Inconsistency between addPolygonMesh and updatePolygonMesh

Your Environment

• Operating System and version: Ubuntu 16.04
• Compiler: gcc (Ubuntu 5.4.0-6ubuntu1~16.04.5) 5.4.0 20160609
• PCL Version: 1.8.1

Context

I am trying to update a polygon mesh with only one polygon in PCLVisualizer. The updatePolygonMesh function returns false here(https://github.com/PointCloudLibrary/pcl/blob/master/visualization/src/pcl_visualizer.cpp#L3235). The reason is because the addPolygonMesh functions handles mesh differently when the size of polygons are 1 and bigger than 1 (see https://github.com/PointCloudLibrary/pcl/blob/master/visualization/src/pcl_visualizer.cpp#L3149). Can I ask why we use a vtkUnstructuredGrid instead of vtkPolyData when there is only one polygon in the mesh?

Expected Behavior

I would expect the updatePolygonMesh call to be successful regardless of how many polygons are there in the mesh.

Current Behavior

However, current updatePolygonMesh has a bug if the existing PolygonMesh has only one polygon when added. Because the code(https://github.com/PointCloudLibrary/pcl/blob/master/visualization/src/pcl_visualizer.cpp#L3235) is checking if there is a vtkPolyData added but addPolygonMesh added a updatePolygonMesh, which will always fail.

Code to Reproduce

Try adding a polygon mesh with just one polygon and then update it by calling updatePolygonMesh, it will fail.

Possible Solution

I personally tried removing the special case when the polygon number is 1 and always use the existing code for cases where there more than 1 polygon, it worked fine. So my question is: why do we handle the cases differently, as when the number of polygons is equal or bigger than 1?

Read visualization/src/pcl_visualizer.cpp at the addPolygonMesh and updatePolygonMesh locations linked in the issue, then reproduce the failure by adding a mesh with one polygon and calling updatePolygonMesh. Compare the vtkPolyData and vtkUnstructuredGrid handling. Done means updating a single-polygon mesh succeeds and the add and update paths use compatible handling.