Puff, Derek T. (1995)
(Biomedical Engineering, UNC-Chapel Hill)
"Human vs. Vision Model
Performance for Two Medical Image Estimation Tasks"
Under the direction of Stephen M. Pizer
A computed method for measuring medical image quality would allow
fast and economical evaluation of image acquisition and display systems
without the need for arduous, expensive human observer experiments.
It would help such a method to be predictive of human assessment if
it reflected the principles thought to govern the operation of the visual
system. This dissertation research implemented and tested the accuracy
of a measure of medical image quality that incorporates a model of human
vision in a simulation of human image interpretation. It was hypothesized
that the model, by performing in a way that reflected the inherent capabilities
and limitations of a human, would be predictive of human performance
as physical properties of the image varied. The core model of shape
perception, a theory for the representation of objects that may serve
as a fundamental perceptual basis for a number of medical image interpretation
tasks, was applied in computing estimates of the depth of a vessel stenosis
in an angiogram and the position of a radiation treatment field relative
to the spinal cord in a portal image. Parameters of those imaging systems
that have significant effects on the physical characteristics of the
images, such as the amount of imaging system blur or the extent of contrast-enhancing
processing, were systematically varied. Model and human task performance
was studied as a function of the parameters in order to assess the extent
to which the model predicted the human results. In most instances, the
analysis suggested that the conformance of the model and human data
was not sufficient to allow use of the visual model as proposed. The
conclusion explores the potential factors in those discrepancies and
reiterates the claim that image quality assessments based upon fundamental
principles of visual perception might eventually be utilized successfully
for medical image interpretation tasks.
Rademacher, Pablo
M. (2003)
"Measuring the Perceived Visual Realism of Images"
Under the direction of Gary Bishop
Electronic
copy available
One of the main goals of computer graphics research is to develop techniques
for creating images that look real - i.e., indistinguishable from photographs.
Most existing work on this problem has focused on image synthesis methods,
such as the simulation of the physics of light transport and the reprojection
of photographic samples. However, the existing research has been conducted
without a clear understanding of how it is that people determine whether
an image looks real or not real. There has never been an objectively
tested, operational definition of realism for images, in terms of the
visual factors that comprise them. If the perceptual cues behind the
determination of realism were understood, then rendering algorithms
could be developed to directly target these cues.
This work introduces an experimental method for measuring the perceived
visual realism of images, and presents the results of a series of controlled
human participant experiments. These experiments investigate the following
visual factors: shadow softness, surface smoothness, number of objects,
mix of object shapes, and number of light sources. The experiments yield
qualitative and quantitative results, confirm some common assertions
about realism, and contradict others. They demonstrate that participants
untrained in computer graphics converge upon a common interpretation
of the term real, with regard to images. The experimental method can
be performed using either photographs or computer-generated images,
which enables the future investigation of a wide range of visual factors.
Rajgopal, Suresh (1992)
"Spatial Entropy--A Unified Attribute to Model Dynamic Communication in
VLSI Circuits"
Under the direction of Kye S. Hedlund
Department of Computer Science Technical Report TR92-041
Electronic copy available
This dissertation addresses the problem
of capturing the dynamic communication in VLSI circuits. There are several
CAD problems where attributes that combine behavior and structure are
needed, or when function behavior is too complex and is best captured
through some attribute in the implementation. Examples include, timing
analysis, logic synthesis, dynamic power estimation, and variable ordering
for binary decision diagrams (BDDs). In such a situation, using static
attributes computed from the structure of the implementation is not
always helpful. Firstly, they do not provide sufficient usage information,
and secondly they tend to exhibit variances with implementations which
is not desirable while capturing function behavior.
The contribution of this research is a new circuit attribute called
spatial entropy. It models the dynamic communication effort in
the circuit by unifying the static structure and the dynamic data usage.
Quantitatively, spatial entropy measures the switching energy in a physical
(CMOS) implementation. A minimum spatial entropy implementation is a
minimum energy implementation. For the purposes of this dissertation
we restrict our scope to combinational circuits. We propose a simple
procedure to estimate spatial entropy in a gate level circuit. It is
characterized in extensive detail and we describe why it is difficult
to compute spatial entropy accurately. We show how it can also be defined
at other levels of abstraction.
We illustrate applications of spatial entropy in BDD variable ordering,
a problem that has traditionally relied on static attribute based solutions.
We also show empirically that spatial entropy can track function behavior
through implementations, by using it to measure gate-count complexity
in Boolean functions.
Ramamurthy, Srikanth (1997)
"A Lock-Free Approach to Object Sharing in Real-Time Systems."
Under the direction of James Anderson
Electronic copy available
This work aims to establish the viability
of lock-free object sharing in uniprocessor real-time systems. Naive
usage of conventional lock-based object-sharing schemes in real-time
systems leads to unbounded priority inversion. A priority inversion
occurs when a task is blocked by a lower-priority task that is inside
a critical section. Mechanisms that bound priority inversion usually
entail kernel overhead that is sometimes excessive.
We propose that lock-free objects offer an attractive alternative
to lock-based schemes because they eliminate priority inversion and
its associated problems. On the surface, lock-free objects may seem
to be unsuitable for hard real-time systems because accesses to such
objects are not guaranteed to complete in bounded time. Nonetheless,
we present scheduling conditions that demonstrate the applicability
of lock-free objects in hard real-time systems. Our scheduling conditions
are applicable to schemes such as rate-monotonic scheduling and earliest-deadline-
first scheduling.
Previously known lock-free constructions are targeted towards asynchronous
systems; such constructions require hardware support for strong synchronization
primitives such as compare-and-swap. We show that constructions for
uniprocessor real-time systems can be significantly simplified--and
the need for strong primitives eliminated--by exploiting certain characteristics
of real-time scheduling schemes.
Under lock-based schemes, a task can perform operations on many shared
objects simultaneously via nested critical sections. For example, using
nested critical sections, a task can atomically dequeue an element from
one shared queue and enqueue that element in another shared queue. In
order to achieve the level of functionality provided by nested critical
sections, we provide a lock-free framework that is based on a multi-word
compare-and-swap primitive and that supports multi-object accesses |
the lock-free counterpart to nested critical sections. Because multi-word
primitives are not provided in hardware, they have to be implemented
in software. We provide a time-optimal implementation of the multi-word
compare-and-swap primitive.
Finally, we present a formal comparison of the various object-sharing
schemes based on scheduling conditions, followed by results from a set
of simulation experiments that we conducted. Also, as empirical proof
of the viability of lock-free objects in practical systems, we present
results from a set of experiments conducted on a desktop videoconferencing
system.
Raskar, Ramesh (2002)
"Projector-Based Three Dimensional Graphics"
Under the direction of Henry Fuchs and Gregory Welch
Department of Computer Science Technical Report TR02-046
