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<html><head><title>3d ray tracing animation applet: ray tracing</title>

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<font size=5 face="Arial"><p>Ray Tracing
<font size=3 face="Arial"><p>
Raytracing is a technique used to produce high quality images. It is based on
the physics of rays of light propagating through a scene and interacting with objects.

<p>A raytracer algorithm has two main functions. An intersection routine determines
if a ray cast from a point intersects any objects. A shading routine determines the color
of a ray based on lighting, and object reflection properties.

<p>Raytracing is a global illumination model. The pixel colors in an image are
individually determined by tracing rays of light as they propagate through a scene.
Rays are cast from a viewpoint, through each pixel on the screen, and tested for
intersection with objects. From the intersection point closest to the viewpoint, shadow
feeler rays are sent to each light source. The Phong reflection model is applied to
determine the intensity and color of the light that comes to the observer from that
intersection point.
[<a href="project_references.html#Owen">Phong</a>]

<p>The Phong model uses the sum of three types of light reflection at a point intersected:
Ambient, Diffuse, and Specular Reflection. It also takes into account light reflected
from other surfaces, and transmitted through the object.

<p>Red, blue, and green Ambiant reflection coefficients model background light, or light
reflecting from nearby matte surfaces.

<p>Diffuse reflection models light shining on a dull surface, where reflected light
is distributes in many directions. This light is independant of the position
of the observer. Instead, the amount of light reflected depends on the orientation of
the surface, relative to the light source. Diffuse light models the intrinsic color
of an object.

<p>Specular reflection models shiny, mirror-like surfaces. The intensity of light seen
by the observer depends on the viewing position relative to the surface.
A specular highight is produced. In addition to red, green, and blue specular
reflection coefficients, a specular exponent determines the 'shinyness' of the surface,
and the resulting size of the specular highlight.

<p>The raytracing algorithm implemented in 3DAA provides for multiple reflections
and transparency. It does so recursively, where at each intersection point, the
intersected object has some specular reflection, a 'reflection ray' is cast in
the direction of the most specular reflection. If the object intersected has some
transparency, a 'transparency ray' is cast into the scene. Both rays use the point
of intersection with the object, as the source of the 'eye ray' for the next
level of recursion. The color returned by recursive calls is incorporated
into the color computed by the Phong model at the original surface's intersection point
with the eye ray.

<p>Here is an excellent illustration of raytracing:
<br><a href="http://www.cs.helsinki.fi/group/goa/render/rt/rtrace1.html">
www.cs.helsinki.fi/group/goa/render/rt/rtrace1.html</a>.
[<a href="project_references.html#Owen">Owen</a>]

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