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Claims  |
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We claim:
1. In a computer generated graphic display system of the type which
generates image display data of a colored vector on a colored background,
including data representing the intensity of each pixel of the display, a
raster scan graphic display for displaying the image data, and
anti-aliasing means for controlling the brightness of each displayed pixel
to smooth out "stair-step" effects inherent in raster scan graphic
displays, the improvement comprising gamma correction means supplied with
the anti-aliased display data of the colored vector for correcting for
non-linearity in the relationship between the pixel intensity data and the
actual intensity of the corresponding pixel of the raster scan display
independently of the background color.
2. In a computer generated graphic display system of the type which
generates anti-aliased image display data for display on a raster scan
graphic display and which includes a color look-up table for storing data
representative of a plurality of predetermined colors, the improvement
comprising gamma correction means for generating data representing a
plurality of gamma-corrected intensities for each red, green, and blue
component of the plurality of colors and wherein said color look-up table
stores such data as a function of the ratio of the intensity of the
foreground color of the display to the intensity of the background color
of the display, and wherein said plurality of intensities are located
within the color look-up table as a function of the anti-aliasing value
corresponding to the calculated intensity value such that a linear
relationship exists between successive anti-aliasing values and successive
intermediated colors progressing from background to foreground as the
colors appear on a raster scan graphic display, whereby independence
between anti-aliased foreground image data and existing background image
dated is facilitated.
3. The computer generated graphic display system as recited in claim 2
further comprising display memory means for storing display data
corresponding to each pixel location of the graphic display, with each
pixel location being represented by a plurality of intermediate order bits
for storing the color data of the foreground of the display, a plurality
of higher order bits for storing color data of the background of the
display, and a plurality of lower order bits for storing anti-aliased
intensity data which modifies the resulting pixel color appearing on the
display to a value inclusively between said foreground and said background
color values individually for each pixel location within said display
memory.
4. A computer generated graphic display system as recited in claims 1 or 2
wherein the anti-aliasing means uses a two pixel wide Fourier window
anti-aliasing algorithm.
5. A computer generated graphic display system as recited in claim 4
wherein the anti-aliasing means causes the intensity (C') of each pixel
used to display a vector on the display means to be inversely proportional
to the pixel's distance from the vector according to the formula:
C'=C(d-2dx)/d
where:
dx=distance between the pixel and the center line of the vector
d=2(cosine of vector slope angle)
C=overall intensity of displayed vector.
6. A computer generated graphic display system as recited in claims 1 or 2
wherein the gamma correction means causes the generation of intensity data
for each pixel according to the formula:
B=kE.sup.1/.gamma.
where:
B=the gamma corrected digital brightness of the pixel value
K=a constant equal to the maximum digital brightness value
E=(Pv-Pmn)/(Pmx-Pmn)
Pv=linear pixel intensity value in question
Pmn=pixel intensity value corresponding to zero brightness
Pmx=pixel intensity value at maximum brightness
.gamma.=gamma correction constant for the particular model of the raster
scan graphic display.
7. The computer generated graphic display system as recited in claim 2
further comprising display memory means for storing display data
corresponding to each pixel location of the graphic display, with each
pixel location being represented by a plurality of intermediate order bits
for storing the color data of the background of the display, a plurality
of higher order bits for storing color data of the foreground of the
display, and a plurality of lower order bits for storing anti-aliased
intensity data which modifies the resulting pixel color appearing on the
display to a value inclusively between said foreground and said background
color values individually for each pixel location within said display
memory.
8. The computer generated graphic display system as recited in claim 2
wherein the raster scan graphic display has an implied background color
and further comprising display memory means for storing display data
corresponding to each pixel location of the graphic display, with each
pixel location being represented by a plurality of intermediate order bits
and higher order bits for storing color data of the foreground of the
display, and a plurality of lower order bits for storing anti-aliased
intensity data which modifies the resulting pixel color appearing on the
display to a value inclusively between said foreground and said implied
background color individually for each pixel location within said display
memory.
9. The computer generated graphic display system as recited in claim 2
wherein the raster scan graphic display has an implied background color
and further comprising display memory means for storing display data
corresponding to each pixel location of the graphic display, with each
pixel location being represented by a plurality of intermediate order bits
and higher order bits for storing color data of the background of the
display, and a plurality of lower order bits for storing anti-aliased
intensity data which modifies the resulting pixel color appearing on the
display to a value inclusively between said foreground and said implied
background color individually for each pixel location within said display
memory.
