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Description  |
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BACKGROUND OF THE INVENTION
This invention relates in general to pseudocolor images and more
particularly, to a real-time analytic pseudocolor system and encoder.
Most of the images obtained in various scientific and medical equipment are
usually in the form of gray-level images. For example, thermal line scan
recorders, laser retina scanners, and multi-format cameras normally
produce only gray-level images. Other relatively widely known density
images such as scanning electron microscopic images, X-ray transparencies
are all gray-level images.
The human visual system can discriminate simultaneously only 15 to 20 gray
levels from a complex black and white image. If the same image is
presented in full color, the visually distinguishable levels can be
increased enormously, up to hundreds or even thousands of different levels
[H.-K. Liu and J. W. Goodman, "A New Coherent Optical Pseudocolor
Encoder," Nouv. Rev. Optique, t.7, No. 5, 1976, 285].
The basic philosophy of pseudocolor is that the eye can perceive many more
colors than gray levels and pseudocolor mapping will effectively extend
the range of the observer's eye [C. H. Radewan, "Digital Image Processing
with Pseudo-Color," Conf. Proc. Acquisition and Analysis of Pictorial
Data, The Modern Science of Imagery, Aug. 19-20, 1974, Soc. Photo-Optical
Instrumentation Engrs. (SPIE), pp. 50-56]. However, this range extension
is not only used for observing an already available density image. Surface
structure, optical interferant pattern, thermal pattern, etc. can be
directly input to the pseudocolor encoder through an appropriate TV camera
resulting in a real-time pseudocolor output.
True color information in the input domain is lost when pseudocolor is
generated in the output domain. However, under some conditions, true color
information is not as important as brightness information. For instance,
most animals are color blind. They do not perceive true color information.
A pseudocolor encoder picks up the input information as an animal's eye
and displays it as a human color perception.
Also, the comparison between two pseudocolor maps is easier than that of
density maps. In thermal imaging, if temperature is encoded with gray
level, it is very difficult to recognize precisely the temperature of a
specific gray level. However, a pseudocolor map may overcome this
difficulty. It is much easier to pinpoint a specific color than a specific
gray level. The pseudocolor encoder has two primary merits: (1) better
discrimination, and (2) better recognition.
Pseudocolor encoding is commonly achieved through two methods: a
sophisticated digital method and a relatively simpler optical method. The
digital computer technique is a logical choice if the images are already
digitized.
In the digital computer technique, the input image is first sampled
resulting in the image consisting of finite image elements; for instance,
512.times.512 elements. The input analog gray level signal at each image
element is digitized to certain quantized gray levels; for instance, 64
gray levels. The quantized gray level of the image element is then stored
in the memory. The computer has a program, which is usually a lookup
table, to assign every quantized gray level with a specific color. The
programmer may assign the color first based on perceptual terms which
relate to attributes of sensations of light and color. The selected
perceptual colors are encoded (color codes) with specific data as outputs
from the lookup table in the computer. A color/graphics adapter card is
implemented in the computer to interface with a video monitor. The adapter
card inputs display information from the computer through data bus and
address bus, and outputs color video and sync signals to drive the color
monitor. The digital computer technique requires a computer for processing
and storing the image data, in addition to a color/graphics adapter card.
Although it is very flexible in programming, it is relatively expensive.
In contrast, optical methods are simpler. In the above-referenced article
"A New Coherent . . . . ", Liu, et al. described a coherent method for
implementing pseudocolor encoding by half-tone screen. The principle of
the optical half-tone screen method is as follows: the positive and the
negative images of the gray-level input are encoded with two primary
colors; for example, red and blue. Then the product of the positive image
and the negative image is encoded with the third primary color, in this
case green. The superposition of these three color-encoded images results
in a pseudocolor image of the gray-level input. The optical system was
improved by Yu, as described in U.S. Pat. No. 4,623,245 ("Yu"), in which a
white-light source was used replacing lasers. Yu also described in U.S.
