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meshmeshlab

MeshLab Face Count

Not sure if I'm supposed to ask this question here, but going to give it a try since MeshLab doesn't seem to respond to issues on GitHub fast..

When I imported a mesh consisting of 100 vertices and 75 quad faces, meshlab somehow recognizes it to have 146 faces. What is the problem here?

Please find here the OBJ file and below the screenshot:

enter image description here

Any help/advice would be greatly appreciated, Thank you!

Tim

Solution

  • Yes, per the MeshLab homepage Stack Overflow is now the recommended place to ask questions. Github should be reserved for reporting actual bugs.

    It is important to understand is that MeshLab is designed to work with large unstructured triangular meshes, and while it can do some things with quad and polygonal meshes, there are some limitations and idiosyncrasies.

    MeshLab essentially treats all meshes as triangular for most operations; when a polygonal mesh is opened, MeshLab creates "faux edges" that subdivide the mesh into triangles. You can visualize the faux edges by turning "Polygonal Modality" on or off in the edge display pane. If you run "Compute Geometric Measures", it will provide different lengths for the edges both with and without the faux edges. This is why MeshLab is reporting a higher number of faces for your model; it is reporting the number of faces after triangulation, i.e. including the faux edge subdivision. As you can see, when dividing the number of quad faces (75) in half, you end up with nearly double the number of triangular faces (146), which makes sense. Unfortunately I don't know of a way to have MeshLab report the number of faces without these faux edges.

    Most filters only work on triangular meshes, and if run on a polygonal mesh the faux edges will be used. A few specific filters (e.g. those in the "Polygonal and Quad Mesh" category) work with quads, and for these the faux edges should be ignored. When exporting, if you check "polygonal" the faux edges should be discarded and the mesh will be saved with the proper polygons, otherwise the mesh will be permanently triangulated per the faux edges.

    Hope this helps!