10. In a graphic display system of the type which includes a computer and
which generates anti-aliased image display data for display on a raster
scan graphic display and which includes a color look-up table for storing
data representative of a plurality of predetermined colors, the
improvement comprising gamma correction means for generating, for each
predetermined color, data representing a plurality of gamma-corrected
intensities and wherein said color look-up table includes an eight bit
color map, three bits of which store antialiasing information and the
remaining, higher order five bits store thirty two predefined colors and
wherein the color look-up table further stores, for each of these colors,
eight gamma corrected intensities calculated by adding the three bit
intensity value to the five upper bit color values to form an eight bit
color map entry number wherein said gamma correction is applied
selectively to the intermediate color values between foreground and
background while leaving all other image data uncorrected which
effectively smooths vectors on a raster scan graphic display without
affecting any other photometric property of the display apparatus.
11. The graphic display system as recited in claim 10 wherein the computer,
in performing a vector draw display, logically shifts a five-bit color
value three bits to the left to place the color value in the high order
position of an eight-bit pixel value then combines said pixel value with a
three bit intensity value, calculated according to a predetermined
anti-aliasing algorithm using an inclusive OR logical operation, which
places the intensity data in the low order position of the eight bit pixel
value, whereby a multicolor anti-aliased vector on a black background can
be displayed.
12. The graphic display system as recited in claim 10 wherein the computer,
in performing a vector draw display, first sets the display background to
a desired color, sets a software flag indicating that inverse video
vectors are to be drawn, calculates the three bit pixel intensity value
according to a predetermined anti-aliasing algorithm to produce an
anti-aliasing pixel intensity value and exclusive OR's said anti-aliasing
pixel intensity value with a predetermined digital value representing full
intensity whereby the anti-aliasing intensity value is altered to produce
the required intermediate color values to apply the anti-aliasing effect
to an inverse video vector.
13. A method of using a digital computer to define anti-aliased, gamma
corrected colors for use in displaying color vectors on a raster scan
graphic display comprising the steps of defining the red, green, and blue
intensities for the foreground and the background of each of the vector
colors, calculating an array of gamma-corrected color intensities, storing
the array of gamma-corrected color intensities in a look-up table in a
digital memory, calculating eight linear intensity values as even
increments from foreground to background for each red, green and blue
component of each vector color, using the linear intensity values as a
subscript into the look-up table to find the corresponding gamma-corrected
value for that color intensity, and storing that color intensity in a
color look-up table, wherein said plurality of intensities are located
within the color look-up table as a function of the anti-aliasing value
corresponding to the calculated intensity value, and storing anti-aliased,
display image data in a bit mapped memory for a raster scan display
screen, reading the display data out of the bit mapped memory and feeding
the data into the color look-up table to produce gamma-corrected display
image data, such that a linear relationship exists between successive
anti-aliasing values and successive intermediated colors progressing from
background to foreground as the colors appear on a raster scan graphic
display, whereby independence between anti-aliased foreground image data
and existing background image data is facilitated.
14. The method as recited in claim 13, further comprising the steps of
exchanging the foreground color with that of the background color in
reading the color look-up table to enable inverse video display of a given
color vector.
15. The method as recited in claim 13, wherein the steps of storing the
anti-aliased, display image data in the bit mapped memory comprises the
further steps of storing display data corresponding to each pixel location
of the graphic display, with each pixel location being represented by a
plurality of intermediate order bits for storing the color data of the
foreground of the display, a plurality of higher order bits for storing
color data of the background of the display, and a plurality of lower
order bits for storing anti-aliased intensity data which modifies the
resulting pixel color appearing on the display to a value inclusively
between said foreground and said background color values individually for
each pixel location within said display memory.
16. The method as recited in claim 15, wherein changing the display image
data stored in the bit mapped memory comprises writing new anti-aliased
display data in the bit mapped memory while simultaneously masking the
plurality of higher order bits to protect the existing background
information.
17. The method as recited in claim 15, wherein changing the display image
data stored in the bit mapped memory comprises writing new anti-aliased
display data in the bit mapped memory while simultaneously masking the
plurality of intermediate order bits to protect the existing foreground
information.