Pat. No. 4,623,245, "System of White-Light Density Pseudocolor Encoding
with Three Primary Colors," that the pseudocolor-encoded image formed at
the output plane can be received by an additional color TV camera and then
depicted on a color TV monitor.
The principle of the optical method is that three optical masks for three
primary colors are generated independently from the same density image
following three simple analytic transform functions. The superposition of
the three color images results in a pseudocolor output. The main
difference between the digital computer technique and the optical
technique is that the optical technique utilizes continuous analytic
transform functions for direct generation of primary color signals instead
of quantized discrete gray-levels such as entries in a look up table. The
disadvantage of the optical technique is that the generation of optical
masks cannot be performed in real-time. Some optical methods employing
liquid crystal televisions are able to perform pseudocoloring in
real-time; however, the color and contrast are severely limited by the
physical properties of the liquid crystal molecules (F. T. S. Yu, S.
Jutamulia, T. W. Lin, X. L. Huang, "Real-Time Pseudocolor-Encoding Using
Low-Cost Liquid Crystal Television," Opt. Laser Tech. 19, 1987, 45). In
another method proposed by Yu (F. T. S. Yu, S. Jutamulia, E. Tam, "Gray
Level Pseudocolor Encoding Using a Liquid Crystal Television," J. Opt.,
(Paris), 19, 1988, 129), primary color images generated by liquid crystal
televisions are optically added. To display the pseudocolor-encoded
pattern on a TV monitor, an additional color TV camera is required to
receive the color images generated on the liquid crystal televisions,
since a conventional TV monitor (non-liquid crystal) exhibits no
birefringence effect for producing color.
SUMMARY OF THE INVENTION
One aspect of the invention is based on the observation that the continuous
analytic transform functions implemented optically by Yu in U.S. Pat. No.
4,623,245 can instead be advantageously implemented electronically and in
real-time. The outputs of transformation are the primary color video
signals. Three primary color images are displayed on the same color
monitor. In the optical method, three primary color images are generated
optically using separate optical masks or liquid crystal televisions. Our
invention also differs from the digital computer technique. Using computer
techniques, the input monochrome video signal is first digitized (sampled
and quantized) and stored in the memory. Secondly, the digitized data
(gray level and address) is processed usually with a lookup table to
assign appropriate color code to each data (color code and address). The
processed data (color code and address) is then input to the
color/graphics adapter card which finally produces the required video and
sync signals for a conventional TV monitor. Our invention, in principle,
directly transforms an input monochrome video signal to the pseudocolor
video and sync signals for a color monitor. No memory or central
processing unit (CPU) is required. As a consequence, a pseudocolor encoder
based on our invention has the following advantages: (1) direct transform
(as the optical method), (2) high definition (as the digital computer
technique), (3) compact (as compared with the optical and digital computer
techniques), and (4) low cost (as compared with the optical and digital
computer techniques).
One aspect of the invention is directed to an apparatus for generating a
first, second and third pseudocolor video signals from monochromatic video
signals having a range of possible amplitudes I between 0 and A, A being a
predetermined constant. The apparatus comprises means for generating a
first pseudocolor video signal whose amplitude is proportional to that of
the monochromatic video signal. The apparatus further comprises means for
generating a second pseudocolor video signal whose amplitude is
proportional to the difference between A and that of the monochromatic
video signal; and means for multiplying the first and second pseudocolor
signals to provide the third pseudocolor video signal.
The three pseudocolor signals may then be sent to a conventional color TV
monitor for displaying three primary color signals; i.e., red (R), green
(G), and blue (B). Three independent red, green and blue images are
actually formed on the display screen. The display screen can be a color
cathode ray tube (CRT), a color liquid crystal flat panel, a color plasma
display, or any color display which is driven by standard video color
signals.
