Last update: Aug. 31, 1998

Publications on Ray Tracing for Light Rendering :

• Priority-driven Ray Tracing, 1997
• A Terrain Rendering Method using Vertical Ray Coherence, 1997.

BibTeX references.

* Adapted from the HyperGraph Notes on Ray Tracing (<- see these for more details & images).

In ray tracing, a ray of light is traced in a backwards direction. That is, we start from the eye or camera and trace the ray through a pixel in the image plane into the scene and determine what it hits. The pixel is then set to the color values returned by the ray.

If the ray hits an object, we want to know if that point on the object is in a shadow. So, when the ray hits an object, a secondary ray, called a "shadow" ray, is shot towards the light sources. If this shadow ray hits another object before it hits a light source, then the first intersection point is in the shadow of the second object.

Also, when a ray hits an object, a reflected ray is generated which is tested against all of the objects in the scene. If the reflected ray hits an object then a local illumination model is applied at the point of intersection and the result is carried back to the first intersection point.

If the intersected object is transparent, then a transmitted ray is generated and tested against all the objects in the scene. As with the reflected ray, if the transmitted ray hits an object then a local illumination model is applied at the point of intersection and the result is carried back to the first intersection point.

The reflected/transmitted rays can generate other reflected/transmitted rays that can generate other reflected/transmitted rays, etc.

Radiosity

Computes the interchange of llight from diffuse surfaces using an approach based on heat transfer principles. The energy equilibrium of a system is modelled independently on the viewer's position and can be therefore computed in a pre-processing step. After the later is performed, view-dependent visible surface determination and shading are performed.

Similarly to ray tracing, efficiency is a problem and different approaches have been proposed in the past to alleviate it. Progressive Refinement, similarly to PSS, generates intermediate images. Importance-Driven radiosity incorporates view-dependency by assigning and importance value to patches in the scene.

Ray Tracing

Computes the color of each pixel by tracing a light ray backwards from the viewer's eye through the scene while determining which object (if any) is intersected by the ray. Reflection and/or transmission of rays is modelled according to the object's surface properties. Different strategies to make the process more efficient have been proposed, such as Progressive Screen Sampling (PSS).

Priority-Driven Ray Tracing (PDRT) assign a priority value to each newly spawned ray and insert it into a priority queue. The priority value can be based on various criteria, such as: contribution, type (primary, reflection, transmission, shadow), origin, tree depth, object complexity, surface properties, light source attributes (intensity, color), etc. PDR is controllable by the user, contrarilly to classical approaches. The priority mechanism gives the user the possibility to steer the rendering and to deliver intermediate images.

Basic Definitions :

• Primary ray: Ray casted from the eye, through a pixel, onto the scene.
• Nth-order ray : Refelected or transmitted rays.
• Shadow-feelers : Rays sent from the intersection point to all light sources..
• Ray tree : Set or rays spawned as descendents of a primary ray. Such a tree is created at each pixel of the image.
• Ray forest : Set of all ray trees.
• Ray casting : Breadth-first traversal of the ray forest which stops at the first level.

Ray plane : Plane perpendicular to the sea level plane and passing through the COP (Center Of Projection).

Ray line : Line of intersection of the ray plane and the projection plane.

Vertical ray coherence : If 2 rays pass through the same ray line, they pass over the identical set of points on the DTM base plane. This property is used, once a ray is casted, as a starting point to cast others rays.

Page created & maintained by Frederic Leymarie, 1998.
Comments, suggestions, etc., mail to: leymarie@lems.brown.edu