Computer Graphics Qualifying Exam
September 24, 2001
This exam consists of six questions. You are required to answer five questions
in the time allotted. Answer each question in a separate blue book.
- In 1988, Forsey and Bartels introduced Hierarchical B-Splines. This technique
was extremely important because it added a new shape representation with a
key feature. More recently, subdivision surfaces have emerged as a useful
and popular surface representation, because they too offer advantages over
- What is the key feature that Hierarchical B-Splines provided (over
shape representations that came before them)? Explain why is this so significant,
using an example to demonstrate your argument.
- Describe two difficulties with spline based surface representations
that are overcome with subdivision surfaces. Explain why these tasks are
difficult with splines.
- A graduate student is planning to write a renderer that will be very photo-realistic.
Her goal is to produce images that depict as many of the lighting effects
seen in the real world as possible, using only the current state of the art
in image synthesis algorithms.
Her advisor (who obviously knows little about graphics) insists that she
write her renderer without using any random number generator, as he believes
this will mean her results are more reproducible. This precludes her use
of several modeling and rendering algorithms.
Name some algorithms that she will not be able to use in her renderer.
Describe the effect that absence of each algorithm will have on the resulting
system's performance (in terms of image quality, computation time, effects
that can be produced, ...)
- In his 1997 paper, Peter Litwinowicz discusses many issues in creating
"painterly" video, and suggests computer vision algorithms that allow for
the automation of this task.
In a sequence of papers beginning with Winkenbach and Salesin's 1994 paper,
the simulation of a pen and ink style has proven to be a popular "artistic
Describe issues that would arise in doing "animated pen-and-ink", and
suggest how methods that appear in the stylized rendering literature, such
as those proposed by Litwinowicz, might be applied.
- The well-known REYES rendering architecture (which is the "standard" implementation
of Renderman) was introduced by Pixar scientists as an efficient and effective
way to produce very high-quality images. REYES introduced a new rendering
algorithm called mico-polygonalization in which geometric primitives were
broken into small (sub-pixel) sized polygons before being considered for shading.
Over the years, this style of "micropolygon-based" renderer has come in and
out of favor as the best choice for doing high-quality renderings.
Current trends in computing hardware might make a micropolygon renderer
a good choice. Discuss why this is true. Hint: consider how a micropolygon-based
renderer might be implemented on modern graphics hardware.
- Physical simulation is a good mechanism for creating some kinds of motion
in computer graphics, and a terrible one for other settings.
Describe four real-world physical effects for which (given the current
state of the art) physical simulation is an appropriate method for creating
motion, and situations in which each might be used. Give examples of at
least two situations where simulation would be a poor tool for creating
motion, and the reasons why.
- The Quicktime VR system, the Light-Field and Lumigraph systems, and Debevec's
Facade system represent three different approaches to image-based modeling
and rendering. For each approach, describe a situation to which it is well
suited. Also explain why the other approaches are not suited to each situation.
Your answer should consist of a description of three scenes, and for each
situation a description of why each system would or would not be appropriate.