Graphics Reference
In-Depth Information
Fig. 9.20 The normal direction of faces has a strong effect on how an object is rendered. On left ,
they face inward, outward on the right
triangles that look nothing like your model. Depending on how serious the error is,
it may be possible to fi x by creating a script to renumber the vertices in your fi le.
9.3.14
Reversed Normals
A polygonal face where the normal vector is pointing away from the front of the
object has a backward, or reversed, normal (Fig. 9.20 ). A face with a reversed nor-
mal will appear to be invisible until you turn the camera to the reverse side of the
object, where the face will suddenly reappear. Whenever you think you are missing
a face, the most likely answer is that it has a reversed normal. Reversed normals are
easy to fi nd and simple to fi x.
Reversed normals are a problem only if you have been incautious with your
technique and have manufactured a great many of them. Then, you must search
every face of your entire model, fl ipping normals one by one until they all face the
right direction. Some programs allow you to unify an object's normals, to make
them all point inside or outside, but it doesn't always work. If your model is a non-
manifold object, a unify algorithm will at least partially fail. If you use such a tool,
you should be prepared to check the result.
Incorrect normals are an extremely serious error and must be fi xed wherever they
are found. Unfortunately, it is very easy to create this error, and you should expect
that any model you make would contain at least some reversed normals until you
have fi xed them.
9.3.15
Transforms Attached
Transforms are modifi ed translation and rotation values in the transformation matrix
of an object. If these values are present, they will cause errors when the object is
placed in a video game scene, or when animation is attached. When a modeler is
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