Decaling (Direct3D 9)
Direct3D applications use decaling to control which pixels from a particular primitive image are drawn to the rendering target surface. Applications apply decals to the images of primitives to enable coplanar polygons to render correctly.
For instance, when applying tire marks and yellow lines to a roadway, the markings should appear directly on top of the road. However, the z values of the markings and the road are the same. Therefore, the depth buffer might not produce a clean separation between the two. Some pixels in the back primitive may be rendered on top of the front primitive and vice versa. The resulting image appears to shimmer from frame to frame. This effect is called z-fighting or flimmering.
To solve this problem, use a stencil to mask the section of the back primitive where the decal will appear. Turn off z-buffering and render the image of the front primitive into the masked-off area of the render-target surface.
Although multiple texture blending can be used to solve this problem, doing so limits the number of other special effects that your application can produce. Using the stencil buffer to apply decals frees up texture blending stages for other effects.