Shadow volume - AbsoluteAstronomy.com

Shadow volume is a technique used in 3D computer graphics

3D computer graphics

3D computer graphics are graphics that use a three-dimensional representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering 2D images...

to add shadows to a rendered scene. They were first proposed by Frank Crow

Franklin C. Crow

Franklin C. Crow or Frank Crow is a computer scientist who has made important contributions to computer graphics, including some of the first practical anti-aliasing techniques. Crow also proposed the shadow volume technique for generating geometrically accurate shadows...

in 1977 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

Stencil buffer

A stencil buffer is an extra buffer, in addition to the color buffer and depth buffer found on modern computer graphics hardware. The buffer is per pixel, and works on integer values, usually with a depth of one byte per pixel...

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
Doom 3
Doom 3 is a science fiction horror video game developed by id Software and published by Activision. An example of the first-person shooter genre, Doom 3 was first released for Microsoft Windows on August 3, 2004. The game was later adapted for Linux, as well as being ported by Aspyr Media for Mac...

, and a particular variation of the technique used in this game has become known as Carmack's Reverse (see depth fail below).

Shadow volumes have become a popular tool for real-time shadowing, alongside the more venerable shadow mapping

Shadow mapping

Shadow mapping or projective shadowing is a process by which shadows are added to 3D computer graphics. This concept was introduced by Lance Williams in 1978, in a paper entitled "Casting curved shadows on curved surfaces"...

. 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).

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

Shadow mapping

).

The basic steps for forming a shadow volume are:

Find all silhouette edge
Silhouette edge
In computer graphics, a silhouette edge on a 3D body projected onto a 2D plane is the collection of points whose outwards surface normal is perpendicular to the view vector. Due to discontinuities in the surface normal, a silhouette edge is also an edge which separates a front facing face from a...

s (edges which separate front-facing faces from back-facing faces)
Extend all silhouette edges in the direction away from the light-source
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)

Stencil buffer implementations

After Crow, Tim Heidmann showed in 1991 how to use the stencil buffer

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:

Render the scene as if it were completely in shadow.
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, as each object can cast its own shadow volume of any potential size onscreen. See optimization below for a discussion of techniques used to combat the fill time problem.

Depth pass

Heidmann proposed that if the front surfaces and back surfaces of the shadows were rendered in separate passes, the number of front faces and back faces in front of an object can be counted using the stencil buffer. If an object's surface is in shadow, there will be more front facing shadow surfaces between it and the eye than back facing shadow surfaces. If their numbers are equal, however, the surface of the object is not in shadow. The generation of the stencil mask works as follows:

Disable writes to the depth
Z-buffering
In computer graphics, z-buffering is the management of image depth coordinates in three-dimensional graphics, usually done in hardware, sometimes in software. It is one solution to the visibility problem, which is the problem of deciding which elements of a rendered scene are visible, and which...

and color buffers.
Use back-face culling.
Set the stencil operation to increment on depth pass (only count shadows in front of the object).
Render the shadow volumes (because of culling, only their front faces are rendered).
Use front-face culling.
Set the stencil operation to decrement on depth pass.
Render the shadow volumes (only their back faces are rendered).

After this is accomplished, all lit surfaces will correspond to a 0 in the stencil buffer, where the numbers of front and back surfaces of all shadow volumes between the eye and that surface are equal.

This approach has problems when the eye itself is inside a shadow volume (for example, when the light source moves behind an object). From this point of view, the eye sees the back face of this shadow volume before anything else, and this adds a −1 bias to the entire stencil buffer, effectively inverting the shadows. This can be remedied by adding a "cap" surface to the front of the shadow volume facing the eye, such as at the front clipping plane. There is another situation where the eye may be in the shadow of a volume cast by an object behind the camera, which also has to be capped somehow to prevent a similar problem. In most common implementations, because properly capping for depth-pass can be difficult to accomplish, the depth-fail method (see below) may be licensed for these special situations. Alternatively one can give the stencil buffer a +1 bias for every shadow volume the camera is inside, though doing the detection can be slow.

There is another potential problem if the stencil buffer does not have enough bits to accommodate the number of shadows visible between the eye and the object surface, because it uses saturation arithmetic

Saturation arithmetic

Saturation arithmetic is a version of arithmetic in which all operations such as addition and multiplication are limited to a fixed range between a minimum and maximum value. If the result of an operation is greater than the maximum it is set to the maximum, while if it is below the minimum it is...

. (If they used arithmetic overflow

Arithmetic overflow

The term arithmetic overflow or simply overflow has the following meanings.# In a computer, the condition that occurs when a calculation produces a result that is greater in magnitude than that which a given register or storage location can store or represent.# In a computer, the amount by which a...

instead, the problem would be insignificant.)

Depth pass testing is also known as z-pass testing, as the depth buffer is often referred to as the z-buffer.

Depth fail

Around the year 2000, several people discovered that Heidmann's method can be made to work for all camera positions by reversing the depth. Instead of counting the shadow surfaces in front of the object's surface, the surfaces behind it can be counted just as easily, with the same end result. This solves the problem of the eye being in shadow, since shadow volumes between the eye and the object are not counted, but introduces the condition that the rear end of the shadow volume must be capped, or shadows will end up missing where the volume points backward to infinity.

