I am trying to use the Hough transform for edge detection, and would like to use gradient images as the basis.
What I have done so far, given the image
I of size
[M,N] and its partial derivatives
gy, is to calculate the gradient angle in each pixel as
thetas = atan(gy(x,y) ./ gx. Similarly I calculate the gradient magnitude as
magnitudes = sqrt(gx.^2+gy.^2).
To build the Hough transform, I use the following MATLAB code:
max_rho = ceil(sqrt(M^2 + N^2)); hough = zeros(2*max_rho, 101); for x=1:M for y=1:N theta = thetas(x,y); rho = x*cos(theta) + y*sin(theta); rho_idx = round(rho)+max_rho; theta_idx = floor((theta + pi/2) / pi * 100) + 1; hough(rho_idx, theta_idx) = hough(rho_idx, theta_idx) + magnitudes(x,y); end end
The resulting Hough transform looks plausible (see http://i.stack.imgur.com/hC9mP.png), but when I try to use its maxima as edge parameters in the original image, the results look more or less random. Did I do something wrong in constructing the Hough transform?
UPDATE: I had a stupid mistake in my code:
rho was calculated as
x*cos(theta)+y*cos(theta) instead of
x*cos(theta)+y*sin(theta). That is, I was using two cosines instead of a cosine and a sine. I have edited the code above and the new resulting image is below. This did not give much better edges though.
@endolith: To plot an edge, given a maximal value in the
rho_idx, theta_idx, I translate the indices to
theta = (theta_idx -1) / 100 * pi - pi / 2; rho = rho_idx - max_rho;
Finally I plot the edge as
y= (rho - x*cos(theta)) / sin(theta).