How to use Haar wavelet to detect LINES on an image?

Question

So I have an image like this:

I want to get something like this (I haven\'t

Answer 1

The following is a complete example of applying Hough Transform to detect lines. I am using MATLAB for the job..

The trick is to divide the image into regions and process each differently; this is because you have different "textures" in your scene (tiles on the upper region of the wall are quite different from the darker ones on the bottom, and processing the image all at once wont be optimal).

As a working example, consider this one:

%# load image, blur it, then find edges
I0  = rgb2gray( imread('http://www.de-viz.ru/catalog/new2/Holm/hvannaya.jpg') );
I = imcrop(I0, [577 156 220 292]);     %# select a region of interest
I = imfilter(I, fspecial('gaussian', [7 7], 1), 'symmetric');
BW = edge(I, 'canny');

%# Hough Transform and show accumulated matrix
[H T R] = hough(BW, 'RhoResolution',2, 'Theta',-90:0.5:89.5);
imshow(imadjust(mat2gray(H)), [], 'XData',T, 'YData',R, ...
       'InitialMagnification','fit')
xlabel('\theta (degrees)'), ylabel('\rho')
axis on, axis normal, colormap(hot), colorbar, hold on

%# detect peaks
P  = houghpeaks(H, 20, 'threshold',ceil(0.5*max(H(:))));
plot(T(P(:,2)), R(P(:,1)), 'gs', 'LineWidth',2);

%# detect lines and overlay on top of image
lines = houghlines(BW, T, R, P, 'FillGap',50, 'MinLength',5);
figure, imshow(I), hold on
for k = 1:length(lines)
    xy = [lines(k).point1; lines(k).point2];
    plot(xy(:,1), xy(:,2), 'g.-', 'LineWidth',2);
end
hold off

You could try the same procedure for other regions while tuning the parameters to get good results..