Selecting a string using hough transform

Problem: Find unwanted string in image using Hough transform.

I did the following:

• Apply directional filter to analyze 12 different directions rotated about 15 ° one after another.
• Apply threshold to get 12 binary images.

Now I need to select any of the two images marked in yellow. Coz, the lines in these two images are the most prominent.

I have tried the following code. It doesn't seem to work.

MATLAB code

    %   Read 12 images into workspace.
input_images  = {imread('1.png'),imread('2.png'),imread('3.png'),...
    imread('4.png'),imread('5.png'),imread('6.png'),...
    imread('7.png'),imread('8.png'),imread('9.png'),...
    imread('10.png'),imread('11.png'),imread('12.png')};

longest_line = struct('point1',[0 0], 'point2',[0 0], 'theta', 0, 'rho', 0);

for n=1:12
    %Create a binary image.
    binary_image = edge(input_images{n},'canny');

    %Create the Hough transform using the binary image.
    [H,T,R] = hough(binary_image);

    %Find peaks in the Hough transform of the image.
    P  = houghpeaks(H,3,'threshold',ceil(0.3*max(H(:))));

    %Find lines
    hough_lines = houghlines(binary_image,T,R,P,'FillGap',5,'MinLength',7);         
    longest_line = FindTheLongestLine(hough_lines, longest_line);
end


% Highlight the longest line segment by coloring it cyan.
plot(longest_line.point1, longest_line.point2,'LineWidth',2,'Color','cyan');

      

        
        
        
      

    

...

Corresponding source code

function longest_line = FindTheLongestLine( hough_lines , old_longest_line)
%FINDTHELONGESTLINE Summary of this function goes here
%   Detailed explanation goes here
    longest_line = struct('point1',[0 0] ,'point2',[0 0],'theta', 0, 'rho', 0);

    max_len = 0;

    N = length(hough_lines);

    for i = 1:N
       % Determine the endpoints of the longest line segment
       len = LenthOfLine(hough_lines(i));

       if ( len > max_len)
          max_len = len;
          longest_line = hough_lines(i);
       end
    end

    old_len = LenthOfLine(old_longest_line);
    new_len = LenthOfLine(longest_line);

    if(old_len > new_len)
       longest_line =  old_longest_line;
    end
end

function length = LenthOfLine( linex )
%LENTHOFLINE Summary of this function goes here
%   Detailed explanation goes here

    length = norm(linex.point1 - linex.point2);
end

      

        
        
        
      

    

Image testing

Here are 12 images, drive.google.com/open?id=0B-2FDw63ZNTnRnEzYlNyS0V4YVE