Alternatives

Introduction


    While ray tracing certainly adds dynamic capabilities to the field of computer graphics, it also has it's limitations.  Other methods of rendering images that model light are also widely used.  Two such methods are ray casting and radiosity.   These methods incorporate a process that is similar to the one used in used in ray tracing, however, they vary in technique and capability.

Ray Casting

    Ray casting is a three-dimensional rendering technique that has served as a foundation in computer graphics, especially in computer games.  Like ray tracing, ray casting traces imaginary rays of light in order to render realistic images of model worlds.  The appeal of choosing ray casting, though, is its capability to render images at extremely high frame rates.
    In ray casting, 3-D worlds are described by 2-D maps which are overhead views of the worlds.  The designer assigns the location and orientation of the point of view (POV).  In order to simulate a realistic human viewpoint, the POV generally encompasses a 60-degree field of vision.  Light sources are then positioned within the model of the world.  Individual rays are traced, and intersections between the rays  and the 3-D world are determined.  This, in turn, determines what, in the 3-D world, should be visible from the POV.
    In its basic structure, the technique of ray casting closely resembles that of ray tracing.  But when a model is ray traced, rays of light are simulated in order to accurately  model real physical environments.  Rays "interact" with the objects they encounter, and light is reflected, refracted, and absorbed within a given space, before the color of an individual pixel is determined.  In ray casting, rays are sent from a viewpoint through pixels of the screen into the model scene and the intersections between the ray and the objects it hits are calculated.  The ray is not "recast" from that point, therefore, mirroring, refraction, and shadowing effects cannot be determined.
    When models are rendered using ray casting as opposed to ray tracing, the accuracy of the method is severly reduced.  Beacuse of the constraints it places on the 3-D world, ray casting sacrifices realism.  The usefulness of ray casting is that it can be performed in real time.   Ray casting is a useful tool in applications such as the preliminary steps of ray tracing, where speed is a primary concern. [This images is re-produced, with permission, from the Stanford Computer Graphics Lab Web Site @ http://graphics.stanford.edu. Reproduction is prohibited.]

Radiosity

Ray tracing certainly adds dynamic effects to the field of computer graphics.  It also has its limitations.  Radiocity is a technique that offers a short cut to ray tracing.  When an image is ray traced, individual rays move throughout a model world traveling from one surface to another.  However, each intersection between the ray  and a surface is only a dimensionless point.  In order to complete a rendering we must still approximate the amount of light that has been reflected, refracted, and absorbed by any given surface between the calculated points.  Radiosity is a way of tracing the general flow of light between two surfaces.  The technique is combined with Gouraud shading to smooth across the calculated patches of light.  The result is a technique that  has significantly different capablities than conventional ray tracing.
    Ray tracing is particularly good at rendering point light, specular reflection, and refraction.  Radiocity, on the other hand, effectively renders models containing area light sources, and can produce diffuse reflections, realistic shadows and color bleeding effects.  It is said that radiocity is view independent, this is perhaps the most important difference between radiocity and ray tracing.  View independence means that once radiocity calculartions have been performed for a given environment, the distribution of light throughout the entire environment is known.  This makes it possible to run walkthrough animations in near-real time!

Table of Contents