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...Image
Processing Fundamentals
8.
Displays
The
displays used for image
processing--particularly the display
systems used
with
computers--have a number of characteristics
that help determine the
quality of
the
final image.
8.1
REFRESH RATE
The
refresh
rate is defined as
the number of complete
images that are written to
the
screen
per second. For standard video
the refresh rate is fixed at
the values given in
Table
3, either 29.97 or 25 images/s.
For computer displays the
refresh rate can
vary
with common values being 67
images/s and 75 images/s. At values above
60
images/s
visual flicker is negligible at
virtually all illumination
levels.
8.2
INTERLACING
To
prevent the appearance of
visual flicker at refresh
rates below 60 images/s,
the
display
can be interlaced as described in Section
2.3. Standard interlace for
video
systems
is 2:1. Since interlacing is not
necessary at refresh rates above 60
images/s,
an
interlace of 1:1 is used
with such systems. In other
words, lines are drawn in
an
ordinary
sequential fashion:
1,2,3,4,...,N.
8.3
RESOLUTION
The
pixels stored in computer
memory, although they are
derived from regions
of
finite
area in the original scene
(see Sections 5.1 and
7.5), may be thought of
as
mathematical
points having no physical
extent. When displayed, the
space between
the
points must be filled in.
This generally happens as a
result of the finite spot
size
of
a cathode-ray tube (CRT).
The brightness profile of a
CRT spot is approximately
Gaussian
and the number of spots that
can be resolved on the
display depends on
the
quality of the system. It is
relatively straightforward to obtain
display systems
with
a resolution of 72 spots per inch
(28.3 spots per cm.) This
number
corresponds
to standard printing conventions. If
printing is not a consideration
then
higher
resolutions, in excess of 30 spots
per cm, are
attainable.
9.
Algorithms
In
this Section we will describe
operations that are
fundamental to digital
image
processing.
These operations can be divided into
four categories: operations
based
on
the image histogram, on simple
mathematics, on convolution, and
on
mathematical
morphology. Further, these
operations can also be described in
terms
44
Table of Contents:
- Introduction
- Digital Image Definitions:COMMON VALUES, Types of operations, VIDEO PARAMETERS
- Tools:CONVOLUTION, FOURIER TRANSFORMS, Circularly symmetric signals
- Perception:BRIGHTNESS SENSITIVITY, Wavelength sensitivity, OPTICAL ILLUSIONS
- Image Sampling:Sampling aperture, Sampling for area measurements
- Noise:PHOTON NOISE, THERMAL NOISE, KTC NOISE, QUANTIZATION NOISE
- Cameras:LINEARITY, Absolute sensitivity, Relative sensitivity, PIXEL FORM
- Displays:REFRESH RATE, INTERLACING, RESOLUTION
- Algorithms:HISTOGRAM-BASED OPERATIONS, Equalization, Binary operations, Second Derivatives
- Techniques:SHADING CORRECTION, Estimate of shading, Unsharp masking
- Acknowledgments
- References