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BACKGROUND OF THE INVENTION
The present invention relates to a technique for determining appropriate
luminance linearization of gray levels for sub-pixel positioning tasks.
Additionally, the present invention relates to a technique for optimizing
grayscale characters for specific display devices.
Traditional characters are analog in nature, their
shapes defined by smoothly-varying boundaries. With the advent of
raster-scan displays and printers, the analog letterforms--produced by
optical and mechanical methods--have been replaced with digital
representations which can only approximate their predecessors. This will
always be the case since character edges have frequencies of infinite
magnitude that can never be exactly reproduced with discrete devices. On
the other hand, since the visual system is band-limited, it is only
necessary to match the quality of the transmitter (i.e., the display
device) to the capabilities of the receiver (i.e., the visual system).
Unfortunately, the available resolution of most current display devices
pale in comparison to the resolving power of the visual system.
Furthermore, pixel point spread functions in display devices usually
differ substantially from the ideal reconstruction kernel (i.e., the sync
function).
An alternative to higher resolution is the use of grayscale technology,
where in addition to black and white pixels, a multitude of gray levels
are realizable. In general, if each pixel is represented with n bits,
2.sup.n different grayscales are available to each pixel (subject to
possible limitations of the display technology). Using gray pixels at the
edges of characters can achieve a more faithful representation of the
master character than any bi-level version could on the same grayscale
device.
Until recently, most text on raster displays used characters represented as
binary matrices, the ones and zeros corresponding to the black and white
dots to be displayed. Typically, only one set of characters was provided,
simple and tuned to the characteristics of the display. Lately, grayscale
technology has allowed the incorporation of gray pixels in the character
description, leading to a perceived quality improvement when comparing the
discrete version of a character with its analog predecessor. With the
advent of higher-resolution bi-level displays as well as grayscale
devices, there is more flexibility in font sizes and styles which are
achievable, but techniques still need to be developed to aid in the
production and evaluation of such fonts.
Numerous factors contribute to the perceived quality of digital characters
displayed on raster-scan devices such as cathode ray tubes. Due to the
characteristic differences between the various display technologies, it is
not possible to design a single set of characters that will have
acceptable image quality on all devices. Quite often, the only approach to
manufacturing suitable character sets for a particular display device is
to have a font designer iteratively modify the characters' bitmaps and
evaluate them on the screen, until the satisfactory results are obtained.
In order to replicate the success of those character matrices, the same
type of display must be used and under similar viewing conditions.
Standard filtering techniques are commonly used to generate a grayscale
character. In this manner, a high-resolution bi-level master character is
convolved with a digital filter and sampled to yield lower-resolution
grayscale character. A typical grayscale video display system is
disclosed, for example, in U.S. Pat. No. 4,158,200 issued June 12, 1979 to
Seitz et al. Other examples of grayscale generation are discussed in
Warnock, "The Display of Characters Using Gray Level Sample Arrays,"
Computer Graphics, Vol. 14, No. 3, July 1980, pp. 302-307, and in Kajiya
et al, "Filtering High Quality Text for Display on Raster Scan Devices,"
Computer Graphics, Vol. 15, No. 3, Aug. 1981, pp. 7-15. These references
are incorporated herein by reference.
For a particular grayscale display, the spatial resolution and number of
intensity levels available is predetermined for the grayscale character
generation process. However, many different filters can be used to
generate a character. Furthermore, even with a single filter, different
versions of the same character can be generated by shifting the sampling
grid of the filtered character relative to the origin of the master.
A technique is needed whereby grayscale linearizations may be tailored to
the response of the human visual system to grayscale character displays.
Additionally, in order to measure character quality objectively and
effectively, font designers need automated tools for both character
generation and image-quality evaluation. Although many systems have been
developed for generating characters, utilities for evaluating them are
sorely lacking. Accordingly, there is a need for systems which may be used
to generate and evaluate high-quality grayscale characters.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a method for determining appropriate
luminance linearization values for sub-pixel edge placement in a grayscale
display device. A first bipartite field having a white portion and a black
portion separated by a sharp transition line is displayed on a first area
of a display device. A second field having a white portion and a black
portion separated by an intermediate gray strip is displayed on a second
area of the display device adjacent the first area. The gray strip is
substantially parallel with the sharp transition line separating the black
and white portions of the first field and has a height which is determined
by the desired sub-pixel edge placement.
