What Are Ray Tracing Calculations?

Point of View

• Unlike the behavior of real light, which begins at a source and travels to the observer, ray tracing starts at the point of view and traces its way to the light source. The software traces many thousands of straight paths from the observer to build a realistic three-dimensional scene. Along the way, the rays encounter objects in the scene, such as walls, people and furniture. The point of view also determines the scene's perspective as the observer has a certain height and looks in a certain direction.
  
  

Objects

• Objects in scenes have different optical properties, including color, degrees of transparency, reflection and refraction. For example, a glass chandelier in the software may consist of dozens of transparent pieces. Light rays undergo refraction as they pass from air to glass and back again. This bends the angle of light rays according to Snell's Law. At obtuse angles inside the glass, light reflects, forming complex paths. The ray tracing software calculates how the angles change for all the light rays in the scene.
  
  

Light Sources

• A ray tracing program can have many light sources in a scene, each with its own color, intensity, direction and focus. A laser light source, for example, produces comparatively simple ray paths, as its light is tightly focused and has a single color and direction. A frosted bulb, on the other hand, is more complex, sending diffuse white light in all directions and creating large illuminated areas and shadows. The software calculates these lighting conditions ray by ray according to the properties of the source.
  
  

Complexity and Speed

• Because the ray tracing software evaluates thousands of complex paths, it takes many fast computers to generate, or render, a scene with a good level of detail. Since each frame of an animated movie might take many minutes of calculations, the movie might take weeks of computer time. For this reason, the movie's director plots the scene very carefully and runs tests at low resolution. These have a primitive, low-quality appearance but the computers can produce these scenes faster. If the director wants to change the scene, he does it at low quality. Once he finalizes the scene, he has the software generate the high-quality version.