18. The method as recited in claim 15, wherein changing the display image
data stored in the bit mapped memory comprises writing new anti-aliased
display data in the bit mapped memory while simultaneously masking the
plurality of intermediate and higher order bits to allow an anti-aliased
line to be erased cleanly with full restoration of the previous
background. |
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Claims |
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Description |
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DESCRIPTION
1. Technical Field
This invention relates to computer generated graphic display systems and,
more particularly, to such a system which employs a cathode ray tube
display and which has correction to minimize distortion caused by
aliasing.
2. Background Art
Picture degradation in raster scan displays caused by aliasing presents the
image of "jaggy" or "staircase" appearance of lines and edges.
Anti-aliasing techniques have long been known which employ algorithms
which vary the intensity of the pixels which are used to display the lines
or edges. However, such algorithms are typically criticized for causing
line defocusing, brightness uniformity problems, inefficiency of
execution, and ineffectiveness for both inverse video and colored vectors
against complex backgrounds. In the past, despite the algorithm employed,
it has been difficult to produce crisp, straight lines on a raster display
taking advantage of raster functions.
Aliasing results from imposing an integer step function (the integer x, y
raster coordinates) onto a real function, such as a vector. Any means of
anti-aliasing lines, then, is an attempt to draw pixels at fractional
coordinates either through an optical illusion using software (such as
high spacial frequency filtering) or by physical micro displacement of the
electron beam (i.e. pixel phasing).
There are several optical phenomena that are the basis for creating the
illusion of fractional pixel coordinates. The most important one in the
present application is the integration of light emitted from separate
sources in close proximity. By controlling the intensity of, for example,
two adjacent pixels, the human eye integrates the light, giving the
impression of a single spot whose center is offset toward the brighter of
the two spots, and whose intensity is equal to the sum of the intensities
of the two spots.
A particularly useful anti-aliasing algorithm was proposed by Fujimoto and
Iwata, "Jag-Free Images on Raster Displays," IEEE Computer Graphics and
Applications (December 1983), pages 26-34. Portions of that article are
recreated in this application and the article is incorporated herein by
reference.
According to Fujimoto and Iwata, a two-pixel wide Fourier sampling window
is imposed on the real vector function to determine the relative
brightness of each member of a pixel pair used to approximate a single
pixel with fractional coordinates. As Fujimoto and Iwata demonstrated, the
brightness calculation can be reduced to a simple, linear, incremental
algorithm, thus incurring very little time overhead. The basis of this
algorithm will be explained in greater detail further in this application.
Unfortunately, although the proposed anti-aliasing algorithm attempts to
correct for aliasing by varying the intensity of pixels as an inverse
function from their distance from the center line of the vector or edge,
the perceived variation in pixel intensity by the observer does not
correlate with the algorithm. This is because there is not a linear
relationship between the brightness of the pixel and the intensity
information supplied to the display device. Thus, the algorithm does not
calculate the individual intensities for each pixel pair according to what
the eye actually sees on the cathode ray tube screen.
SUMMARY OF THE INVENTION
The above and other problems which complicate anti-aliasing correction on
cathode ray tube displays are overcome by the present invention of a
computer generated graphic display system of the type which generates
image display data, including data representing the intensity of each
pixel of the display, a raster scan graphic display for displaying the
image data, and anti-aliasing means wherein the improvement comprises
gamma correction means supplied with the anti-aliased display data for
correcting for non-linearity in the relationship between the pixel
intensity data and the actual intensity of the corresponding pixel of the
raster scan display.
In a preferred embodiment, the system includes a color look-up table for
storing data representative of a plurality of predetermined colors. The
gamma correction means causes the color look-up table to store, for each
predetermined color, data representing a plurality of gamma-corrected
intensities. In one particular embodiment, the gamma-corrected intensities
for each red, green, and blue component of the plurality of colors are
stored in the color look-up table as a function of the ratio of the
intensity of the foreground color of the display to the intensity of the
background color of the display.
