Shadow Buffers

NVIDIA’s GeForce3 Ti Graphics Processing Units (GPU) are changing the face of lighting techniques for PC graphics. NVIDIA’s new GPUs are the first to bring hardware acceleration for realistic shadows to consumer PCs. Special hardware features in the texture unit of the GeForce3 Ti family enable these GPUs to create and use shadow buffers for real-time shadows. The technique of using shadow buffers has been field proven for such animated films as Columbia Pictures computer generated (CG) motion picture Final Fantasy: The Sprits Within. Until now, shadow buffering was only available on professional rendering equipment outside of the consumers’ reach (like SGI’s Octane workstation).

NVIDIA shadow buffer technology closes the gap between PC gaming and cinematic style special effects for realistic, real-time shadows. This outlines the techniques used to create and utilize shadow buffers. It also describes the unique hardware features in the GeForce3 Ti GPUs that enable this groundbreaking capability and its benefits for end users.

Realistic Shadows Captivate Users

The fundamental goal of computer graphics is to create the perception of an alternate reality. Graphic imagery becomes more compelling as it gets closer to replicating a real world visual experience. Until now, lighting techniques for real-time computer graphics were missing a key element: realistic shadows. Artists and photographers have always known the power of lighting and shadows. In fact, black and white film is still used for artistic photography because of the powerful effects that lighting and shadows, even without color, have on people. These same effects can be used to captivate users and draw them into real-time graphics environments.

Creating Real-Time Shadows

The NVIDIA shadow buffer technique used by the GeForce3 Ti processors involves creating a “map” of which objects are lighted in the scene. This map is stored in a shadow buffer so that it can be used later and accessed like a texture through the special shadow buffer hardware located in the texture engine within the GeForce3 Ti GPUs. This is a block diagram that shows where the shadow buffer hardware is located in the GeForce3 Ti GPUs.

While mapping the lighted areas instead of the shadowed areas may seem odd, this technique is faster and simplifies the hardware because, while there is only one lit pixel per XY location, there may be many shadowed pixels behind it. Shadow buffer support requires a number of new hardware features in the GeForce3 Ti GPU, but also leverages the general render-to-texture and texture-mapping capabilities previously implemented in the NVIDIA texture engines. As a result, shadow buffers enable the GeForce3 Ti to use texture-filtering techniques to create soft edges on shadows.

None of this would be possible however, if the GeForce3 Ti GPU did not have the special hardware features required to create and use a shadow buffer. These special hardware features include the ability to:

• Designate an area of graphics memory as a shadow buffer.
• Render the shadow map into the shadow buffer.
• Read data from the shadow buffer for the mathematical calculation to determine if a specific pixel in the frame buffer is completely shadowed, partially shadowed or not shadowed at all.

To create a shadow map, the GeForce3 GPU renders the scene from the point of view of the light, as if the viewer were located at the same position and looking inthe same direction as the light. As the GeForce3 renders the scene, it stores the depth information, or Z-value, for each pixel in the shadow buffer. The hardware has a special algorithm to bias this Z-value to avoid undesirable artifacts from precision errors. All of the objects in the scene are rendered, but only the smallest biased Z-value for each pixel is kept to ensure that the shadow buffer represents only the objects that are closest to the light.

Once a shadow map is created and stored in the shadow buffer, it is ready to be used. When the GeForce3 Ti GPU renders the scene, it performs the following steps:

• Transform the pixel into the coordinate system defined by the light source.
• Use the texture filtering techniques to read multiple Z-values stored in the shadow buffer from locations that surround the XY location of the pixel. The multiple Z-values are used for two important calculations:
  • Using a proprietary shadow algorithm, the multiple Z-values are compared to the single Z-value for the pixel to determine what fraction of the pixel is in shadow. This enables up to 256 discrete levels of shadow to create soft edges for shadows.
  • Using texture-filtering algorithms, the multiple Z-values are filtered down to a single Z value.
• The results of steps 2a and 2b are used by the nfiniteFX engine as input data for pixel shading operations. If the Z value of the pixel is equal to (or very close to equal to) the Z value stored in the shadow buffer, then the pixel is lit. Otherwise, the pixel is shadowed with the degree of shadow determined by the result of step 2a. The nfiniteFX engine can also turn off specular highlights for any pixel that is in shadow, enhancing the realism of the image.

The following images illustrate this process visually. To render the scent in Image 1, it is necessary to first create the shadow map showing in Image 2. Not the direction of the light source in Image 1 and you'll see how teh image in Image 2 is rendered from the light's point of view.

Note that Image 2 simply looks like a gray-scale image of the scene from the point of view of the light source.

Image 1		Image 2		Image 3

Image 3 shows the shadow buffer image rendered in full color to highlight the fact that everything that is "in view" of the light, will not be shadowed; everything else is shadowed.

NVIDIA Shadow Mapping Techniques

There are many techniques for rendering shadows and each of them have their strengths. However, shadow mapping with NVIDIA shadow buffers has a number of key advantages over alternative shadow rendering techniques:

Complex Scenes Works equally well for arbitrarily complex scenes (many objects or complex shapes).
Soften shadow edges Shadow mapping can soften shadow edges using the proprietary shadowfiltering algorithm with up to 256 discrete levels of shadowing based on the percentage coverage of the pixel, if the pixel area is partially shadowed and partially lighted.
Self Shadow Objects can “self shadow” (a persons arm can cast a shadow on their chest.) Note the wings on the spacecraft cast a shadow on other parts of the craft
Compatible with multi-texture rendering NVIDIA shadow buffers are compatible with multi-texture rendering. Stenciled shadow volumes can force multi-pass rendering, reducing performance if the application is either fill rate limited or memory bandwidth limited.

Conclusion

NVIDIA shadow buffers represent a giant leap forward for the PC graphics industry. The unique hardware features of the GeForce3 Ti GPUs make realistic, real-time shadows available for the first time ever on consumer PCs and bridge the gap between cinema style shadow effects and the state-of-the-art for PC graphics.

Appendix A - How Shadow Buffers Work

Here we use a heavy black line to show which values get stored in the shadow buffer, leaving other locations 'in shadow' because they are 'behind' some other object or part of an object.
Here we show how objects that are hidden from the light (i.e. shadowed) can be visible to the eye when the scene is rendered from the eye’s perspective. Note that the yellow circle is visible to the eye, but is hidden from the light. That simply means that it is ‘in shadow.’
The specific test to determine whether the pixel is shadowed or lighted is the comparison of the distance Zlight to the filtered Z value from the shadow buffer. The upper image explicitly shows these two values via two arrows with the length of the arrows representing the distance of the object from the image plane, which has a value of Z=0. An alternate scenario would be where the yellow circle moves toward the light source until it is closer to the light than the green square. As shown in the lower image, the yellow circle would cast a shadow on part of the green square, because A approx/= B.