A viewer is positioned at a distance from the display calculated from the
height of the gray strip and a predetermined visual angle, and the
grayscale setting of the gray strip is varied. The grayscale setting which
minimizes apparent line discontinuities between the interface of black and
white portions of the first and second fields is selected, and a luminance
linearization value is set in accordance with the selection.
The predetermined visual angle may be obtained by placing an observer at
various arbitrary distances from the display device and varying the
grayscale settings. The distance at which the fewest number of grayscale
settings provides no apparent line discontinuities is determined. This
distance and the height of the gray strip then determine the desired
visual angle.
The response of a plurality of viewers may be measured so that an average
response may be calculated. This average response is useful for
determining appropriate factory settings for luminance linearization
values.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects, features and advantages of the present invention will become
apparent to the skilled artisan from the following detailed description of
the preferred embodiment, when read in view of the accompanying drawings,
in which:
FIG. 1 schematically illustrates a technique for generating grayscale
character images from a master character;
FIG. 2 illustrates a sampling grid used by the grayscale convolution filter
of FIG. 1 for generating grayscale character images;
FIGS. 3A-3E graphically illustrate various weighting schemes that may be
used in calculating pixel luminance values for a grayscale character
image;
FIG. 4 schematically illustrates a sampling grid including overlapping
sampling areas;
FIG. 5 shows a grayscale character image produced by filtering in
accordance with the sampling grid of FIG. 4;
FIGS. 6A and 6B illustrate a grayscale character image produced in
accordance with a particular sampling grid;
FIGS. 7A and 7B are similar to FIGS. 6A and 6B, respectively, and
illustrate a grayscale character image produced in accordance with a
sampling grid which is shifted with respect to a master character;
FIG. 8 illustrates a gray scale character display system;
FIG. 9 illustrates a system for modelling the generation, display, and
observation of a gray scale character image;
FIGS. 10A and 10B graphically illustrate pixel point spread functions used
in the system of FIG. 9;
FIG. 11 graphically illustrates an optical blur function used in the system
of FIG. 9;
FIG. 12 graphically illustrates a cortical blur function used in the system
of FIG. 9;
FIG. 13 schematically illustrates a system for generating grayscale
characters from ideal representations of character images;
FIG. 14 illustrates a CRT screen display useful in calibrating luminance
linearization for grayscale sub-pixel edge placement;
FIG. 15 illustrates a modified CRT screen display similar to that of FIG.
14.
FIG. 16 illustrates the method of determining the luminance linearization
values according to the present invention; and
FIG. 17 illustrates the method of calculating the visual angle according to
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A technique for generating grayscale character images is illustrated
schematically in FIG. 1. Briefly, the technique utilizes a conventional
master character generator 2 to provide a bi-level representation of a
master character. The master character generator may operate in a
conventional manner to generate high-resolution master characters.
Preferably, the outline around a particular character defined by a
parametric function is scan converted to produce a high precision bit
matrix representation. Of course, other well-known techniques of master
character generation such as imaging and various analytic methods are also
available.
A digital signal output from the master character generator 2 is input to a
grayscale convolution filter 4. The grayscale convolution filter 4
operates in a conventional manner to produce grayscale character images
which may be stored in a grayscale character memory 6 for future use.
The grayscale convolution filter 4 may compute an appropriate pixel
intensity setting by weighting contributions from an area of the master
character centered on the pixel. Referring now to FIG. 2, a
high-resolution bi-level master character M is overlaid with a sampling
grid G. The grid G comprises an array of sampling areas SA which may be
centered on pixels in a character display matrix. Each sampling area SA
further includes an array of individual samples.
In operation, the convolution filter 4 may compute pixel intensity values
by weighting intensity contributions from the individual samples within
the sampling area SA. The weighted sum of the intensity contributions from
the area centered on the pixel whose value is to be determined is rounded
to the nearest of the possible pixel intensity settings.
One simple weighting scheme is to equally weight the intensity contribution
by each sample within the sampling area. Samples from outside the sampling
area are given zero weight. This weighting scheme is illustrated
graphically in FIG. 3A. Other possible weighting schemes are graphically
illustrated in FIGS. 3B-3E. For example, in the weighting scheme
illustrated in FIG. 3B, samples from the central portion of the sampling
area are given greater weight than outlying samples. Samples from beyond
the sampling area are, again, given no weight. The filters illustrated in
FIGS. 3B and 3C appear in the aforementioned Warnock paper. FIG. 3D
illustrates a two-dimensional radially symmetric sinusoidal filter, and
FIG. 3E illustrates a two-dimensional radially symmetric gaussian filter.
