How to detect lines in this pic using opencv?

Question

I don't know why opencv can't find out the lines using HoughLines, the result is follow,

But if I use another picture from the internet,

voila. Any ideas?

Answer 1

It seems that Hough detect the lines extremities. A gradient is probably computed during the process.

A solution would be to:

1. Binarize the image
2. Compute the black region skeleton
3. Skeleton pruning
4. Apply the Hough Transform.

Doing that, each line will be reduced to 1 pixel width.

[EDIT] Here is an example:

1. The test image binarized
2. The skeleton (simple thinning).
3. The result (I skipped the pruning).