Triangle Interpolation

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During rendering, the pipeline interpolates vertex data across each triangle. Several types of data can be interpolated.

• Diffuse color
• Specular color
• Diffuse alpha (triangle opacity)
• Specular alpha
• Fog factor (taken from specular alpha for fixed function vertex pipeline and from fog register for programmable vertex pipeline)
• Texture coordinates

The interpolation of the vertex data is dependent on the current shading mode, as shown in the following table.

Shading mode	Description
Flat	Only the fog factor is interpolated in flat shade mode. For all other interpolated values, the color of the first vertex in the triangle is applied across the entire face.
Gouraud	Linear interpolation is performed between all three vertices.

The color and specular interpolated values are treated differently, depending on the color model. In the RGB color model, the system uses the red, green, and blue color components in the interpolation.

The alpha component of a color is treated as a separate interpolated value because device drivers can implement transparency in two different ways: by using texture blending or by using stippling.

Use the ShadeCaps property of the Caps structure to determine what forms of interpolation the current device driver supports.

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