Given a brightness signal from a monochrome TV camera, a red and a blue
signal can be derived as the replica (positive) and the inverse (negative)
signal from the original brightness signal, respectively. A green signal
is generated as the product of the positive and negative signals. The
above-described system is much simpler than the system invented by Yu in
U.S. Pat. No. 4,623,245, in which the color-encoded pattern is generated
by an optical setup. To perform the monochrome to pseudocolor conversion,
a system employing the pseudocolor encoder of this invention will require
only a conventional monochrome TV camera, a conventional color TV monitor,
and specifically-designed electronic circuits in the encoder to implement
the analytic transforms. It is worth noting that in the encoder of the
invention, the generation of pseudocolor images can be freely changed by
changing the transform functions, which is not possible in the optical
method.
Unlike the optical method of Yu, the analytic transform functions that can
be implemented using the pseudocolor apparatus of this invention is not
limited to the particular function in U.S. Pat. No. 4,623,245 described
above. Instead, functions providing improved discrimination and
recognition characteristics over the function utilized by Yu may be
implemented using the system of this invention. Thus another aspect of the
invention is directed towards an apparatus for generating a first, second
and third pseudocolor video signals from monochromatic video signals
having a range of possible amplitudes I between 0 and A, said range
comprising n sections, n being a positive integer. The apparatus comprises
means for generating a first, second and third pseudocolor video signals
S1, S2, S3 whose amplitudes are constants or functions of I in said n
sections, said functions being substantially linear or sinusoidal, wherein
at least the amplitudes of the pseudocolor video signals in one section
are not constants and are functions of I that are different from the
constant or function in at least one other section. The apparatus further
comprises means for outputting the three pseudocolor video signals. By
dividing the range of possible amplitudes of the monochromatic video
signals into sections and selecting an analytic function within each
section for transforming a monochromatic video signal into three
pseudocolor video signals, where the transform function for each section
is distinct and independent of those of other sections, it is possible to
generate additional pseudocolors to the three primary pseudocolors red,
blue and green, to further enhance the viewer's ability to discriminate
and recognize gray-levels. Thus in the preferred embodiment, the range of
possible values of the monochromatic video signal is divided into seven
sections and seven sets of distinct analytic transform functions are
adopted, one for each section, to transform the monochromatic video signal
into three pseudocolor video signals. In this manner, the full range of
pseudocolors utilizing the whole color space may be implemented.
From the above, it is evident that one key advantage the invention has over
the optical method of Yu lies in the fact that the apparatus of this
invention permits a user to select any particular function from a set of
functions (where the amplitudes of the pseudocolor signals are either
constants or substantially linear or sinusoidal functions of the
monochromatic video signal amplitude), in order to achieve any particular
objective in gray-level discrimination and recognition.
Another aspect of the invention is directed to a system for providing
pseudocolor images of objects comprising a monochromatic television camera
for providing monochromatic video signals in response to objects, a
pseudocolor encoder for generating a first, second and third pseudocolor
video signals in response to the monochromatic video signals in real-time
and color monitor means responsive to said three pseudocolor video signals
for displaying pseudocolor images of the objects. The system further
comprises a housing for holding the camera, the encoder and the monitor
means. The system is compact and portable and is therefore very convenient
for users. This is in contrast to conventional systems where the
television camera, the pseudocolor encoder, and the color monitor all have
their own casings and are generally bulky. Such conventional systems are
inconvenient for transport and are therefore less flexible compared to the
portable system of this application.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a pseudocolor image system to illustrate the
invention.
FIG. 2A is a block diagram illustrating in more detail an analog embodiment
of the pseudocolor encoder of FIG. 1.
FIG. 2B are schematic circuit diagrams illustrating in more detail the
embodiment of FIG. 2A.
FIG. 3 is a graphical illustration of R, G and B analytic transform
functions for transforming a monochromatic input into three pseudocolor
outputs.
FIG. 4 is a graphical illustration of analytic transform functions for
transforming monochromatic video signals into three pseudocolor video
signals.
FIG. 5 is an illustration of a three-dimensional color space formed by red,
green and blue primary colors.
FIG. 6 is a schematic illustration of a pseudocolor map of the transform
functions of FIG. 4 represented in the color space.
FIG. 7 is a pseudocolor map arrived at by modifying the pseudocolor map of
FIG. 6 to maximum saturation and brightness.