Disable writes to the depth and color buffers.
Use front-face culling.
Set the stencil operation to increment on depth fail (only count shadows behind the object).
Render the shadow volumes.
Use back-face culling.
Set the stencil operation to decrement on depth fail.
Render the shadow volumes.

The depth fail method has the same considerations regarding the stencil buffer's precision as the depth pass method. Also, similar to depth pass, it is sometimes referred to as the z-fail method.

William Bilodeau and Michael Songy discovered this technique in October 1998, and presented the technique at Creativity, a Creative Labs developer's conference, in 1999. Sim Dietrich presented this technique at a Creative Labs developer's forum in 1999. A few months later, William Bilodeau and Michael Songy filed a US patent application

Software patent

Software patent does not have a universally accepted definition. One definition suggested by the Foundation for a Free Information Infrastructure is that a software patent is a "patent on any performance of a computer realised by means of a computer program".In 2005, the European Patent Office...

for the technique the same year, , entitled "Method for rendering shadows using a shadow volume and a stencil buffer" issued in 2002. John Carmack of id Software

Id Software

Id Software is an American video game development company with its headquarters in Richardson, Texas. The company was founded in 1991 by four members of the computer company Softdisk: programmers John Carmack and John Romero, game designer Tom Hall, and artist Adrian Carmack...

independently discovered the algorithm in 2000 during the development of Doom 3
Doom 3
Doom 3 is a science fiction horror video game developed by id Software and published by Activision. An example of the first-person shooter genre, Doom 3 was first released for Microsoft Windows on August 3, 2004. The game was later adapted for Linux, as well as being ported by Aspyr Media for Mac...

. Since he advertised the technique to the larger public, it is often known as Carmack's Reverse.

Exclusive-or

Either of the above types may be approximated with an exclusive-or

Exclusive disjunction

The logical operation exclusive disjunction, also called exclusive or , is a type of logical disjunction on two operands that results in a value of true if exactly one of the operands has a value of true...

variation, which does not deal properly with intersecting shadow volumes, but saves one rendering pass (if not fill time), and only requires a 1-bit stencil buffer. The following steps are for the depth pass version:

Disable writes to the depth and color buffers.
Set the stencil operation to XOR on depth pass (flip on any shadow surface).
Render the shadow volumes.

Optimization

One method of speeding up the shadow volume geometry calculations is to utilize existing parts of the rendering pipeline to do some of the calculation. For instance, by using homogeneous coordinates
Homogeneous coordinates
In mathematics, homogeneous coordinates, introduced by August Ferdinand Möbius in his 1827 work Der barycentrische Calcül, are a system of coordinates used in projective geometry much as Cartesian coordinates are used in Euclidean geometry. They have the advantage that the coordinates of points,...

, the w-coordinate may be set to zero to extend a point to infinity. This should be accompanied by a viewing frustum
Viewing frustum
In 3D computer graphics, the viewing frustum or view frustum is the region of space in the modeled world that may appear on the screen; it is the field of view of the notional camera. The exact shape of this region varies depending on what kind of camera lens is being simulated, but typically it is...

that has a far clipping plane that extends to infinity in order to accommodate those points, accomplished by using a specialized projection matrix. This technique reduces the accuracy of the depth buffer slightly, but the difference is usually negligible. Please see 2002 paper Practical and Robust Stenciled Shadow Volumes for Hardware-Accelerated Rendering, C. Everitt and M. Kilgard
Mark Kilgard
Mark J. Kilgard is a graphics software engineer working at Nvidia.Prior to joining Nvidia, Mark Kilgard worked at Compaq and Silicon Graphics. While at Silicon Graphics, he authored the OpenGL Utility Toolkit, better known as GLUT, to make it easy to write OpenGL-based 3D examples and demos. The...

, for a detailed implementation.

Rasterization time of the shadow volumes can be reduced by using an in-hardware scissor test to limit the shadows to a specific onscreen rectangle.

NVIDIA
NVIDIA
Nvidia is an American global technology company based in Santa Clara, California. Nvidia is best known for its graphics processors . Nvidia and chief rival AMD Graphics Techonologies have dominated the high performance GPU market, pushing other manufacturers to smaller, niche roles...

has implemented a hardware capability called the depth bounds test that is designed to remove parts of shadow volumes that do not affect the visible scene. (This has been available since the GeForce
GeForce
GeForce is a brand of graphics processing units designed by Nvidia. , there have been eleven iterations of the design. The first GeForce products were discrete GPUs designed for use on add-on graphics boards, intended for the high-margin PC gaming market...

FX 5900 model.) A discussion of this capability and its use with shadow volumes was presented at the Game Developers Conference
Game Developers Conference
The Game Developers Conference is the largest annual gathering of professional video game developers, focusing on learning, inspiration, and networking...

in 2005.

Since the depth-fail method only offers an advantage over depth-pass in the special case where the eye is within a shadow volume, it is preferable to check for this case, and use depth-pass wherever possible. This avoids both the unnecessary back-capping (and the associated rasterization) for cases where depth-fail is unnecessary, as well as the problem of appropriately front-capping for special cases of depth-pass.

External links

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