The system interpolates the characteristics of a triangle's vertices across the triangle when it renders a face. The following are the triangle interpolants:
·Color
·Specular
·Fog
·Alpha
All of the triangle interpolants are modified by the current shade mode:
Flat No interpolation is done. Instead, the color of the first vertex in the triangle is applied across the entire face.
Gouraud Linear interpolation is performed between all three vertices.
Phong Vertex parameters are reevaluated for each pixel in the face, using the current lighting. The Phong shade mode is not currently supported.
The color and specular interpolants are treated differently, depending on the color model. In the RGB color model (D3DCOLOR_RGB), the system uses the red, green, and blue color components in the interpolation. In the monochromatic model (D3DCOLOR_MONO), the system uses only the blue component of the vertex color.
For example, if the red component of the color of vertex 1 were 0.8 and the red component of vertex 2 were 0.4, in the Gouraud shade mode and RGB color model the system would use interpolation to assign a red component of 0.6 to the pixel at the midpoint of the line between these vertices.
The alpha component of a color is treated as a separate interpolant because device drivers can implement transparency in two different ways: by using texture blending or by using stippling.
An application can use the dwShadeCaps member of the D3DPRIMCAPS structure to determine what forms of interpolation the current device driver supports.