DBSCAN clustering algorithm

[griegler]

Implementation of the DBSCAN clustering algorithm for PointCloud. Implementation is not threaded for now, but could be a simple addition.

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This change is 

[yxlao]

Great work! Added some small comments.

Reviewed 6 of 6 files at r1.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @griegler)

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src/Open3D/Geometry/PointCloudCluster.cpp, line 52 at r1 (raw file):

        std::vector<int> nbs;
        std::vector<double> dists2;
        kdtree.SearchRadius(points_[idx], eps2, nbs, dists2);

from https://github.com/intel-isl/Open3D/blob/e151b0bc0e95e7c3605fd7627335b648b2f555a3/src/Open3D/Geometry/KDTreeFlann.cpp#L152-L153, looks like the esp is squared internally by SearchRadius, so we shall probably use esp instead of esp2?

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src/Open3D/Geometry/PointCloudCluster.cpp, line 60 at r1 (raw file):

        }

        std::unordered_set<int> nbs_next(nbs.begin(), nbs.end());

maybe it's cleaner to use std::deque<int>, orstd::stack<int> (if we conver to DFS) ? seems like we're not using the "set" property here.

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src/Open3D/Geometry/PointCloudCluster.cpp, line 78 at r1 (raw file):

            labels[nb] = cluster_label;

            kdtree.SearchRadius(points_[nb], eps2, nbs, dists2);

same, esp2 -> esp?

[griegler]

Reviewable status: 5 of 6 files reviewed, 3 unresolved discussions (waiting on @yxlao)

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src/Open3D/Geometry/PointCloudCluster.cpp, line 52 at r1 (raw file):

Previously, yxlao (Yixing Lao) wrote…

from https://github.com/intel-isl/Open3D/blob/e151b0bc0e95e7c3605fd7627335b648b2f555a3/src/Open3D/Geometry/KDTreeFlann.cpp#L152-L153, looks like the esp is squared internally by SearchRadius, so we shall probably use esp instead of esp2?

thanks, changed it to eps.

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src/Open3D/Geometry/PointCloudCluster.cpp, line 60 at r1 (raw file):

Previously, yxlao (Yixing Lao) wrote…

maybe it's cleaner to use std::deque<int>, orstd::stack<int> (if we conver to DFS) ? seems like we're not using the "set" property here.

the set property is used in the insert step. In deque, or stack it would possibly insert some indices multiple times.

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src/Open3D/Geometry/PointCloudCluster.cpp, line 78 at r1 (raw file):

Previously, yxlao (Yixing Lao) wrote…

same, esp2 -> esp?

done.

[yxlao]

Reviewable status: 5 of 6 files reviewed, 3 unresolved discussions (waiting on @griegler and @yxlao)

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src/Open3D/Geometry/PointCloudCluster.cpp, line 60 at r1 (raw file):
From here, are we already checking that nbs_visited.count(qnb) == 0 before inserting?

                    if (nbs_visited.count(qnb) == 0) {
                        nbs_next.insert(qnb);
                    }

[griegler]

Reviewable status: 5 of 6 files reviewed, 1 unresolved discussion (waiting on @yxlao)

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src/Open3D/Geometry/PointCloudCluster.cpp, line 60 at r1 (raw file):

Previously, yxlao (Yixing Lao) wrote…

From here, are we already checking that nbs_visited.count(qnb) == 0 before inserting?

                    if (nbs_visited.count(qnb) == 0) {
                        nbs_next.insert(qnb);
                    }

but nbs_visited and nbs_next are different. A point can be inserted multiple times into nbs_next before it is actually visited (and inserted into nbs_visited).

[yxlao]

Reviewable status: 5 of 6 files reviewed, 1 unresolved discussion (waiting on @griegler and @yxlao)

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src/Open3D/Geometry/PointCloudCluster.cpp, line 60 at r1 (raw file):

Previously, griegler (Gernot) wrote…

but nbs_visited and nbs_next are different. A point can be inserted multiple times into nbs_next before it is actually visited (and inserted into nbs_visited).

i see, this could be more efficient, thanks for the explanation