Smooth shading is a method of shading a region with a color gradient. Including color information, along with the bounds of drawing primitive, specifies the color gradient. GDI linearly interpolates the color of the inside of the primitive passed on the color endpoints. Color and vertex information is included with position information in the TRIVERTEX structure.
Use the GradientFill function to fill a triangle or rectangle structure. To fill a triangle with smooth shading, call GradientFill with the three triangle endpoints. To fill a rectangle with smooth shading, call GradientFill with the upper-left and lower-right rectangle coordinates. GradientFill references the TRIVERTEX, GRADIENT_RECT, and GRADIENT_TRIANGLE structures.
For an example, see Drawing a Shaded Triangle.