Adding point cloud sampling examples (#770)

docs/modules/mapping/mapping_main.rst

@@ -9,6 +9,7 @@ Mapping
    gaussian_grid_map/gaussian_grid_map
    ray_casting_grid_map/ray_casting_grid_map
    lidar_to_grid_map_tutorial/lidar_to_grid_map_tutorial
+   point_cloud_sampling/point_cloud_sampling
    k_means_object_clustering/k_means_object_clustering
    circle_fitting/circle_fitting
    rectangle_fitting/rectangle_fitting

docs/modules/mapping/point_cloud_sampling/point_cloud_sampling_main.rst

@@ -0,0 +1,70 @@
+.. _point_cloud_sampling:
+
+Point cloud Sampling
+----------------------
+
+This sections explains point cloud sampling algorithms in PythonRobotics.
+
+Point clouds are two-dimensional and three-dimensional based data
+acquired by external sensors like LIDAR, cameras, etc.
+In general, Point Cloud data is very large in number of data.
+So, if you process all the data, computation time might become an issue.
+
+Point cloud sampling is a technique for solving this computational complexity
+issue by extracting only representative point data and thinning the point
+cloud data without compromising the performance of processing using the point
+cloud data.
+
+Voxel Point Sampling
+~~~~~~~~~~~~~~~~~~~~~~~~
+.. figure:: voxel_point_sampling.png
+
+Voxel grid sampling is a method of reducing point cloud data by using the
+`Voxel grids <https://en.wikipedia.org/wiki/Voxel>`_ which is regular grids
+in three-dimensional space.
+
+This method determines which each point is in a grid, and replaces the point
+clouds that are in the same Voxel with their average to reduce the number of
+points.
+
+API
+=====
+
+.. autofunction:: Mapping.point_cloud_sampling.point_cloud_sampling.voxel_point_sampling
+
+
+Farthest Point Sampling
+~~~~~~~~~~~~~~~~~~~~~~~~~
+.. figure:: farthest_point_sampling.png
+
+Farthest Point Sampling is a point cloud sampling method by a specified
+number of points so that the distance between points is as far from as
+possible.
+
+This method is useful for machine learning and other situations where
+you want to obtain a specified number of points from point cloud.
+
+API
+=====
+
+.. autofunction:: Mapping.point_cloud_sampling.point_cloud_sampling.farthest_point_sampling
+
+Poisson Disk Sampling
+~~~~~~~~~~~~~~~~~~~~~~~~~
+.. figure:: poisson_disk_sampling.png
+
+Poisson disk sample is a point cloud sampling method by a specified
+number of points so that the algorithm selects points where the distance
+from selected points is greater than a certain distance.
+
+Although this method does not have good performance comparing the Farthest
+distance sample where each point is distributed farther from each other,
+this is suitable for real-time processing because of its fast computation time.
+
+API
+=====
+
+.. autofunction:: Mapping.point_cloud_sampling.point_cloud_sampling.poisson_disk_sampling
+
+
+