FIG. 8 is a graphical illustration of red, green and blue analytic
transform functions implementing the pseudocolor map of FIG. 7.
FIG. 9 is a pseudocolor map illustrating transform functions fully
utilizing the color space with maximum saturation and maximum brightness
with increasing brightness in the initial step.
FIG. 10 are graphical illustrations of red, green and blue analytic
transform functions implementing the pseudocolor map of FIG. 9.
FIG. 11 is a pseudocolor map arrived at by modifying the map of FIG. 9,
illustrating red, green and blue analytic transform functions with maximum
saturation and nearly constant brightness.
FIG. 12 is a graphical illustration of the red, green and blue transform
functions of FIG. 11.
FIG. 13 is a graphical illustration of analytic transform functions for
transforming a monochromatic video signal into three pseudocolor outputs.
FIG. 14 is a block diagram of a hybrid analog-digital pseudocolor encoder
illustrating the preferred embodiment of the pseudocolor encoder of FIG.
1.
FIG. 15 is a schematic view of a compact, hand-held pseudocolor system
comprising a pseudocolor encoder, a television camera, and a color liquid
crystal display.
FIG. 16 is a block diagram of the pseudocolor system of FIG. 15.
FIGS. 17A, 17B are schematic circuit diagrams of pre-processing circuits
within the red, green and blue processors of the analog pseudocolor
encoder of FIG. 2A.
FIGS. 18A, 18B, 18C are schematic circuit diagrams of portions of the red,
green and blue processors of FIG. 2A.
FIG. 19 is a schematic circuit diagram of a detailed implementation of the
hybrid analog-digital pseudocolor encoder of FIG. 14.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is a system 20 including three primary parts as shown
in FIG. 1. A monochrome TV camera 22 or other video signal generator such
as a computer, inputs a monochrome video signal into the electronic system
or encoder 24 which generates three primary color signals from the input
monochrome signal. The monochrome video signal includes a sync signal
which is separated by the electronic system or encoder 24 in the
generation process. The generated color signals, together with the sync
signal, are fed to a color TV monitor 26 such that a pseudocolor-encoded
image can be displayed. Of course, these R, G and B signals, together with
the sync signal, can be combined in a standard NTSC color signal by a
standard video signal converter.
The full analog electronic system is depicted schematically in FIG. 2A. The
input of the electronic system is a B/W composite video signal that may
come from a monochrome TV camera or a computer. The input signal is fed
into the amplifier 30 and then the separator 32 which separates the B/W
video (brightness) signal from the sync signal. The sync signal is
directly connected through an output buffer 34 to the output port 36. The
brightness signal is then split to three identical signals as inputs for
the processor R, processor G and processor B. The outputs from the
processors R, G and B are connected through output buffers 34 to the
output port 38 of the electronic system which provides red (R), green (G)
and blue (B) signals for a color TV monitor.
Following the algorithm o the optical method mentioned previously, the R, G
and B signals can be obtained as follows. If the brightness signal is
denoted by I, 0.ltoreq.I.ltoreq.A, signals R, G and B are:
##EQU1##
where A is a predetermined constant.
To get these outputs, the processor R is only an unity gain amplifier, the
processor G includes a multiplier, amplifiers, and a differential
amplifier, and the processor B is a differential amplifier as shown in
FIG. 2B. The functions of the resultant R, G and B versus I are shown in
FIG. 3. The color output from the transformation expressed in Equations
(1) to (3) is not visually attractive. This transformation is used in the
optical method because it is difficult to implement optically a
transformation different from that of Equations (1) to (3). In other
words, Equations (1) to (3) can be easily realized optically.
In the Equations (1) to (3) above, it will be observed that the three
pseudocolor signals R, G, B may be interchanged so that, for example, G is
given by Equation (1), B by Equation (2) and R by Equation (3), or
actually by any other possible permutation. For this reason, the three
Equations may be generalized by replacing R, G, B by signals S1, S2, S3
instead. Thus, in this application, where transform functions are
described or illustrated in the figures by reference to R, G, B, it will
be understood that these functions may be generalized by replacing them
with S1, S2, S3 instead. All such variations are within the scope of the
invention. In fact, in many of the equations below, the transform
functions are expressed in terms of S1, S2 and S3.