Although each illustrated filter is symmetrical, asymmetrical filters may
be used in appropriate settings, and in fact need not be generated
analytically. The grayscale character images generated by the grayscale
convolution filter 4 will, of course, depend upon the particular filtering
scheme which is used.
Although the individual sampling areas SA of FIG. 2 do not overlap, actual
sampling grids will usually include overlapping sampling areas as
illustrated in FIG. 4. FIG. 5 illustrates a computed grayscale character
image resulting from filtering the master character in accordance with the
sampling grid represented schematically in FIG. 4. It will be appreciated
that the number of sampling areas illustrated in FIG. 4 is reduced for
purposes of illustration.
The position of the sampling grid with respect to the master character will
also affect the computed grayscale character image. FIGS. 6A and 6B
schematically illustrate a sampling grid and the resultant grayscale
image, respectively. FIGS. 7A and 7B schematically illustrate a grayscale
image computed from the sampling grid of FIG. 6A which is shifted with
respect to the master character. As can be seen from a comparison of FIGS.
6B and 7B, the computed grayscale image is clearly affected by sampling
grid position. Orientation of the sampling grid will likewise affect the
computed grayscale image.
Turning to FIG. 8, one conventional manner of displaying grayscale image
patterns will now be described. This description is, of course, merely
exemplary. The skilled practitioner will appreciate that alternate schemes
are available.
A character generator 8 may receive a character code from a microprocessor
(not shown) or the like. The character code may be processed by the
character generator 8 to obtain an address value. This address value is
then used as a look-up value in a character font memory wherein grayscale
character information is stored. Alternatively, the grayscale character
information may be calculated in real time by the character generator
directly from the master character information. Such a technique is
discussed in Naiman et al, "Rectangular Convolution for Fast Filtering of
Characters," Computer Graphics, Vol. 21, No. 4, July 1987, pp. 233-242,
which is hereby incorporated by reference.
The character generator 8 preferably generates a serial stream of digital
image point values corresponding to the grayscale intensity settings. Of
course, in appropriate systems, a parallel image point signal may be used.
The serial stream of image point intensity setting values from the
character generator 8 is supplied to a digital-to-video converter 10 which
converts the digital stream to an analog video signal. The analog video
signal would of course include an appropriate horizontal synchronization
rate and vertical blanking interval. The analog video signal, in turn,
controls a display device 12 which displays grayscale characters in
response to the analog video signal. The grayscale character may then be
observed by a viewer V.
As is well known in the art, the relationship between pixel intensity
settings and the luminance values actually realized on a display device is
nonlinear. Several techniques have been developed for compensating for
luminance non-linearities. Examples of such techniques are described in
Catmull, "A Tutorial on Compensation Tables," Computer Graphics, Vol. 13,
No. 2, Aug. 1979, pp. 1-7, and in Cowan, "An Inexpensive Scheme for
Calibration of a Colour Monitor in Terms of CIE Standard Coordinates,"
Computer Graphics, Vol. 17, No. 3, July 1983, pp. 315-321.
Although recent indications suggest that a single compensation table may be
inadequate for the entire display surface, it is usually adequate for any
localized area. Accordingly, a linearization table may be provided between
the character generator 8 and the digital-to-video converter 10 to
compensate for nonlinearities in display device luminance. The luminance
produced on a display surface is also somewhat dependent on adjacent pixel
settings. Additional factors such as shadow mask interference (in color
monitors) may also affect pixel luminance.
For each pixel, an area on the screen is illuminated, wherein the intensity
profile, also known as the point spread function, is centered on the pixel
location and decreases monotonically from the center. Modelling a pixel
point spread is somewhat difficult. Fortunately, the spectral power
distribution of screen phosphors is invariant over emission levels, and
the intensity profile is scale invariant, i.e., the intensity profile
maintains its shape at different settings, module a multiplicative factor.
Difficulties arise, however, due to the fact that the intensity profile is
spatially variant, i.e., it may have a different shape in a different
portion of the display. Furthermore, pixel point spread functions are
designed to overlap with those of neighboring pixels and, thus, the
intensity profile may not be spatially independent.
Typically, a single point spread function will be determined as a general
description of the point spread at all portions of the screen.
