Make plane segmentation from point cloud deterministic (#6580)

feat: move sampling outside parallel for loop
this change makes the results deterministic when a seed is used

Co-authored-by: Ben Thompson <benjamin.thompson@dexterity.ai>
Co-authored-by: Jonathan Kuck <jonathan@dexterity.ai>

cpp/open3d/geometry/PointCloudSegmentation.cpp

@@ -163,6 +163,12 @@ std::tuple<Eigen::Vector4d, std::vector<size_t>> PointCloud::SegmentPlane(
 
     size_t num_points = points_.size();
     RandomSampler<size_t> sampler(num_points);
+    // Pre-generate all random samples before entering the parallel region
+    std::vector<std::vector<size_t>> all_sampled_indices;
+    all_sampled_indices.reserve(num_iterations);
+    for (int i = 0; i < num_iterations; i++) {
+        all_sampled_indices.push_back(sampler(ransac_n));
+    }
 
     // Return if ransac_n is less than the required plane model parameters.
     if (ransac_n < 3) {
@@ -187,8 +193,8 @@ std::tuple<Eigen::Vector4d, std::vector<size_t>> PointCloud::SegmentPlane(
             continue;
         }
 
-        const std::vector<size_t> sampled_indices = sampler(ransac_n);
-        std::vector<size_t> inliers = sampled_indices;
+        // Access the pre-generated sampled indices
+        std::vector<size_t> inliers = all_sampled_indices[itr];
 
         // Fit model to num_model_parameters randomly selected points among the
         // inliers.