| Microsoft DirectX 8.1 (C++) |
The ShadowVolume sample uses stencil buffers to implement real-time shadows. In the sample, a complex object is rendered and used as a shadow caster to cast real-time shadows on itself and on the terrain below.
Stencil buffers are a depth-buffer technique that can be updated as geometry is rendered, and used again as a mask for drawing more geometry. Common effects include mirrors, shadows (an advanced technique), dissolves, and so on.
Not all cards support all features for stencil-buffer techniques. Some hardware has no, or limited, stencil-buffer support. For more information on stencil buffers, refer to the Microsoft® DirectX® SDK documentation.
Source: (SDK root)\Samples\Multimedia\Direct3D\StencilBuffer\ShadowVolume
Executable: (SDK root)\Samples\Multimedia\Direct3D\Bin
The following table lists the keys that are implemented. You can use menu commands for the same controls.
| Key | Action |
|---|---|
| ENTER | Starts and stops the scene. |
| SPACEBAR | Advances the scene by a small increment. |
| F2 | Prompts the user to select a new rendering device or display mode. |
| ALT+ENTER | Toggles between full-screen and windowed modes. |
| ESC | Exits the application. |
Creating real-time shadows is a fairly advanced technique. In each frame, or as the geometry or lights in the scene are moved, an object called a shadow volume is computed. A shadow volume is a 3-D object that is the silhouette of the shadowcasting object, as protruded away from the light source.
In this sample, the 3-D object that casts shadows is a bi-plane. The silhouette of the plane is computed in each frame. This technique uses an edge-detection algorithm in which silhouette edges are found. This can be done because the normals of adjacent polygons will have opposing normals with respect to the light vector. The resulting edge list (the silhouette) is protruded into a 3-D object away from the light source. This 3-D object is known as the shadow volume, as every point inside the volume is inside a shadow.
Next, the shadow volume is rendered into the stencil buffer twice. First, only forward-facing polygons are rendered, and the stencil-buffer values are incremented each time. Then the back-facing polygons of the shadow volume are drawn, decrementing values in the stencil buffer. Normally, all incremented and decremented values cancel each other out. However, because the scene was already rendered with normal geometry, in this case the plane and the terrain, some pixels fail the zbuffer test as the shadow volume is rendered. Any values left in the stencil buffer correspond to pixels that are in the shadow.
Finally, these remaining stencil-buffer contents are used as a mask, as a large all-encompassing black quad is alpha-blended into the scene. With the stencil buffer as a mask, only pixels in shadow are darkened
This sample uses common Microsoft DirectX® code that consists programming elements such as helper functions. This code is shared with other samples in the DirectX SDK. You can find the common headers and source code in (SDK root)\Samples\Multimedia\Common.