FIG. 9 illustrates a system for modelling the generation, display, and
observation of grayscale character images. A grayscale character generator
14 includes a master character generator 16 which provides a series of
digitized signals representative of the master character. These digitized
signals are supplied to a grayscale processor 18 for grayscale filtering
and resampling in the manner discussed above. Accordingly, the output of
the grayscale processor 18, and thus the output of grayscale character
generator 14, is a set of pixel intensity settings corresponding to the
computed grayscale values.
The pixel intensity settings are supplied to a display model 20.
Preferably, the display model includes a luminance linearization circuit
22 and a point spread filter 24 connected in series. In order to obtain an
indication of the light pattern on a display surface when a grayscale
character is presented, a luminance linearization function L is
implemented in the luminance linearization circuit 22. The output of the
linearization circuit 22 is then convolved with the pixel point spread
function.
The luminance linearization function L tailors the intensity setting
request from the grayscale processor to the physical characteristics of a
particular display device. In a display model, however, it is possible to
assume that the request of the grayscale processor was actually met by the
display device. Accordingly, the point spread filter may be applied
directly to the output of the grayscale processor, as indicated in FIG. 9.
Of course, the linearization function must be applied before sending the
intensity request to the screen of an actual display device.
For simplicity, it may be assumed that pixel luminance is spatially
invariant with respect to position on the CRT screen and is independent of
adjacent pixel settings. In other words, an assumption may be made that
actual pixel luminance depends only upon the intensity setting and is
independent of the particular pixel position on the screen and the
intensity setting of adjacent pixels.
The linearized gray scale image is convolved with the pixel point spread
function by the point spread filter 24 to produce a signal S which is
representative of the light stimulus coming from the display device. A
pixel point spread function for a typical monochrome gray scale display
device is graphically illustrated in FIG. 10A. FIG. lOB illustrates the
point spread function for one type of color monitor displaying a white
pixel. Of course, other color monitors would include different pixel point
spread functions.
Typically, a single point spread function may be determined which generally
characterizes the entire display. It is also possible, however, to
determine different point spread functions for various portions of the
screen. Of course, in the limit, separate point spread functions may be
determined for each pixel on the display device, and functional
relationships between adjacent pixels may be developed.
By specifying the resolution at which the point spread function is measured
or, alternatively, by using an analytic representation of the pixel point
spread function, the precision at which the stimulus signal S is given can
be controlled. Analytic representations of pixel point spread functions
are discussed, for example, in Infante, "Ultimate Resolution and
Non-Gaussian Profiles in CRT Displays," Proceedings of the SID, Vol. 27,
No. 4 (1986), pp. 275-280.
Once a useful representation of a character displayed on a gray scale
device is obtained, it is desired to determine what a typical human eye
would actually observe in terms of the pattern imaged on the retina when
viewed from a given distance. Additionally, it is desired to determine how
a typical human visual system responds to the stimulus in terms of
sensitivity to the incoming frequencies. The former relates to the optics
of the ocular media, whereas the latter relates to psychophysical
measurements of cortical image processing.
Accordingly, the stimulus signal S is supplied to a visual system model 26.
The visual system model 26 includes an optical blur function circuit 28
which models the optical aspects of the visual system. The optical blur
function V.sub.o, or optical point spread, describes how a point light
source is imaged onto the retina, in terms of visual angle. As is well
known in the art, visual angle is the angular subtense of an image
measured at the retina. Although an appropriate optical blur function
depends on the diameter of the pupil and the spectral power of the light,
a single optical blur function suffices for monochromatic, broadband light
sources, when the eye is in good focus and has a pupil diameter of 2 mm.
See Westheimer, G., "The Eye as an Optical Instrument," Handbook for
Perception and Human Performance, Vol. 1, Sensory Processes and
Perception, Eds. Boff, K. R., Kaufman, L., and Thomas, J. P., John Wiley &
Sons, 1986, pp. 4.1-4.20.
A two-dimensional optical blur function V.sub.o representing the filtering
of a point light source passing through the lens of the eye is illustrated
in FIG. 11. The dimensions of the grid are 30.times.30 minutes of arc, and
the blur function approaches zero at approximately two minutes of arc from
the center. Convolving the stimulus representation S with the optical blur
function V.sub.o yields a description of the lens-blurred character image
I.sub.o on the retina. Like the pixel point spread function discussed
above, error in the character image representation can be controlled by
setting the precision at which the optical blur function V.sub.o is
defined.
