|Microsoft DirectX 9.0|
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 spotlights, and is described with a vector.
C++ applications set the light direction in the Direction member of the light's D3DLIGHT9 structure. Direction member is of type D3DVECTOR. Direction vectors are described as distances from a logical origin, regardless of the light's position in a scene. Therefore, a spotlight that points straight into a scene—along the positive z-axis—has a direction vector of <0,0,1> no matter where its position is defined to be. Similarly, you can simulate sunlight shining directly on a scene by using a directional light whose direction is <0,-1,0>. Obviously, you don't 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.