More issues with igl::readMESH

[evouga]

Describe the bug

I've hit a few more bugs in the .mesh parser:

• Blank lines might include some whitepace before the newline; this is not detected as a comment by the parser.
• The parser chokes on "Edges" sections in the file.

Here's a failure case "from the wild": https://www.dropbox.com/s/6ovsblr1rnrslm6/cup_85k.mesh?dl=0

I wrote my own parser, from the .mesh spec, as a replacement for my personal project; I didn't create a pull request since

• this code is barely tested
• it's still not fully spec-compliant (for example, the code assumes that the three vertex coordinates appear on the same line, whereas I believe the spec allows them to be split by newlines)
• meshes "in the wild" might violate the spec, and I'm not convinced my parser is strictly more permissive than libigl's.

But I'm leaving the code here in case it's useful:

template <typename DerivedV, typename DerivedF, typename DerivedT>
bool readMESH(
   const std::string mesh_file_name,
   Eigen::PlainObjectBase<DerivedV>& V,
   Eigen::PlainObjectBase<DerivedT>& T,
   Eigen::PlainObjectBase<DerivedF>& F)
{
   V.resize(0, 0);
   T.resize(0, 0);
   F.resize(0, 0);
   auto nextNonComment = [](std::istream& fs, std::string& s) -> bool
   {
       while (true)
       {
           std::getline(fs, s);
           if (!fs)
               return false;
           for (int i = 0; i < s.length(); i++)
           {
               if (s[i] == '#')
                   break;
               else if (!std::isspace(s[i]))
                   return true;
           }
       }
       // unreachable
       return false;
   };

   std::ifstream ifs(mesh_file_name);
   if (!ifs)
   {
       std::cerr << "Error: couldn't read .mesh file: " << mesh_file_name << std::endl;
       return false;
   }

   std::string line;

   // check that first word is MeshVersionFormatted
   if (!nextNonComment(ifs, line))
   {
       std::cerr << "Error: missing MeshVersionFormatted line" << std::endl;
       return false;
   }
   std::stringstream ss(line);
   std::string format;
   int versionid;
   ss >> format >> versionid;
   if (!ss || format != "MeshVersionFormatted")
   {
       std::cerr << "Error: expected \"MeshVersionFormatted (1 or 2)\", read " << line << std::endl;
       return false;
   }
   if (versionid != 1 && versionid != 2)
   {
       std::cerr << "Error: MeshVersionFormatted must be 1 or 2" << std::endl;
       return false;
   }

   // check that third word is Dimension
   if (!nextNonComment(ifs, line))
   {
       std::cerr << "Error: missing Dimension line" << std::endl;
       return false;
   }
   ss = std::stringstream(line);
   std::string dimension;    
   ss >> dimension;
   if (!ss || dimension != "Dimension")
   {
       std::cerr << "Error: expected \"Dimension\", read " << line << std::endl;
       return false;
   }
   int dim;
   ss >> dim;
   if (!ss)
   {
       // not spec compliant, but occurs in the wild
       if (!nextNonComment(ifs, line))
       {
           std::cerr << "Error: missing dimension number" << std::endl;
           return false;
       }
       ss = std::stringstream(line);
       ss >> dim;
       if (!ss)
       {
           std::cerr << "Error: missing dimension number" << std::endl;
           return false;
       }
   }
   if (dim != 3)
   {
       std::cerr << "Error: only Dimension 3 supported not " << dim << std::endl;
       return false;
   }

   while (true)
   {
       if (!nextNonComment(ifs, line))
       {
           std::cerr << "Error: file ended before \"End\" line" << std::endl;
           return false;
       }

       ss = std::stringstream(line);
       std::string sectionid;
       ss >> sectionid;
       if (sectionid == "End")
           return true;
       else if (sectionid == "Vertices")
       {
           // Not spec compliant, but since this was in the IGL code, needed in some cases?
           int nverts;
           ss >> nverts;
           if (!ss)
           {
               // On next line
               if (!nextNonComment(ifs, line))
               {
                   std::cerr << "Error: missing number of vertices following \"Vertices\"" << std::endl;
                   return false;
               }
               ss = std::stringstream(line);
               ss >> nverts;
               if (!ss)
               {
                   std::cerr << "Error: missing number of vertices following \"Vertices\"" << std::endl;
                   return false;
               }
           }
           V.resize(nverts, 3);
           for (int i = 0; i < nverts; i++)
           {
               if (!nextNonComment(ifs, line))
               {
                   std::cerr << "Error: read " << i << " vertices, was expecting " << nverts << std::endl;
                   return false;
               }
               int ref;
               ss = std::stringstream(line);
               ss >> V(i, 0) >> V(i, 1) >> V(i, 2) >> ref;
               if (!ss)
               {
                   std::cerr << "Error: expected \"x y z ref\", read " << line << std::endl;
                   return false;
               }
           }
       }
       else if (sectionid == "Triangles")
       {
           // Not spec compliant, but since this was in the IGL code, needed in some cases?
           int ntris;
           ss >> ntris;
           if (!ss)
           {
               // On next line
               if (!nextNonComment(ifs, line))
               {
                   std::cerr << "Error: missing number of triangles following \"Triangles\"" << std::endl;
                   return false;
               }
               ss = std::stringstream(line);
               ss >> ntris;
               if (!ss)
               {
                   std::cerr << "Error: missing number of triangles following \"Triangles\"" << std::endl;
                   return false;
               }
           }
           F.resize(ntris, 3);
           for (int i = 0; i < ntris; i++)
           {
               if (!nextNonComment(ifs, line))
               {
                   std::cerr << "Error: read " << i << " triangles, was expecting " << ntris << std::endl;
                   return false;
               }
               int ref;
               DerivedF::Scalar v1, v2, v3;
               ss = std::stringstream(line);
               ss >> v1 >> v2 >> v3 >> ref;
               if (!ss)
               {
                   std::cerr << "Error: expected \"v1 v2 v3 ref\", read " << line << std::endl;
                   return false;
               }
               F(i, 0) = v1 - 1;
               F(i, 1) = v2 - 1;
               F(i, 2) = v3 - 1;
           }
       }
       