Additional filtering occurs due to photoreceptor sampling and cortical
image processing. It is known that the combined effects of optical and
psychophysical filtering are captured by the human contrast sensitivity
function, which describes the band-pass spatial filtering properties
imposed upon every stimulus the visual system encounters. Like optical
blur, contrast sensitivity depends on the amount of light entering the eye
as well as the temporal frequency of the stimulus S.
For a particular contrast sensitivity function, a band-pass point spread
function V.sub.c may be derived which, when convolved with the stimulus
signal S yields a representation I.sub.c of the image after luminance
contrast that the visual system cannot detect has been filtered out. A
cortical blur circuit 30 is provided to convolve the stimulus signal S
with the cortical blur function V.sub.c. A two-dimensional cortical blur
function V.sub.c derived from a human contrast sensitivity function is
illustrated in FIG. 12. The dimensions of the grid are 30.times.30 minutes
of arc. The cortical blur function V.sub.c becomes negative at
approximately five minutes of arc from the center and returns to zero at
approximately fifteen minutes of arc.
Given the optical point spread and the contrast sensitivity functions
appropriate for the viewing conditions, the visual system response may be
represented by convolving the stimulus signal S with filters V.sub.o and
V.sub.c to yield signals I.sub.o and I.sub.c, which correspond to the
images perceived by the visual system, either in terms of the physical
mapping of the retina, or the psychophysical response to the stimulus,
respectively. Although the illustrated visual system model 26 includes
only an optical blur circuit and a cortical blur circuit, additional
circuits which model intermediate stages of human visual processing may
also be provided.
Turning again to FIG. 9, a computing system 32 receives the signals I.sub.o
and/or I.sub.c as inputs. This computing system may be any appropriate
system including, for example, a processor and associated circuitry. A
feedback control loop is provided between the computing system 32 and the
grayscale processor 18, and between the computing system 32 and the
luminance linearization circuit 22.
In operation, the computing system may compare the signals I.sub.o and
I.sub.c against ideal representations. These comparisons could be used in
an iterative process wherein one or more of the parameters controlling the
generation and display may be adjusted to optimize the modelling system
output. For example, the computing system might instruct the grayscale
processor to vary filter type, shift or re-orient the sampling grid,
adjust overlap of sampling areas, etc. Linearization values may also be
adjusted. In this way, the individual portions of the system may be
tailored for optimal performance. For example, for a given display device,
an optimal grayscale generator may be determined. Additionally, an
appropriate luminance linearization for a certain display device may be
determined.
A parallel output may be provided from the grayscale processor to a
conventional display device. Thus, the product of the modelling system may
be visually monitored and an observer of the conventional display device
may actively take part in the system optimization. Similarly, the output
of the luminance linearization circuit may be provided to a
digital-to-video converter for display.
Referring now to FIG. 13, a modelling system may be used to backsolve for
grayscale character images. If a representation of an ideal retinal or
cortical image is available, this representation may be used in a visual
system model 34 to solve for an output signal S.sub.I which when convolved
with the visual system filter would match the ideal image. The output
signal S.sub.I of the visual system model would then represent the ideal
stimulus to the visual system.
The signal S.sub.I may then be provided to the display model 36. The signal
S.sub.I would be used to solve for a signal which when convolved with the
point spread function would provide the ideal stimulus signal S.sub.I. The
inverse of the luminance linearization function L would then be applied to
determine the ideal input to the display. The output signal G.sub.I from
the display device model 36 would then define the ideal grayscale
character image. This ideal grayscale character image may then be stored
in a grayscale storage device 38. In this way, a set of ideal grayscale
character images may be developed.
In accordance with another feature of the present invention, appropriate
linearization of grayscale intensity settings for accurately controlling
sub-pixel positioning of image edges, such as in grayscale characters, may
be determined. Furthermore, display devices may be interactively
calibrated for particular users.
Referring to FIGS. 14, 16 and 17, the left half of a CRT screen 40 may be
presented with a bipartite field including an upper white portion 42 and a
lower black portion 44. A sharp black/white transition 46 is provided
between the respective portions. The right half of the CRT screen in
provided with a similar bipartite field including an upper white portion
48 and a lower black portion 50. However, instead of a sharp transition
between the respective fields, an intervening strip 52 of gray pixels
separates the fields on the right half of the CRT screen. It should be
noted that the vertical line in FIG. 14 separating the left and right
halves of the screen is merely for illustrative purposes.
