detecting straight lines using hough transform, you can measure the angle of each line and if it was in the range select it.

 % after extracting straight lines using Haugh transform.
 for k = 1:length(lines)

    xy = [lines(k).point1; lines(k).point2];

    deltaY = xy(2,2) - xy(1,2);
    deltaX = xy(2,1) - xy(1,1);
    % calculate the angle of line
    angle = atan2(deltaY, deltaX) * 180 / pi;
    if (angle > 45 && angle < 115)

        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');        
    end
end

How to detect lines in certain angle range using Hough transform

1 Answers1