In order to allow anti-aliased lines to be displayed on complex
backgrounds, the preferred embodiment includes display memory means for
storing display data corresponding to each pixel location of the graphic
display, with each pixel location being represented by a plurality of
lower order bits for storing gamma-corrected intensity data, a plurality
of intermediate order bits for storing the color data of either the
foreground or the background of the display, and a plurality of higher
order bits for storing color data of the background or the foreground of
the display. This allocation makes it possible to draw anti-aliased
vectors against an existing background using a masked write, thus
eliminating the need for a prior read. Where the color of the background
information is stored in the higher order bits, by write protecting the
higher order bits, each pixel can be automatically anti-aliased to the
background present at that pixel location. Masking all but the higher
order bits allows the background data to be changed without degradation or
recalculation of the anti-aliased foreground. Masking only the
intermediate order bits allows an anti-aliased line to be erased cleanly
with full restoration of the previous background, thus allowing the
animation of anti-aliased objects over a complex background.
It is therefore an object of the present invention to provide a computer
generated graphic display which incorporates anti-aliasing and gamma
correction for non-linearity of a display means.
It is a further object of the present invention to provide a computer
generated, anti-aliased and gamma-corrected display which allows
anti-aliased vectors in multiple colors to be drawn on a black background.
It is a yet further object of the present invention to provide a computer
generated graphic display which allows inverse video anti-aliasing.
It is yet a further object of the invention to provide a computer generated
graphic display which allows the drawing of multi-color vectors
anti-aliased to multi-color backgrounds.
Still another object of the invention is to provide a computer generated
graphic display which makes it possible to draw anti-aliased vectors
against a complex background.
The foregoing and other objectives, features and advantages of the
invention will be more readily understood upon consideration of the
following detailed description of certain preferred embodiments of the
invention, taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is an enlarged computer drawn simulation of an uncorrected vector
as displayed on a raster scan display;
FIG. 1B illustrates a plurality of uncorrected vectors drawn from a common
origin as displayed on a 19 inch color monitor;
FIG. 2 is an anti-aliasing algorithm flow chart for a two pixel Fourier
window anti-aliasing algorithm for one octant, as developed by Fujimoto
and Iwata;
FIG. 3 is a typical gamma correction curve showing the monitor response to
increasing signal levels and the gamma correction necessary to produce a
linear, corrected response;
FIG. 4A is an enlarged computer simulation of one anti-aliased vector line
as displayed on a raster scan display;
FIG. 4B illustrates a plurality of radially diverging, anti-aliased vector
lines as displayed on a 19 inch color monitor;
FIG. 5A is an enlarged computer simulation of a gamma-corrected,
anti-aliased vector line as displayed on a raster scan display;
FIG. 5B is a drawing illustrating a plurality of radially diverging
gamma-corrected, anti-aliased vector lines as displayed on a 19 inch color
monitor;
FIG. 6 is a block diagram illustrating the environment of the present
invention;
FIG. 7 is a block diagram of the raster engine depicted in FIG. 6;
FIG. 8 is a table showing eight gamma-corrected eight-bit intensity values
together with the linear intensities for the red, green, and blue
components of an orange line anti-aliased to a blue background (for
gamma=2.67); and
FIG. 9 is an illustration of a bit plane usage for multi-color vector
anti-aliasing.
BEST MODE FOR CARRYING OUT THE INVENTION
An example of the need for anti-aliasing is shown in FIGS. 1A and 1B.
Referring to FIG. 1A, an uncorrected line vector as simulated by a
computer drawing is illustrated. It will be seen that, if the display
attempts to produce the line by evenly illuminating a series of four
pixels at different horizontal scan lines in a generally diagonal
direction, a jagged line image results. This is best shown in FIG. 1B
which is an illustration of a plurality of vector lines radiating from a
single point on a monitor.
While the present invention is intended to work with any anti-aliased
computer generated raster scan graphic display, the inventor has found
that an anti-aliasing algorithm as developed by Fujimoto and Iwata, in the
reference described above, is particularly efficient. The anti-aliasing
approach adopted by Fujimoto and Iwata is described in the
above-referenced article. This specification will only briefly refer to it
for purposes of explaining the improvement of the present invention.
