Computing & the Arts

2. Computer Graphics and Drawing

Ray tracing: Rendering realistic pictures of a scene

We need to model (create) the scene (objects), its projection on a 2D image (retinal plan or window), and the eye or camera (the observer) which will capture the rendered image (viewpoint, focal length).

From [Rademacher:Ray:1997]

The color of a pixel in a window is due to a light ray hitting a colored object and bouncing back toward the eye through that pixel locus.


Tracing from the light source: many rays are useless.	Approximation: Tracing from the eye, through the window, towards an object, directly to the light source.

Issue: Computational complexity versus (perceived) Realism.


More realistic: Follow the ray thru reflections and refractions (partial transparency).	Modeling of reflection and refraction via Snell's Laws (geometric optics), where each medium (outside vs. object's inside) has a particular (homogeneous) refractive index (ratio of the speed of light in a vacuum over its speed in this medium).

Bidirectional Reflectance Distribution Function (BRDF):

Gives the reflectance of a target as a function of illumination geometry and viewing geometry.

From BRDF@BU

N.B.: The same target can look different under varying illumination conditions ...

We can approximate complex surface (or volumetric) geometry by sampling the BRDF of known material, such as velvet:

From [Rusinkiewicz:BRDF:1997]

From [Rusinkiewicz:BRDF:2004]

Power Point Presentation

Another "trick:" Texture mapping

Mapping a rectangular texture onto a sphere (globe).
[Rademacher:Ray:1997]

N.B.: The implicit goal of CG is not to necessarily mimic (from approximations to exactness) the physics of geometric optics for real scenes, light sources and eyes or cameras, but rather to render "pictures which look real." (Hence the need to also understand human visual perception.)

Lecture Notes by Fredo Durand & Julie Dorsey (MIT):

Photorealism versus non-photorealism in CG

Course notes from "The Art and Science of Depiction," MIT, Spring 2001.

References

Durand:Art:2001

Durand, F. & Dorsey, J., "The Art and Science of Depiction," MIT, Spring 2001.

Rademacher:Ray:1997

Rademacher, P., "Ray Tracing: Graphics for the Masses," ACM Crossroads, Summer 1997.

Rusinkiewicz:BRDF:1997

Rusinkiewicz, S., "A Survey of BRDF Representation for Computer Graphics," Winter 1997.

Rusinkiewicz:BRDF:2004

Lawrence, J., Rusinkiewicz, S., & Ramamoorthi, R., "Efficient BRDF Importance Sampling Using a Factored Representation," SIGGRAPH 2004.

Links

Cohen:LectureNotes:1999

Cohen, J. "Lecture Notes on Rendering Techniques," 1999.

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Last updated: Nov. 19, 2007