3D graphics eBook - Course Materials Repository
3D graphics eBook - Course Materials Repository 3D graphics eBook - Course Materials Repository
Shadow mapping 183 External links • Hardware Shadow Mapping (http:/ / developer. nvidia. com/ attach/ 8456), nVidia • Shadow Mapping with Today's OpenGL Hardware (http:/ / developer. nvidia. com/ attach/ 6769), nVidia • Riemer's step-by-step tutorial implementing Shadow Mapping with HLSL and DirectX (http:/ / www. riemers. net/ Tutorials/ DirectX/ Csharp3/ index. php) • NVIDIA Real-time Shadow Algorithms and Techniques (http:/ / developer. nvidia. com/ object/ doc_shadows. html) • Shadow Mapping implementation using Java and OpenGL (http:/ / www. embege. com/ shadowmapping) Shadow volume Shadow volume is a technique used in 3D computer graphics to add shadows to a rendered scene. They were first proposed by Frank Crow in 1977 [1] as the geometry describing the 3D shape of the region occluded from a light source. A shadow volume divides the virtual world in two: areas that are in shadow and areas that are not. The stencil buffer implementation of shadow volumes is generally considered among the most practical general purpose real-time shadowing techniques for use on modern 3D graphics hardware. It has been popularised by the video game Doom 3, and a particular variation of the technique used in this game has become known as Carmack's Reverse (see depth fail below). Example of Carmack's stencil shadowing in Doom 3. Shadow volumes have become a popular tool for real-time shadowing, alongside the more venerable shadow mapping. The main advantage of shadow volumes is that they are accurate to the pixel (though many implementations have a minor self-shadowing problem along the silhouette edge, see construction below), whereas the accuracy of a shadow map depends on the texture memory allotted to it as well as the angle at which the shadows are cast (at some angles, the accuracy of a shadow map unavoidably suffers). However, the shadow volume technique requires the creation of shadow geometry, which can be CPU intensive (depending on the implementation). The advantage of shadow mapping is that it is often faster, because shadow volume polygons are often very large in terms of screen space and require a lot of fill time (especially for convex objects), whereas shadow maps do not have this limitation. Construction In order to construct a shadow volume, project a ray from the light source through each vertex in the shadow casting object to some point (generally at infinity). These projections will together form a volume; any point inside that volume is in shadow, everything outside is lit by the light. For a polygonal model, the volume is usually formed by classifying each face in the model as either facing toward the light source or facing away from the light source. The set of all edges that connect a toward-face to an away-face form the silhouette with respect to the light source. The edges forming the silhouette are extruded away from the light to construct the faces of the shadow volume. This volume must extend over the range of the entire visible scene; often the dimensions of the shadow volume are extended to infinity to accomplish this (see optimization below.) To form a closed volume, the front and back end of this extrusion must be covered. These coverings are called "caps". Depending on the method used for the shadow volume, the front end may be covered by the object itself, and the rear end may sometimes be omitted (see depth pass below).
Shadow volume 184 There is also a problem with the shadow where the faces along the silhouette edge are relatively shallow. In this case, the shadow an object casts on itself will be sharp, revealing its polygonal facets, whereas the usual lighting model will have a gradual change in the lighting along the facet. This leaves a rough shadow artifact near the silhouette edge which is difficult to correct. Increasing the polygonal density will minimize the problem, but not eliminate it. If the front of the shadow volume is capped, the entire shadow volume may be offset slightly away from the light to remove any shadow self-intersections within the offset distance of the silhouette edge (this solution is more commonly used in shadow mapping). The basic steps for forming a shadow volume are: 1. Find all silhouette edges (edges which separate front-facing faces from back-facing faces) 2. Extend all silhouette edges in the direction away from the light-source 3. Add a front-cap and/or back-cap to each surface to form a closed volume (may not be necessary, depending on the implementation used) Illustration of shadow volumes. The image above at left shows a scene shadowed using shadow volumes. At right, the shadow volumes are shown in wireframe. Note how the shadows form a large conical area pointing away from the light source (the bright white point). Stencil buffer implementations After Crow, Tim Heidmann showed in 1991 how to use the stencil buffer to render shadows with shadow volumes quickly enough for use in real time applications. There are three common variations to this technique, depth pass, depth fail, and exclusive-or, but all of them use the same process: 1. Render the scene as if it were completely in shadow. 2. For each light source: 1. Using the depth information from that scene, construct a mask in the stencil buffer that has holes only where the visible surface is not in shadow. 2. Render the scene again as if it were completely lit, using the stencil buffer to mask the shadowed areas. Use additive blending to add this render to the scene. The difference between these three methods occurs in the generation of the mask in the second step. Some involve two passes, and some only one; some require less precision in the stencil buffer. Shadow volumes tend to cover large portions of the visible scene, and as a result consume valuable rasterization time (fill time) on 3D graphics hardware. This problem is compounded by the complexity of the shadow casting objects,
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Shadow volume 184<br />
There is also a problem with the shadow where the faces along the silhouette edge are relatively shallow. In this<br />
case, the shadow an object casts on itself will be sharp, revealing its polygonal facets, whereas the usual lighting<br />
model will have a gradual change in the lighting along the facet. This leaves a rough shadow artifact near the<br />
silhouette edge which is difficult to correct. Increasing the polygonal density will minimize the problem, but not<br />
eliminate it. If the front of the shadow volume is capped, the entire shadow volume may be offset slightly away from<br />
the light to remove any shadow self-intersections within the offset distance of the silhouette edge (this solution is<br />
more commonly used in shadow mapping).<br />
The basic steps for forming a shadow volume are:<br />
1. Find all silhouette edges (edges which separate front-facing faces from back-facing faces)<br />
2. Extend all silhouette edges in the direction away from the light-source<br />
3. Add a front-cap and/or back-cap to each surface to form a closed volume (may not be necessary, depending on<br />
the implementation used)<br />
Illustration of shadow volumes. The image above at left shows a scene shadowed using shadow volumes. At right, the shadow volumes are<br />
shown in wireframe. Note how the shadows form a large conical area pointing away from the light source (the bright white point).<br />
Stencil buffer implementations<br />
After Crow, Tim Heidmann showed in 1991 how to use the stencil buffer to render shadows with shadow volumes<br />
quickly enough for use in real time applications. There are three common variations to this technique, depth pass,<br />
depth fail, and exclusive-or, but all of them use the same process:<br />
1. Render the scene as if it were completely in shadow.<br />
2. For each light source:<br />
1. Using the depth information from that scene, construct a mask in the stencil buffer that has holes only where<br />
the visible surface is not in shadow.<br />
2. Render the scene again as if it were completely lit, using the stencil buffer to mask the shadowed areas. Use<br />
additive blending to add this render to the scene.<br />
The difference between these three methods occurs in the generation of the mask in the second step. Some involve<br />
two passes, and some only one; some require less precision in the stencil buffer.<br />
Shadow volumes tend to cover large portions of the visible scene, and as a result consume valuable rasterization time<br />
(fill time) on <strong>3D</strong> <strong>graphics</strong> hardware. This problem is compounded by the complexity of the shadow casting objects,
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