Normal transformations
Whenever vertices are transformed, normal vectors should also be transformed, so they point in the right direction. We could think of using the Model-View matrix that transforms vertices to do this, but there is a problem: The Model-View matrix will not always keep the perpendicularity of normals.
This problem occurs if there is a unidirectional (one axis) scaling transformation or a shearing transformation in the Model-View matrix. In our example, we have a triangle that has undergone a scaling transformation on the y-axis. As you can see, the normal N' is not normal anymore after this kind of transformation. How do we solve ...
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