OpenCV Hough行不显示

Question

I am trying to get the lines from the following image: original

My code uses the following function:

import cv2
lines = cv2.HoughLinesP(image, .1, np.pi/360, 10,
                        minLineLength, maxLineGap)

Where minLineLength = 20, and maxLineGap = 10. I am obtaining the following: after hough lines

Why are the horizontal lines not showing up in this case, despite being clearly visible to the naked eye?

Answer 1

The problem is the resolution that you use for rho.

A resolution of 0.1 is extremely small. Indeed for an image of size (N, N) pixels, the number of possible values for rho is N * N * sqrt(2) / rho_resolution (sometimes the double of this depending of how the angle is defined).

The number of possible values is materialized as bins in the Hough accumulator. The larger this number, the less the different pixels of the image will accumulate is the same bins. With a very small value for the resolution of rho, you will end up with a sparse accumulator, with a few low values here and there.

Bring this value up to 1 or 2 pixels of resolution and you should see more significant lines appear.

Answer 2

If HoughLinesP() is not detecting some line, even though it is clearly visible, it is because that line surely does not meet some of your criteria. Most probably your minLineLength or maxLineGap . If you reduce those criteria you will start detecting more lines.

Also, as some comments indicate, your rho parameter may be too small (0.1). As that parameter is measured in pixels , giving it a non-integer value may produce unexpected results, try giving it a value of 1.

If you want you can take a look at this example from the docs where they explain and show how to use both HoughLines() and HoughLinesP() .