Summary

Fixes a bug where the OpenCL-accelerated implementation of cv::HoughLines ignored min_theta and max_theta parameters, always computing angles from 0 instead of the specified range.

Problem Description

When calling cv::HoughLines with OpenCL acceleration and specifying min_theta/max_theta parameters (e.g., to detect only horizontal lines with theta ∈ [80°, 100°]), the function would fail to find any lines. This is because:

1. The C++ code correctly computed numangle based on the theta range
2. But the OpenCL kernels always computed angles starting from 0, ignoring min_theta
3. This caused a mismatch between the accumulator indexing and actual angles

Changes Made

C++ code (hough.cpp):

• Added min_theta parameter to ocl_fillAccum function signature
• Updated calls to ocl_fillAccum in ocl_HoughLines (passes min_theta) and ocl_HoughLinesP (passes 0.0 for default [0, π] range)
• Updated getLinesKernel.args to pass min_theta to the kernel

OpenCL kernels (hough_lines.cl):

• Updated fill_accum_global and fill_accum_local kernels to accept min_theta parameter
• Changed angle computation from theta * theta_idx to min_theta + theta * theta_idx
• Updated get_lines kernel to correctly compute detected line angles as min_theta + y * theta

Testing

The fix resolves the issue described in #28036 where horizontal line detection with min_theta=80°, max_theta=100° failed with OpenCL but worked without it.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch (4.x)
• There is a reference to the original bug report and related work (cv::HoughLines with OpenCL use min_theta/max_theta incorrectly #28036)
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
• The feature is well documented and sample code can be built with the project CMake

Fixes #28036