Default value of min_points_per_voxel changed during migration

[ymd-stella]

The default value of min_points_per_voxel is changed in migration (#398).

The default value in melodic was 1.

perception_pcl/pcl_ros/cfg/VoxelGrid.cfg

Line 19 in 283496f

gen.add ("min_points_per_voxel", int_t, 0, "The minimum number of points required for a voxel to be used.", 1, 1, 100000)

After migration, it was changed to 2, but I could not find any reference to the reason for this.

perception_pcl/pcl_ros/src/pcl_ros/filters/voxel_grid.cpp

Line 72 in e8ce252

min_points_per_voxel_desc.name, rclcpp::ParameterValue(2), min_points_per_voxel_desc);

If this is set to 2 or higher and the leaf_size is small relative to the density of the point cloud, part of the pointcloud may disappear, which can be a surprise to someone unaware of this parameter. Therefore, I think this default value should be restored.