The Fujimoto/Iwata algorithm utilizes the special properties of the vector
edge, namely, its slope and thickness, which is geometrically equal to
zero. The Fourier window is adopted for the spacial filter and the window
is assumed to have a value of 1.0 in the region of plus or minus 0.5 of
the pixel and a zero everywhere else. In the present implementation, the
filter function is normalized so that the enclosed volume is 1. It is also
assumed that the size d of the Fourier window is a function of the slope
of the vector. For example, the smallest possible window involving shading
of one pixel on each side of the vector means that d equals two times the
cosine of the angle of the slope of the vector. Each pixel's intensity is
obtained by convoluting the filter function and the line intensity. Each
pixel intensity is inversely proportional to the distance from the center
line of the vector, that is:
C'=C(d-2dx)/d
where
C'=intensity of pixel under consideration;
C=overall vector intensity;
dx=distance between pixel and vector center line;
d=fourier window size=2(cosine of slope of vector)
The intensity of the pixel as it approaches the center line of the vector
will converge to the overall vector intensity. If the thickness of the
vector is assumed to be finite, then this property is satisfied only when
the size of the filter function is equal to or less than the thickness of
the vector.
In producing a jag-free vector that is as thin as possible, the thickness
of the displayed vector is limited by the smallest possible size of the
Fourier window. When generating a pixel on one side of the vector, one
must ensure that its counterpart exists on the other side of the vector.
This sets a limit on the window size for a vector with a given slope as
described above. The intensity of each pixel is a linear function of its
distance from the center line of the vector, as mentioned above, and thus
the principle of superposition can be applied, and the intensity of each
pixel can be calculated incrementally without explicit calculation of this
distance.
FIG. 4A illustrates a computer simulation of a vector line on a raster scan
display after it has been processed by the Fujimoto/Iwata algorithm. It
will be noted that the pixels lying along the center line have the maximum
intensity with the pixels which are located further and further away from
the center line having decreasing intensity.
A flow chart of the Fujimoto/Iwata algorithm is shown in FIG. 2. The logic
shown is for a Fortran program and is restricted to the vectors in the
first octant (i.e., 0<y<x); the extension to other regions being obvious.
Note that in anti-aliasing schemes, one must use whichever value of the
pixel coordinates (x, y) is changing the most often. Thus, in
anti-aliasing schemes of this type, it is necessary to divide the screen
into eight sectors or octants.
In the flow chart, the variable names RX, RY and RINT are related to the
current write pixel coordinates and the intensity of the pixels,
respectively. The variables LX, LY, and LINT refer to the left pixel
coordinates and their intensity. The inputs SX, SY, EX, EY, and VINT refer
to the coordinates of the starting and ending pixels of the vectors and
the vector intensity. Note that the intensity in this program is the
incremental accumulator and that the pixel coordinates are the byproducts
of the iteration counters.
The algorithm has an initialization section 10 which precedes the main body
20 of the procedure. This initialization section 10 provides for subpixel
addressability in that it allows one to place the end points of the vector
between the pixels. A pixel's coordinate and intensity are simultaneously
generated in the main part 20 of the algorithm. It is the pixels'
intensity control variable LINT that directly governs the generation of
the coordinates. The control variable simultaneously incorporates both
physical and geometrical meanings.
As mentioned above, the net result of the Fujimoto/Iwata anti-aliasing
algorithm produces pixel intensity data which determines how bright each
pixel on the display screen is to be for displaying a particular vector or
edge. However, as explained above, this digital data cannot be directly
converted to an analog voltage and applied to the cathode ray tube gun to
produce a corresponding linear increase or decrease in the intensity of
the pixel. The reason for this is that the relationship between the
cathode current of the cathode ray tube and the amount of light produced
by the phosphor in a given CRT is not linear.
FIG. 3 illustrates a typical gamma correction plot. For increasing signal
level, that is, digital pixel intensity data and hence cathode current,
the luminance does not increase in a linear fashion. If this linear
intensity data is corrected according to the gamma correction curve, the
net result is a linear, corrected response, as illustrated by the dashed
line in the figure. FIGS. 4A and 4B show the "twisted rope" uniformity
problem and ineffective straightening which results from failing to
correct for gamma even though the anti-aliasing algorithm has been
applied.
Accurate gray scale selection is imperative in the effectiveness of the
anti-aliasing illusion. The algorithm cannot stop with the linear
brightness relationship arrived at through the incremental or distance
calculation approach of Fujimoto/Iwata. Rather, the algorithm must produce
pixel intensities according to what the human eye perceives. Therefore,
the algorithm must account for the relationship between digital values
presented in the digital to analog controller (DAC) of the signal
generator used to produce the display and the resulting brightness
appearing on the cathode ray tube. In other words, to achieve the maximum
effect of the anti-aliasing illusion, the algorithm must calculate the
individual intensities for each pixel pair according to what the eye will
see on the display screen.
