fix infinite loop in segment_plane if num_points < ransac_n

[rxba]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

Just a small fix, but if the number of points in the pointcloud is smaller than ransac_n, segment_plane is caught in an infinite loop and does not terminate.
It's a bit of an unlikely scenario, but I did stumble across it because I was fitting planes on clustering results, and one of those point clusters only had two points.

The reason for the infinite loop is because RandomSampler is called with num_points

Open3D/cpp/open3d/geometry/PointCloudSegmentation.cpp

Lines 164 to 165 in fcc396e

	size_t num_points = points_.size();
	RandomSampler<size_t> sampler(num_points);

, so in the case of num_points < ransac_n the condition can never become true in RandomSampler:

Open3D/cpp/open3d/geometry/PointCloudSegmentation.cpp

Lines 37 to 45 in fcc396e

	while (valid_sample < sample_size) {
	const size_t idx = utility::random::RandUint32() % total_size_;
	// Well, this is slow. But typically the sample_size is small.
	if (std::find(samples.begin(), samples.end(), idx) ==
	samples.end()) {
	samples.push_back(idx);
	valid_sample++;
	}
	}

Code to reproduce:

import open3d as o3d
import numpy as np

num_points = 2
points = np.random.rand(num_points, 3)
pcd = o3d.geometry.PointCloud()
pcd.points = o3d.utility.Vector3dVector(points)
plane_model, inliers = pcd.segment_plane(distance_threshold=0.01,
                                         ransac_n=3,
                                         num_iterations=1000)

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

• I just moved the already existing checks for this to before the RandomSampler is called so this error can actually be caught
• I also removed some of the comments around this part of the code, as they are redundant in my opinion

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[benjaminum]

Thanks for this bugfix @rxba !