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Vertex tweening blends two user-provided positions (or normals). Tweening is mutually exclusive from vertex blending with weights. Tweening is performed before lighting and clipping, and is enabled by setting D3DRS_VERTEXBLEND to D3DVBF_TWEENING. The resulting vertex position is computed by:
POSITION = POSITION1 * (1.0 - TWEENFACTOR) + POSITION2 * TWEENFACTOR
The same approach is used for calculating normals. For more information, see Using Vertex Tweening.