Gamma is the characteristic that mathematically describes the relationship
between the cathode current density and the amount of light produced by
the phosphor in a given CRT of the display. It is defined by the following
formula:
B=kE.sup.1/.gamma.
Where
B=the gamma corrected digital brightness of the pixel value to store in the
color look-up table
K=a constant equal to the maximum digital brightness value (255 for an
8-bit DAC)
E=the desired linear brightness such that 0.ltoreq.E.ltoreq.1
.gamma.=gamma
E is described as follows:
##EQU1##
for a pixel value continuum where the "pixel minimum" is the pixel value
corresponding to zero brightness, "pixel maximum" is the pixel value
corresponding to full brightness, "pixel value" is the linear pixel value
in question that falls within the given continuum.
For the monitor used during the algorithm development, a gamma value of
2.67 was experimentally determined as the optimum for anti-aliasing. This
gamma value has remained constant for all production units of that monitor
model (Sony GDM-1901) tested over a span of 1.5 years. One additional
benefit of proper gamma compensation is the same gamma correction function
can be applied to accurate color mixing that is continuous throughout the
gray scale. This allows simple shading of objects in the display without
changing the apparent color mix (i.e. orange does not look reddish for
dark shades but rather looks like dark orange.
The effect of gamma correction of anti-aliased display data is illustrated
in FIGS. 5A and 5B. Effective gamma correction produces smooth lines with
good uniformity at very nearly the same line width as raw vectors on a
stroke display.
Referring now to FIG. 6, a block diagram of a system which incorporates the
present invention is illustrated. In this system, a host computer 30
transfers raw graphic data 40 via a bus or other communications interface
to a raster engine 50 which produces RGB and sync signals for a display
monitor 60. The raster engine can be any type of signal generator for a
CRT display. One such raster engine is depicted in FIG. 7. Since the
operation of the raster engine should be well understood by those skilled
in the art and in and of itself forms no part of the invention, it will
not be described in detail.
Suffice to say that the raw graphics data is received by the raster engine
50 through one of the many input/output devices labeled generally 70 in
FIG. 7. This data is passed along a CPU bus 80 to the CPU 90 which
processes the data and supplies it to a refresh memory controller 100 and
a clock sync generator 110 which produces the vertical sync (Vsync),
horizontal sync (Hsync), horizontal blanking (H.BLK), and vertical
blanking (V.BLK) signals for the display monitor 60, as well as
synchronizing the operation of the refresh memory controller 100.
The refresh memory controller 100 loads the data into a refresh memory unit
120 which includes a bit mapped memory for the display screen of monitor
60. The output of the refresh memory 120 is fed to a look-up table 130,
whose function will described in greater detail hereinafter, which
produces the digital display data in parallel form to a parallel to serial
converter 140. Under the control of the clock sync generator 110, the
parallel to serial converter 140 supplies display data in serial form to a
D-to-A converter 150 which produces the RGB signals for the monitor 60.
The CPU 90 has access to the anti-aliasing program 160 and a gamma
correction program 170 which are shown in separate blocks in the figure.
The anti-aliasing program is preferably the Fujimoto/Iwata algorithm shown
in FIG. 2. It will be understood, however, that these programs are simply
stored in ROM or can be loaded in through the input-output devices 70. As
will be explained in greater detail hereinafter, in supplying the refresh
memory controller with the display data, the CPU executes the
anti-aliasing algorithm of Fujimoto/Iwata and supplies anti-aliased pixel
intensity data to the refresh memory 120. Gamma correction, however, is
provided by loading the look-up table 130 with gamma corrected intensity
values.
Because non-linear functions are time consuming to calculate, the gamma
correction is much more efficiently performed in the initial loading of
the color look-up table 130 rather than in performing in real time the
calculation shown above. Assuming an 8-bit color map in the color look-up
table 130, three bits may be assigned for anti-aliasing information and
the remaining five bits may be used to define up to 32 different vector
colors. The CPU loads the color look-up table 130 with 32 programmatically
defined colors corresponding to the high order five bits of the color map.
