extract lines from canny edge detection

Question

In openCV after applying canny edge detection I'd like to further process the result (show only horizontal lines, remove short lines, etc..). But the result of canny is just another image. I'd like to get an array of lines describing the detected edges

I'm aware of the famous Hough Line Transform, but the result is not always good, that's why I'd like to manually process canny result. input:

enter image description here

output canny only:

enter image description here

output canny then Hough line transform

enter image description here

This is Hough line transform result(red lines) for detecting edges of stairs. 4th line from below is not detected correctly, although canny edge detected an edge.

Any idea how to extract edges from canny image?

Your question not so clear. Everyone uses HoughLines to find lines. So better expand your question with sample images. And tell why you can't apply HoughLines with help of images. — Abid Rahman K
I added an image of not so ideal result of houghman and a description. mainly canny detects an edge, but houghman sometimes only detects part of this edge — Moataz Elmasry
Can you please upload the result of canny, just before applying houghtransform, if possible upload original image also. — Abid Rahman K
so I updated original post to include original image and canny without houghman — Moataz Elmasry
BTW: The Hough transform gets it's name from Paul Hough - not Houghman. — Chris Bennet

mevatron mevatron · Accepted Answer · 2012-06-05T18:00:55

A few things you can try to improve your results:

Apply a Region of Interest

Your image looks to have some bordering window effects. I removed them with a region of interest resulting in an image that looks like this (I tweaked it until it looked right, but if you're using some kind of kernel operator it's window size probably better defines this ROI):

enter image description here

Use standard Hough transform

It also seems you're using the probabilistic Hough transform. So, you're only getting line segments instead of an interpolated line. Consider using the standard transform to get the full theoretical line (rho, theta). Doing this I got an image like shown below:

enter image description here

Here is a code snippet I used to generate the lines (from Python interface):

(mu, sigma) = cv2.meanStdDev(stairs8u)
edges = cv2.Canny(stairs8u, mu - sigma, mu + sigma)
lines = cv2.HoughLines(edges, 1, pi / 180, 70)

Filter lines based on angle

You can probably filter out poor lines by taking the most frequently occurring line angles, and throwing away outliers. This should narrow it down to the most visible steps.

Hope that helps!

extract lines from canny edge detection

2 Answers

Apply a Region of Interest

Use standard Hough transform

Filter lines based on angle