However, a variety of transform functions can be implemented
electronically, which will produce more attractive color output. For
example, the following equations:
##EQU2##
where the monochromatic signal amplitude I takes on values between 0 and
A, a predetermined constant.
The processors R, G and B can be made using analog electronic circuits
consisting of subtractors, inverters and amplifiers. The functions of the
resultant R, G and B versus I is given in FIG. 4. For instance, we can see
from FIG. 4 that when I is maximum (bright), that is equal to A, we will
get red color; when I is of an intermediate value e.g. 0.5A, we will see
green color; and when I is minimum or 0 (dark), we will see blue color.
Between pure red and pure green, there is a gradually changing mixture of
red and green. Similarly, an area of gradually changing color is in
between pure green and pure blue.
Better Algorithms
The three primary colors form a color space as shown in FIG. 5 [C. H.
Radewan, "Digital Image . . . "]. It is possible to represent all the
colors that can be produced as points in the three dimensional color
space. This color space can be related to normally perceived color
attributes; namely: hue, saturation and brightness. Hue is the attribute
of a color perception denoted by blue, red, green, yellow and so on.
Saturation is the relative difference from gray or white. Brightness is a
function of the radiant intensity.
FIG. 3 shows blue and red hues with maximum saturation (pure color);
however, the green hue never achieves maximum saturation. This is the
reason that the optical method does not give an attractive color output.
FIG. 4 can be represented in the color space as shown in FIG. 6. This
shows a pseudocolor map with maximum saturation and nearly constant
brightness. This map can be modified to maximum saturation and maximum
brightness as shown in FIG. 7. The functions of R, G and B versus I to
give the maximum saturation and brightness of FIG. 7 are given in FIG. 8.
This can be expressed mathematically as follows:
##EQU3##
where the monochromatic signal amplitude I takes on values between 0 and
A, a predetermined constant.
The color change as a function of the brightness is as follows:
Blue.fwdarw.Pale Blue.fwdarw.Green.fwdarw.Yellow.fwdarw.Red. As shown in
FIGS. 6 and 7, we still do not utilize the whole color space. An algorithm
which fully utilizes the color space is illustrated in FIG. 9. This is a
version of maximum saturation and maximum brightness with increasing
brightness in the initial step. The color change is:
Black.fwdarw.Purple.fwdarw.Blue.fwdarw.Pale Blue.fwdarw.Green
Yellow.fwdarw.Red.fwdarw.White. The functions of R, G and B versus I (or
more generally the S1, S2, S3 functions) are illustrated in FIG. 10, while
the mathematical expression is given below.
##EQU4##
where the monochromatic signal amplitude I has a range between 0 and A, a
predetermined constant.
An alternative is illustrated in FIG. 11, which is a pseudocolor mapping
with maximum saturation and nearly constant brightness. The R, G and B
functions (or more generally S1, S2, S3 functions) by which the mapping of
FIG. 11 is achieved are illustrated in FIG. 12. The analytic equations are
given as follows:
##EQU5##
where the monochromatic signal amplitude I has a range between 0 and A, a
predetermined constant.
Many of Equations (4) to (15) are simple linear equations. Thus, they can
be very easily implemented by an analog electronic circuit using
amplifiers, differential amplifiers, voltage dividers, comparators, etc.