Then, for each of these vector colors, eight gamma corrected intensities
are calculated and loaded into the color look-up table 130 by adding the
three bit intensity value to the five upper bit color values to form the
eight bit color map entry number. For the three bit binary intensity, 000
represents zero intensity, 111 represents full intensity, and the steps
inbetween are the gamma-corrected intensities. When the CPU performs a
vector draw, the anti-aliasing routine 160 shifts the color value three
bits to the left, then for each calculated pixel, inclusive OR's the
proper three bit intensity producing the eight bit pixel value. The result
is multicolor anti-aliased vectors on a black background.
Inverse video anti-aliasing results directly from the gamma-corrected color
look-up table 130. The CPU 90 first sets the background to the desired
color and sets a software flag indicating that inverse video vectors are
to be drawn. When the anti-aliasing routine 160 calculates the pixel
intensity, the three bit value is exclusive OR'ed with 111.sub.2
(effectively subtracting the intensity from the maximum value). This
inverts the intensity level within the gamma correction. The result is
anti-aliased black lines on a solid color background. Using the multicolor
vector color map, up to 32 background colors may be used in one screen of
the monitor 60 with black anti-aliased vectors appearing in each
background region. The resulting line width obtained is dependent upon the
rise and fall times and bandwidth of the video amplifier of the monitor
(not shown).
To enable anti-aliased raster operation such as solid background and filled
polygons it is necessary to be able to draw multicolor vectors
anti-aliased to multicolor backgrounds. This can be done in the color
look-up table loading routine without any changes to the anti-aliasing
algorithm itself.
The basic concept is to blend each vector color to a background color
instead of blending the vector color to black. This blending must be
gamma-corrected and continuous in both directions so the inverse video
capability can reverse the colors, thus doubling the number of color
combinations.
The first step is to define the RGB intensities for the foreground and the
background of each of the vector colors. Next, an array of gamma-corrected
color intensities is calculated to simplify the proper color look-up table
values with intensity subranges (for gamma-corrected intensities of
partial bright colors). Next, eight linear intensity values are calculated
as even increments from foreground to background for each RGB component of
each vector color. This is illustrated in FIG. 8 which depicts the content
of the look-up table for the example of gamma-corrected RGB intensities
for orange lines anti-aliased to a blue background. Each of the linear
intensity values is used as a subscript into the gamma correction table to
find the proper gamma-corrected value for that intensity, which is then
stored in the color look-up table 130. By using this method, inverse video
is defined as exchanging the foreground color with that of the background.
For the table shown in FIG. 8, a gamma value of 2.67 was used. It should
be understood, however, that the gamma value depends on the particular
monitor. Typical values of gamma range from 2.3 to 2.8 for color monitors.
Referring now more particular to FIG. 9, using the bit plane definition
shown in the figure, allows anti-aliased lines to be easily drawn on
complex backgrounds. For example, an anti-aliased vector can be drawn
against an existing background using a masked write, thus eliminating the
need for a prior read. Write protecting bits 6 and 7 will protect the
background information. Each pixel will then be automatically anti-aliased
to the background color present at that pixel location. Masking bits 0
through 5 allows the background data to be changed without degradation or
recalculation of the anti-aliased foreground. Masking bits 3 through 7
allow an anti-aliased line to be erased cleanly with full restoration of
the previous background, thus allowing the animation of anti-aliased
objects over a complex background. This bit plane definition represents
the organization of the refresh memory 120.
In short, using this method effectively treats complex backgrounds the same
as black. The background complexity has no effect on drawing time or
effectiveness of anti-aliasing. Both foreground and background data are
independently changeable.
If, however, in a raster engine other than that depicted in FIG. 7 is used
which does not allow masked writes, then one must use the less efficient
read-modify-write procedure in order to preserve the background data. In
fact, if animation over a background is desired requiring background
restoration, the full write/erase cycle consists of
read-store-modify-write-recall-write; as opposed to mask write-mask write
for a maskable bit plane architecture. Therefore, maskable bit plane-write
enable is an important for efficiency in the signal generator when
utilizing this technique.
Although the present invention has been shown and described with respect to
preferred embodiments, various changes and modifications which are obvious
to a person skilled in the art to which the invention pertains are deemed
to lie within the spirit and scope of the invention.
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