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MODELLING CONCEPTS
Planar faces.
Faces should ideally always be planar, meaning that all of the vertices should lie on the same geometric plane. They should be flat in relation to each other. Any other arrangement than planar causes problems for renderers and other systems which may need to triangulate your mesh (such as game engines). You will not see the shape you hoped for at render time if you have non planar faces.
In the example below, the green face is planar but the red face has one of the vertices moved down in the z axis which has created a non-planar face.
There are many methods of triangulating non-planar faces (which always happens at render time). These triangulation algorithms can be calculated on import, export, on-demand using modifiers or on-the-fly in other software packages. The method used by different software and rendering engines cannot be easily predicted.
Green: Planar Face - Red: Non-Planar Face
The non-planar face in the example above could be triangulated in one of two ways as shown below. (one uses the longest diagonal and one using the shortest diagonal. When a system triangulates on-the-fly and the mesh has deformation animation, the method used can change depending on the shape size and orientation of the face in relation the other faces around it.
The problem is amplified even further with n-gons and a face with even 5 sides will have many possible ways to triangulate and this often causes flickering.
If the mesh is triangulated using the 'triangulation' modifier in blender, animating the shape of the mesh can result in triangles popping in and out of existence during the animation.
Animators for games will normally expect a triangulated version of your model - derived from the framework mesh. If anything needs to be changed because of creative or technical concerns, the framework mesh will be adapted and a newly triangulated version will be provided.
Two possible directions of triangulation of a non-planar face
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