It will be understood, however, that the amplitudes of the three
pseudocolor video signals may also be sinusoidal (sine or cosine type)
functions of the monochromatic video signal amplitude I; such and other
similar functions are within the scope of the invention. One set of
functions for the transforming the monochromatic video signal amplitude I
with values in a range 0 and A, A being a predetermined constant, into
three pseudocolor video signal amplitudes are illustrated in FIG. 13 and
given by the equations below:
##EQU6##
It will be noted that in Equations (4) to (18) and as illustrated in the
accompanying figures, if any, the possible range of values for I, the
monochromatic video signal amplitude, is divided into sections. In
Equations (4) to (6) and FIG. 4, for example, the range is divided into
four sections (0 to 0.25A, 0.25A to 0.5A, 0.5A to 0.75A and 0.75A to A),
where the amplitudes of the three pseudocolor video signals are either
constants or linear functions of I in each of the four sections. Thus
constant values or functions may be defined for each of the four sections
for each of the three pseudocolor video signal amplitudes, where the
constants or functions defined for each section may be different from
those of other sections, except that, preferably, the constants or
functions are such that at each of the junctions (0.25A, 0.5A and 0.75A)
between a pair of adjacent sections, the amplitudes of the pseudocolor
video signal amplitudes take on the same values according to the constants
or functions in the pair of adjacent sections. It will also be noted that
the amplitudes of the three pseudocolor video signals in at least one
section are not constants and are different from the constants or
functions in at least one other section. The above generalization applies
also to the case of other equations such as Equations (7) to (18) and
their accompanying figures.
Hybrid Analog-Digital System
The analytic transformation given by Equations (1) to (18) can also be
performed by digital processors. This hybrid analog-digital system differs
from conventional computer techniques. The system has no memory to hold
the data frame by frame. In other words, there are no data bus and address
bus. The input data is processed on-line (real-time), as in the case of
the full analog system. However, the transformation is performed digitally
in a specific read only memory (ROM). The ROMs can be written with a
programmable ROM writer. This gives an additional merit to the hybrid
analog-digital system that the transform functions can be changed without
physically changing the electronic circuit. There are A/D converter and
D/A converter prior to and after ROMs. The complete block diagram of the
hybrid analog-digital system is illustrated in FIG. 14. The monochrome
(B/W composite) video signal is first input to an amplifier. The amplified
signal is fed into a video sync separator. The separated B/W video signal
goes to the A/D converter, split to three inputs for B/W to red, B/W to
green, and B/W to blue transform ROMs. The red (R), green (G) and blue (B)
transformed outputs are passed through D/A converters resulting in analog
signals for R, G and B video signals. The separated sync signal is input
to the sync controller together with a clock signal. The sync controller
outputs clock signals for A/D converter, transform ROMs and D/A converter,
and composite sync signal for color monitor.
Compact Hand-Held Pseudocolor Encoder
The electronic systems illustrated in FIGS. 2 and 14 can be made and
packaged in a box as an adapter from a monochrome TV camera or other video
signal sources to a color TV monitor for producing pseudocolor images. As
is mentioned previously, the present invention does not require a bulky
optical setup or computer system; therefore, it can be made in a compact
form.
The present invention includes a compact hand-held pseudocolor encoder
based on the method of analytic transformation mentioned previously. The
schematic and block diagrams are illustrated in FIGS. 15 and 16,
respectively. The compact hand-held pseudocolor encoder is composed of a
monochrome TV camera, an electronic encoding system (full analog or hybrid
analog-digital), and a color liquid crystal TV monitor assembled in a
single unit. This compact system can be used in industrial and medical
applications. An infrared or thermal imaging TV camera can be used as well
as an ordinary TV camera. A myriad of applications can be found for the
compact hand-held pseudocolor encoder and the adapter-type pseudocolor
encoder. If desired, the encoder may be used to perform the
transformations as defined in any one set of the Equations (1) through
(18) above.
This invention is particularly advantageous since a complex color transform
function can be implemented by means of a number of simpler functions in n
sections, n being a positive integer. In this manner, the simple function
in each section can be implemented with relatively simple electronic
circuits. For this reason, the simple circuits can be accommodated on a
printed circuit board with a dimension less than 5.47 inches by 8.27
inches in the pseudocolor encoder. Even when the television camera and the
liquid crystal television monitor together with the pseudocolor encoder
are placed within the same housing as shown in FIGS. 15 and 16, the system
that results is still compact. Applicants have found that the housing for
all three components can be made to be less than 10 inches in length, six
inches in width, and six inches in height. The total weight of the three
components and the housing can be reduced to less than 900 grams from 10
kilograms or more for conventional systems employing television camera,
image processing (computer) unit and television monitor as separate units.
