The behavior of min_theta in the function HoughLines seems to be wrong, at least in the python opencv library that I refer to in this issue. The problem is that min_theta needs to be too low for my opinion, as otherwise lines won't get detected.

System information (version)

• OpenCV => 4.5.5
• Operating System / Platform => Arch Linux 64-bit
• Python => 3.10.4

Detailed description

Horizontal lines can only be detected if the min_theta is less or equal 0.0001. The expected behavior would be, that min_theta can get as large as pi / 2 minus a sufficient tolerance.

Steps to reproduce

import cv2 as cv
import math

img = cv.imread('houghlines_test.png', cv.IMREAD_GRAYSCALE)
lines = cv.HoughLines(img, 1, math.pi/180, 90, min_theta=0.0001, max_theta=1.58)
print(lines)
lines = cv.HoughLines(img, 1, math.pi/180, 90, min_theta=0.001, max_theta=1.58)
print(lines)

houghlines_test.png:

Output:

[[[100.          1.5708964]]]
None

Expected output would be two times the first line.

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues,
  forum.opencv.org, Stack Overflow, etc and have not found any solution
• I updated to the latest OpenCV version and the issue is still there
• There is reproducer code and related data files: videos, images, onnx, etc