Light Direction
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4/8/2010
A light's direction property determines the direction that the light emitted by the object travels, in world space. Direction is used only by directional lights and is described with a vector.
C++ applications set the light direction in the Direction member of the light's D3DMLIGHT structure. The Direction member is of type D3DMVECTOR. Direction vectors are described as distances from a logical origin, regardless of the light's position in a scene. You can simulate sunlight shining directly on a scene by using a directional light whose direction is <0,-1,0>. You do not have to create lights that shine along the coordinate axes; you can mix and match values to create lights that shine at more interesting angles.
Note
Although you do not need to normalize a light's direction vector, always be sure that it has magnitude. In other words, do not use a <0,0,0> direction vector.