The weight reduction is possible by adopting state-of-the-art miniaturized
camera and liquid crystal television technologies and the simple
electronic implementation of the pseudocolor transform functions described
above. The portable compact unit of this invention will make possible a
number of new applications. For example, a security guard may take this
portable unit equipped with thermal camera to check if there is a thermal
abnormality caused, for example, by fire. The military may use the unit
for night vision. The medical industry or industrial production may use
the unit for detecting thermal and X-ray distribution or light
distribution. Due to the relatively simple electronic implementation, the
cost of the total system is low as compared to conventional complex
systems.
FIGS. 17A, 17B are schematic circuit diagrams of pre-processing circuits
with red, green and blue processors of the analog pseudocolor encoder of
FIG. 2A for implementing the transform functions of FIG. 8 and Equations
(7) through (9). In reference to FIG. 8, the range of values from 0 to A
for the monochromatic video signal I is divided into four sections: 0 to
0.25A, 0.25A to 0.5A, 0.5A to 0.75A and 0.75A to A. Then four different
sets of separate functions are defined, one set for each of the four
sections. The pre-processing circuit of FIGS. 17A, 17B generate the
control signals for selecting the particular set of functions depending on
the magnitude of the monochromatic video signal amplitude I. For this
purpose the monochromatic video signal I is fed to an 8 bit A/D converter
102. The two most significant bits (7 and 6) of the output of the
converter 102 are connected to four AND-gates 112, 114, 116 and 118, where
bit 6 of the output converter is connected to gates 114-118 through
inverters 122, 124 as shown in FIG. 17A. The outputs of the four AND-gates
are four control signals AA, AB, AC, AD. The conditions under which any
one of the four control signals will be high (or on) or low (or off) are
indicated in FIG. 17A. The four control signals are used to control analog
switches 132-138 of FIG. 17B as shown. When signal AA is high or on, for
example, switch causes the monochromatic video signal I to be connected to
the node BA; where signal AA is low or off, node BA is connected to ground
instead. Similarly, nodes BB, BC, BD are connected to the monochromatic
input when their corresponding control signals AB, AC, AD are high, but
are connected to ground if their corresponding control signals are low or
off. The R-processor, the G-processor, and B-processor are shown in FIGS.
18A-18C, where .alpha. has the value V.sub.MAX /0.25. The control signals
AA, AB, AC, AD and the nodes BA, BB, BC, BD in the preprocessing circuits
of FIGS. 17A, 17B are connected to the three processes as shown in FIGS.
18A-18C. The three processors each contains five to seven operational
amplifiers 202, a number of resistors with the values as shown, and a
switch 204. The R and B-processors each has two parallel branches, where
normally only one branch is operative at any time. Thus where I is within
the range 0.75A and A, the node BA is connected to the monochromatic
input, and the top branch of R-processor in FIG. 18A is operative to apply
the maximum voltage V.sub.MAX to the R output as shown in FIG. 18A. If I
is not within this range, node BA is connected to ground in reference to
FIG. 17B. If I is such that 0.5A.ltoreq.I<0.75A, control signal AB is high
or on, thereby connecting 0.5A to the input of the lower branch. The
output of this branch is .alpha. (I-0.5A) which has the shape of a
slanting straight line shown in FIG. 8. Where 0.ltoreq.I<0.5A, the inputs
of both branches are connected to ground so that the output of the
R-processor at node R is "0". The circuits of the G-processor and
B-processor in FIGS. 18B, 18C operate in a similar manner to implement the
transform functions of FIG. 8. Circuits similar to those shown in FIGS.
17A, 17B, 18A-18C may be designed to implement transform functions other
than those illustrated in FIG. 8.
FIG. 19 is a schematic circuit diagram illustrating in more detail the
design of the hybrid analog-digital pseudocolor encoder of FIG. 14.
While the invention has been described above by reference to several
embodiments, it will be understood that various modifications may be made
without departing from the scope of the invention which is limited only by
the scope of the appended claims.
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