MODELLING CONCEPTS
Non-Manifold
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Non-manifold meshes are considered to be bad practice in modelling.
3D printing is not possible with non-manifold meshes as the shapes they create cannot exist in reality. Several modifiers will not work with non-manifold geometry or will further compound problems with your geometry.
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Zbrush and several other external softwares will not tolerate non-manifolds at all.
Thickness and Skinning modifiers will have problems with non-manifold constructs.
Several physics and fluid simulation operations will not work correctly with non-manifolds.
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UV unwrapping can become very complicated with non-manifold geometry.
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​Any arrangement of faces which could not exist in the real word is considered non-manifold. everything must have thickness and be watertight.
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We call this being 2-manifold and it is essential that all of your meshes conform to this standard. The rules are simple:
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Every edge must be shared by exactly two faces. No faces can intersect each other.
That's it really. if you follow these two rules your meshes will be 2-manifold.
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Your meshes will spend a lot of their development time being non-maniold as there will be exposed edges around the boundary until you are done. You must be aware that the use of modifiers around these non manifold edges cannot be correctly considered and only the manifold portions of your mesh are what you are working on.
It is often preferable to work on a non-manifold shape than it is to work on a watertight model but hopefully I can make that clear through examples as we go forward with tutorials!
The following simple mesh is manifold.
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Simple Manifold Mesh
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The mesh below has had one edge extruded. This Mesh is non-manifold as the edge marked in yellow is shared by 3 faces and the edges marked in blue are connected to only one face. Adding The subdivision surface modifier will leave you with a non manifold mesh with shading errors.
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The next mesh has two 3d sections but they are connected by only one edge. This means this one edge is shared by 4 faces. This is a construct which could not exist in the real world and again the subdivision surface modifier cannot create a geometry which will render correctly.
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Extruding an edge creates Non-Manifold Geometry
One edge shared by four faces
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The next mesh has the top face removed resulting in four edges (marked in blue) which are only attached to one face each. This is the most common problem found in modelling. It means the mesh has zero thickness. Another construct which cannot exist in real life.
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Removing a face leaves exposed Non-Manifold edges
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The next mesh has one face at the top of the model moved and scaled so that it intersects other faces. Another construct which cannot exist in real life Which results in impossibly hard areas in the subdivided mesh.
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The next mesh has one vertex floating by itself not attached to any edges or faces (the small white vertex to the right of the cube!!). This is often used as "Ghost Geometry" to control effects (such as the array modifier). It is considered non-manifold and can cause problems with other modifiers and physics operations. It will also cause problems with 3d-printing.
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It can be useful but you must ensure that these non manifold vertices are removed when the model is finalised and exported to other software.
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Overlapping polygons creating Non-manifold topology
Unattached vertices or edges create non-manifold objects
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There are uses for non-manifold constructs. The next mesh has one face at the top of the model rotated and scaled so that it intersects other faces. This means the cage is non-manifold, but the overlap does not contribute to errors in the subdivided mesh. The resulting geometry is a valid manifold shape. This can and does happen regularly and is considered correct but animators in a CG pipeline should be warned that this situation exists as further deformation to this cage could (more easily than usual) result in non-manifold geometry.
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Overlapping faces on the "cage" but the final object is manifold