Software that renders images from three-dimensional data may execute the task using a variety of mathematical methods. The simplest forms of rendering might show the objects in your three-D data only as lines. More complex rendering will shade the objects according to light source information. The most sophisticated (and nearly photo-realistic) rendering is accomplished by way of raytracing.
Software that renders a scene with ray-tracing considers several variables in constructing the image. First it establishes the location of the "eye" of the viewer. Then it considers the directions in which the eye is looking, and the width of the field of view. The image is built one pixel at a time. Each pixel corresponds to a "mathematical ray" going from the eye out into the scene. The software tries to determine the color and value of each individual pixel by calculating what, if anything, that particular pixel's ray hits in the scene.
In some ways, each ray is like the inverse of a light beam flowing into the eye. If the mathematical ray emanating out of the computer eye hits a reflective object, it will bounce away in a new direction. In it's journey through the 3-D scene, a ray may bounce off of several objects, be refracted as it passes through transparent or translucent objects, be attenuated as it passes simply through distance (as in fog). After the computer has traced the path of the ray through the entire scene, adding up all of the interactions that the ray had with objects in the scene, it can determine the color and value of the pixel in the field of view corresponding to that ray. That was pretty clear, no?
Simply put, raytracing gives one of the most accurate renderings possible, nearly perfectly real. A few things that cannot be rendered even with raytracing: Although a light source can be seen in the reflection on an object (like a mirror, say), light is not actually reflected - that is, you cannot use mirrors in a scene to actually redirect illumination in the scene. Raytracing all by itself cannot render depth of field, the blurriness of objects beyond the focus of the eye. In a raytraced rendering, all things are perfectly sharp.
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