       else if (sectionid == "Quadrilaterals")
       {
           // Not spec compliant, but since this was in the IGL code, needed in some cases?
           int nquads;
           ss >> nquads;
           if (!ss)
           {
               // On next line
               if (!nextNonComment(ifs, line))
               {
                   std::cerr << "Error: missing number of quadrilaterals following \"Quadrilaterals\"" << std::endl;
                   return false;
               }
               ss = std::stringstream(line);
               ss >> nquads;
               if (!ss)
               {
                   std::cerr << "Error: missing number of quadrilaterals following \"Quadrilaterals\"" << std::endl;
                   return false;
               }
           }
           if (nquads != 0)
           {
               std::cerr << "Error: Only tetrahedral meshes are supported (file contains Quadrilaterals)" << std::endl;
               return false;
           }
       }        
       else if (sectionid == "Tetrahedra")
       {
           // Not spec compliant, but since this was in the IGL code, needed in some cases?
           int ntets;
           ss >> ntets;
           if (!ss)
           {
               // On next line
               if (!nextNonComment(ifs, line))
               {
                   std::cerr << "Error: missing number of tetrahedra following \"Tetrahedra\"" << std::endl;
                   return false;
               }
               ss = std::stringstream(line);
               ss >> ntets;
               if (!ss)
               {
                   std::cerr << "Error: missing number of tetrahedra following \"Tetrahedra\"" << std::endl;
                   return false;
               }
           }
           T.resize(ntets, 4);
           for (int i = 0; i < ntets; i++)
           {
               if (!nextNonComment(ifs, line))
               {
                   std::cerr << "Error: read " << i << " tetrahedra, was expecting " << ntets << std::endl;
                   return false;
               }
               int ref;
               DerivedT::Scalar v1, v2, v3, v4;
               ss = std::stringstream(line);
               ss >> v1 >> v2 >> v3 >> v4 >> ref;
               if (!ss)
               {
                   std::cerr << "Error: expected \"v1 v2 v3 v4 ref\", read " << line << std::endl;
                   return false;
               }
               T(i, 0) = v1 - 1;
               T(i, 1) = v2 - 1;
               T(i, 2) = v3 - 1;
               T(i, 3) = v4 - 1;
           }
       }
       else if (sectionid == "Hexahedra")
       {
           // Not spec compliant, but since this was in the IGL code, needed in some cases?
           int nhexes;
           ss >> nhexes;
           if (!ss)
           {
               // On next line
               if (!nextNonComment(ifs, line))
               {
                   std::cerr << "Error: missing number of hexahedra following \"Hexahedra\"" << std::endl;
                   return false;
               }
               ss = std::stringstream(line);
               ss >> nhexes;
               if (!ss)
               {
                   std::cerr << "Error: missing number of hexahedra following \"Hexahedra\"" << std::endl;
                   return false;
               }
           }
           if (nhexes != 0)
           {
               std::cerr << "Error: Only tetrahedral meshes are supported (file contains Hexahedra)" << std::endl;
               return false;
           }
       }
       else if (sectionid == "Edges")
       {
           // Not spec compliant, but since this was in the IGL code, needed in some cases?
           int nedges;
           ss >> nedges;
           if (!ss)
           {
               // On next line
               if (!nextNonComment(ifs, line))
               {
                   std::cerr << "Error: missing number of edges following \"Edges\"" << std::endl;
                   return false;
               }
               ss = std::stringstream(line);
               ss >> nedges;
               if (!ss)
               {
                   std::cerr << "Error: missing number of edges following \"Edges\"" << std::endl;
                   return false;
               }
           }
           for (int i = 0; i < nedges; i++)
           {
               if (!nextNonComment(ifs, line))
               {
                   std::cerr << "Error: read " << i << " edges, was expecting " << nedges << std::endl;
                   return false;
               }
               int e1, e2, ref;
               ss = std::stringstream(line);
               ss >> e1 >> e2 >> ref;
               if (!ss)
               {
                   std::cerr << "Error: expected \"e1 e2 ref\", read " << line << std::endl;
                   return false;
               }
           }
       }
       else
       {
           std::cerr << "Error: unknown section ID " << sectionid << std::endl;
           return false;
       }
   }

   // unreachable
   return false;
}

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