CGAL read_OFF函数因顶点顺序丢弃面的原因及解决方法咨询

The Root Cause #

Let me break this down for you: even though the OFF format doesn’t enforce vertex order rules for faces, CGAL’s read_OFF function has default validation checks that filter out faces it considers "invalid". For triangular faces specifically, it checks the signed area of the face relative to the kernel’s coordinate system (usually a right-handed system).

If your vertex order results in a face with a negative signed area (meaning the vertices are ordered clockwise under the right-hand rule), CGAL will automatically skip that face. In your example:

• The face 0 3 1 has a positive signed area (counter-clockwise order, matching the right-hand rule), so it’s retained.
• The face 0 1 3 has a negative signed area (clockwise order), so it gets filtered out by default.

This behavior is intended to help maintain orientable mesh consistency, even though the OFF spec doesn’t require it.

How to Ensure All Faces Are Read #

To force CGAL to read every face defined in your OFF file—regardless of vertex order or signed area—you need to disable the default validation checks. Here’s how to modify your code:

Replace the basic read_OFF call:

CGAL::IO::read_OFF(file_name, test_mash);

With this version that skips validation:

bool success = CGAL::IO::read_OFF(
    file_name,
    test_mash,
    CGAL::parameters::skip_validation(true)
);

For older CGAL versions (pre-5.0), you might need this simpler overload instead:

bool success = CGAL::IO::read_OFF(file_name, test_mash, false); // false = don't skip invalid faces

Key Notes to Keep in Mind #

• Disabling validation means CGAL will also read truly degenerate faces (like collinear vertices) if they exist in your OFF file. Make sure your input data doesn’t have these if you plan to run mesh operations later.
• If you need to work with an orientable mesh after reading, you can manually adjust vertex orders to ensure consistent face orientations using CGAL’s mesh processing tools.
• For debugging, add verbose output to see exactly which faces are skipped and why:

  bool success = CGAL::IO::read_OFF(
      file_name,
      test_mash,
      CGAL::parameters::skip_validation(true).verbose(true)
  );
  

内容的提问来源于stack exchange，提问作者Echtzeitpepega