Outlines and Silhouettes (Direct3D 9)
You can use the stencil buffer for more abstract effects, such as outlining and silhouetting.
If your application applies a stencil mask to the image of a primitive that is the same shape but slightly smaller, the resulting image contains only the primitive's outline. The application can then fill the stencil-masked area of the image with a solid color, giving the primitive an embossed look.
If the stencil mask is the same size and shape as the primitive you are rendering, the resulting image contains a hole where the primitive should be. Your application can then fill the hole with black to produce a silhouette of the primitive.