A Guide to Clean Geometry

Mesh Lint Addon for Blender:

What is It?
The Mesh Lint add-on locates bad geometry in the objects you model and even has a built-in feature that provides for real-time monitoring (so bad geometry can be detected immediately as it is created). Features listed directly from the home site include:

• Tris
• N-gons
• Nonmanifold Elements
• Interior Faces
• 6+-Poles
• Default Names (like Cube.002 )
• Unapplied Scale

Resources:
Mesh Lint Add-On Tutorial - watch this tutorial for help with the download and add-on usage
Download Mesh Lint Add-On

1.) Degenerate Faces:

Degenerate faces are faces in which 3 or more vertices of a face lie along the same line. Degenerate faces have zero area because the area cannot be calculated for the face.

• How to Find: To find degenerate faces, you can use the CryBlend tool 'Find Degenerate Faces' (use this tool in OBJECT Mode with your object selected) - it will select the degenerate faces and you can proceed to fix them.

• How to Fix: The best way is to manually grab each vertex and move them in such a way that produces a nice clean face (a convex face with non-zero area).

• Quick Fix: The quick-and-dirty method to eliminate degenerate faces is to use the Shrink/Fatten tool with a small factor like .01.

2.) Non-Manifold Geometry:

Case 1: Lamina Faces - Faces that share the same edges and vertices (essentially two faces which share the same space).

Case 2: Three or more faces share the same edge on an object
Case 3: Two or more faces share the same vertex, but share no edge
Case 4: Two or more adjacent faces have oppositely directed normals
General: Anything that isn't water tight (you can't pour water into it); this includes perimeter edges but you can ignore these instances.

• How to Find: Use the Mesh Lint add-on.

• How to Fix: If you have a lot of non-manifold geometry, it is not worth fixing them unless they generate noticeable issues. If you have non-manifold geometry in a character or other soft-body model, it is recommended you fix the non-manifold geometry so it does not negatively affect deformations.
  Case 1: Select the overlapping face in EDIT mode. With the face selected, press 'Y' to separate the face and 'X' or 'DELETE' to delete it.
  Case 2: In EDIT mode, delete the excess faces if you can so that no more than 2 faces share the same edge. If you cannot delete the excess faces, you will need to reconstruct your mesh in this area.
  Case 3: In EDIT mode, grab the two vertices (one on each face) that need to be connected to connect the faces by an edge. With these two vertices selected, press 'ALT' 'M' and choose 'At Center' to merge the vertices. Now the faces should be connected by an edge instead of just one vertex. If you cannot do this, you will need to reconstruct your mesh in this area.
  Case 4: To help you fix the normals you can go to the properties panel and under 'Display' find the heading 'Normals' and choose the cube-with-an-orange-face icon to display face normals. Select the problem faces in EDIT mode and select 'Mesh --> Normals --> Flip Normals'. It may be sufficient to select your entire mesh and press 'CTRL' 'N' and 'CTRL' 'SHIFT' 'N' to recalculate the inside and outside normals, but if you run into issues, just use the first solution which will always work.

• Quick Fix: If you intend to export your object using CryBlend to be used in the CryEngine, you will be able to export your object fine (generating only warnings about the non-manifold geometry), but the physics for your object may malfunction. If you are exporting a character or soft-body it is recommended you perform the repairs, but if you are exporting a static object, the non-manifold geometry will only affect the physics calculations on the object. Thus, it is possible to fix the physics issues by making the material physics for your object 'physNone', creating a simple-geometry proxy (avoiding non-manifold geometry), and adding material physics 'physProxyNoDraw' to your proxy mesh. This will still generate non-manifold geometry warnings when exporting, but your physics should work fine (using the proxy instead of the mesh with non-manifold geometry).

3.) Triangles:

Triangles aren't always a big deal but you should generally try to avoid them, using quads instead. As a general rule, avoid triangles in deformable or soft-body meshes (like characters) at all costs; you don't have to worry if they are in a hard-body model like a gun as much.

• How to Find: Use the Mesh Lint add-on. If you don't have the add-on, you can find triangles in EDIT mode by deselecting all faces in your mesh and selecting 'Select --> Select Faces by Sides' and then in the 'Select Faces by Sides panel that appears in the toolbar at the left choose 3 for the number of sides. This will select all 3-sided faces (or triangles).

• How to Fix: Use loop cuts 'CTRL' 'R', knife tool 'K', or dissolving 'X' of edges to get two triangles next to each other. You can then dissolve the edge connecting the triangles to create a quad. Remember, two triangles make a quad. If you only have one triangle to work with, try using loop cuts that end up at the triangle to develop the triangle into a quad.

• Quick Fix: This may or may not eliminate your triangles. In EDIT mode, select your mesh and select 'Mesh --> Faces --> Beautify Fill' and then 'Mesh --> Faces --> Tris to Quads' (or 'ALT' 'F' and then 'ALT' 'J'). If this doesn't fix your problem, try some of the techniques recommended in the 'How To Fix' section above.