The pixel height of the intervening grayscale strip 52 depends upon the
desired sub-pixel image placement. For example, if a 50% sub-pixel
placement is desired, two grayscale pixel rows would be included in the
grayscale strip 52. For a 33% pixel placement, three pixel rows would be
included, and four pixel rows would be included for a 25% sub-pixel
placement, as will be discussed below in greater detail.
Prior to luminance calibration of a grayscale device, it is useful to
determine the visual angle at which grayscale images first begin working.
At great distances, all possible grayscale settings along a character edge
would appear the same due to filtering by the visual system. At very short
distances, on the other hand, an observer will always be able to delineate
the gray area. At some particular visual angle, only one or a few
grayscale settings will align the character image edge.
In order to determine the appropriate visual angle for a particular viewer,
the viewer will be placed at a first arbitrary distance from the CRT
screen and the grayscale setting of the intermediate gray strip 52 is
varied over a range of settings. The range of grayscale settings at which
the apparent black/white transition between white portion 48 and black
portion 50 appears aligned with the sharp transition 46 is determined. The
viewer is then moved to a second arbitrary distance and the range of
effective grayscale settings is again determined. The distance at which
the range of effective grayscale settings is minimized and the height of
the gray strip determine the visual angle at which sub-pixel positioning
through grayscale imaging begins working.
Once an appropriate visual angle has been determined, the luminance of a
display device may be calibrated with respect to a particular user in
terms of appropriate linearization of luminance values for sub-pixel edge
placement. If a 50% sub-pixel edge placement is desired, the gray strip 52
will include two rows of pixels, with one row being vertically displaced
above the black/white transition 46 and one row being vertically displaced
below the black/white transition 46.
A viewer is set at a distance determined by the pixel height and the
predetermined visual angle. The grayscale setting for the pixels of the
gray area 52 is adjusted and the gray scale setting which minimizes
detectability of any edge discontinuities between black/white transition
46 and the border between white portion 48 and black portion 50 is
determined. This determined grayscale setting corresponds to an
appropriate grayscale setting for 50% sub-pixel edge placement.
The process set forth above could then be repeated for various sub-pixel
edge placements. If it was desired to determine appropriate grayscale
settings for 33% edge placement, a third row of grayscale pixels may be
added to the gray strip 52. In order to maintain the predetermined visual
angle, the distance at which the viewer is positioned is adjusted to
accommodate the increased height of the gray strip 52. Again, the
grayscale intensity setting of the gray strip 52 is adjusted until edge
detectability is minimized between the black/white transition 46 and the
apparent border of the white portion 48 and the black portion 50. The
grayscale setting which minimizes edge discontinuity determines the
appropriate linearization value for 33% sub-pixel edge placement.
For a 25% sub-pixel edge placement, four rows of pixels will be used in the
gray strip 52, with, for example one row of pixels vertically displaced
above the black/white transition 46 and three rows of pixels vertically
displaced below the black/white transition 46. After adjusting viewer
position to maintain the predetermined visual angle, the luminance
linearization value for 25% sub-pixel edge placement is determined. Of
course, this process may be repeated for additional sub-pixel edge
placements.
It is noted that the transitions between black and white portions in FIG.
14 are along a horizontal line. In this way, the display illustrated in
FIG. 14 is tailored to the characteristics of a conventional CRT monitor
which includes a horizontal scan direction. If the transition between the
black and white portions was along a vertical line, the edge would often
appear less sharp due to inherent limitations of CRT display technology.
Additionally, in order to provide two edge discontinuities, the image
illustrated in FIG. 14 may be rotated to obtain the image of FIG. 15. With
the provision of two possible edge discontinuities, more accurate
luminance linearization settings be determined. Additionally, by centering
the gray strip 52 with respect to the CRT screen, any adverse effects
resulting from slight vertical displacement of the scanning beam during a
horizontal scan are minimized.
The processes described above in connection with FIGS. 14 and 15 may be
repeated for a number of individuals. An average grayscale image edge
response may then be calculated. In turn, this average may be used to
determine appropriate factory linearization settings for grayscale display
devices.
Although the preceding discussion focused on CRT screen displays, the
features and advantages of the present invention may likewise be applied
to other appropriate grayscale display technologies.
The principles, preferred embodiments and modes of operation of the present
invention have been described in the foregoing specification. The
invention which is intended to be protected herein, however, is not to be
construed as being limited to the particular forms disclosed, since these
are to be regarded as illustrative rather than restrictive. Variations and
changes may be made by those skilled in the art without departing from the
spirit of the invention.
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