[io] Specify no face elements in PLY files (from point cloud) to make them interoperable with VTK

[tin1254]

Fix loading ply file by loadPolygonFilePLY as vtk requires face element as mentioned in #1470

I tested the fix with ply file here and the following code, it doesn't throw Cannot read geometry from vtk now. And the saved ply can be opened by meshlab without issue

#include <pcl/io/ply_io.h>
#include <pcl/io/vtk_lib_io.h>

int main(){
    pcl::PointCloud<pcl::PointXYZ> test_ply;
    pcl::PolygonMesh test_mesh;
    pcl::io::loadPLYFile("XXX.ply",test_ply);
    pcl::io::savePLYFile("XXX.ply",test_ply);
    pcl::io::loadPolygonFilePLY("XXX.ply",test_mesh);
    return 0;
}

[kunaltyagi]

6 years for a fix... I think the record is 8 years

[larshg]

Should we add a inline comment to why it has to be there?

lars makes a good point

[tin1254]

Should we add a inline comment to why it has to be there?

Done

[mvieth]

Great, thank you!
Minor side note: the written files were only incompatible with the VTK reader, right? Meshlab was never the problem (unlike the title suggests)?

[tin1254]

Great, thank you!
Minor side note: the written files were only incompatible with the VTK reader, right? Meshlab was never the problem (unlike the title suggests)?

Yeah, Meshlab was only used to ensure I didn't break the ply writer, I'll change the title