Apparatus for combining graphics and video images in multiple display formats

An apparatus provides a signal for operating a display device to display combined video and graphics images in plural, selectable scene formats. The apparatus permits selection of a scene format containing an image and has respective sources providing video and graphics image signals. Based upon a selected scene format and the graphics image signal, the apparatus determines when, during display of the scene, an image is to be displayed, and selects from the video and graphics image signals to provide a display signal whose characteristics are based upon the selected image signal. The selectable formats include one displaying a video image alone, one displaying a graphics image superimposed on a video image, one including a graphics image embedded in a predetermined background, and one corresponding to a uniform foreground which curtains an image behind it.

BACKGROUND OF THE INVENTION 
The invention is in the field of image display composition, and more 
particularly is in the field of systems which combine graphics and video 
images for display on a single display device. 
In systems which utilize a display to represent and explain services to 
interested persons, it is often desirable to display a scene including 
video and graphics imagery. For example, the display of program material 
with spoken narrative explanation will be incomprehensible to a deaf 
person. However, the provision of written narrative in a section of the 
program display will permit a deaf person to comprehend both the visual 
and verbal content of the program. Manifestly, the written portion must be 
presented on the same display as the visual program if the deal viewer is 
to appreciate the verbal and visual segments simultaneously. 
One way of providing the written verbal narrative on the same display is to 
utilize a conventional graphics generator to generate common alphanumeric 
characters. Normally, the operation of the generator is directed by a 
program controller which would insure that the written verbal content 
output by the generator is synchronized with the visual presentation. 
In other applications, it is useful to provide a display system which can 
display not only written program narration, but a board set of graphics 
images and symbols which can be displayed in any combination with video 
images. Such a system would be flexible and adaptable to a wide variety of 
uses. 
Therefore, it is evident that there exists a need for a display apparatus 
having the capability of enabling the display of both graphics and video 
images in a variety of combinations. 
SUMMARY OF THE INVENTION 
The present invention is an apparatus which, under the control of a format 
selection processor, combines video and graphics image signals from 
respective sources to produce a display signal suitable for causing a 
display device to show the images in a variety of selectable scene 
formats. 
The apparatus of the invention includes a video signal generator which 
provides a signal representative of a video image, a graphics generator 
which provides a graphics signal representative of a graphics image, and a 
control device which selects one of a plurality of scene formats in which 
one or more of the images is to be displayed and provides a signal 
representative of the selected format. A format control and composition 
circuit responds to the format signal by selectively processing the video 
and graphics signals to produce a display signal which causes the display 
to present the selected scene format containing the image to be displayed. 
The format control composition circuit processes image signals by 
selecting, in response to the format signal, and in synchronism with the 
video signal, a portion of an image signal representative of a portion of 
an image to be displayed. The circuit then produces a display signal 
consisting of a sequence of selected image signal portions. The display 
signal is provided by the format control and composition circuit in 
synchronism with the video signal. 
The invention also contemplates a method of combining video and graphics 
signals to produce a display signal which causes a display device to 
display one or more of the images in multiple, selectable formats. The 
method includes the steps of selecting a display format, selecting, in 
response to the selected format, and in synchronism with the video signal, 
a portion of an image signal representative of a portion of an image to be 
displayed, and synchronously providing a sequence of selected image signal 
portions as the display signal. 
Therefore it is an object of the present invention to provide an improved 
apparatus for providing a display signal which causes a display device to 
combine video and graphics images and to display the combined images in 
multiple, selectable formats. 
A further objective of the present invention is to provide an apparatus 
which combines video and graphics signals in selectable ways which permit 
the display of the combined images in multiple, selectable formats. 
A further object of the present invention is to provide a method for 
combining video and graphics image signals to produce a display signal 
which causes a display device to display one or more of the images in 
multiple, selectable formats. 
Other objects and further advantages of the disclosed invention will become 
more apparent when the following detailed description is taken together 
with the described drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The system of the invention is illustrated in a typical operational 
environment in FIG. 1. In FIG. 1 a conventional three-gun, color CRT 
display 10 is included in a system which interacts with a user to perform, 
for example, an automobile leasing transaction. In order to inform the 
customer of the automobiles which are available to be leased, moving video 
images of the automobiles are displayed on the screen of the display 10. 
One or more graphics images can be displayed together with the images of 
the automobile to indicate the customer's acceptance or non-acceptance of 
a pictured automobile. For example, the graphics images may consist of the 
words YES and NO which are enclosed in respective boxes on the lower part 
of the display screen. 
A customer indicates his response to touching one of the boxes and a 
conventional touch screen apparatus 11 provides an indication of the box 
which has been touched. 
Depending upon the customer's decision, another image of an automobile may 
be projected together with the decision boxes should the customer decide 
not to accept the initially displayed automobile. Alternatively, if the 
customer decides to accept the automobile, another image may be shown, for 
example, a scene comprising a listing in alphanumeric graphic images of 
the different rental plans available for the selected automobile. 
Displays such as that indicated by 10 and touch screen systems which 
perform the functions required of the touch screen apparatus 11 are 
well-known in the art. In the arrangement just described, the display is 
required to display scenes which comprise both video and graphics images 
separately or in combination. The touch screen apparatus 11 provides a 
signal indicating when a particular display is to be changed. In this 
operational application, the apparatus of the invention (indicated by 12 
in FIG. 1) can respond to the signals provided by the touch screen 
apparatus 11 by selecting a scene format in which one or more video or 
graphics images are to be displayed. The apparatus 12 further selects one 
or more images to be displayed in the selected format and composes a 
display signal which is effective to cause the display 10 to display the 
selected images in the selected format. 
The apparatus 12 includes a control processor 13 which is connected 
conventionally to a typical bus interface 14 comprising address, data, and 
control lines. The bus 14 interconnects the processor 13 with the touch 
screen 11. The bus 14 also interconnects the processor 13 with a graphics 
generator 16 and a display format control and composition circuit 18. The 
processor accesses a laser disc player 20 through the control and 
composition circuit 18, and the disc player 20 is connected to a 
conventional NTSC decoder 22. 
The processor 13 can comprise, for example, a microprocessor which is 
conventionally programmed to execute a format and image selection routine, 
described in greater detail below. In the preferred embodiment, the 
control processor 13 includes a microprocessor available from Zylog, Inc., 
under the tradename Z-80. 
The graphics generator 16 is connected to be controlled over the bus 
interconnection 14 by the processor 13. Both FORMAT SELECT and GRAPHICS 
SELECT signals are provided by the processor to the graphics generators. 
The FORMAT SELECT signal indicates the scene format which has been 
selected for display. If the selected scene is to contain a graphics 
image, the image is indicated by the GRAPHICS SELECT signal. In response 
to the FORMAT and GRAPHICS SELECT signals, the generator generates and 
provides a graphics image signal representative of a graphics image 
selected by the processor 13, The operation of the graphics generator 16 
is synchronized to the video image signal and to the operation of the 
display 10 by the provision of an SYNCHRONIZATION signal from the control 
and composition circuit 18. 
Selection of video images is made by the processor 13 which passes a VIDEO 
SELECT signal to the control and composition circuit 18. The VIDEO SELECT 
signal is indicative of a storage location on a laser disc, not shown, 
which is conventionally used by the laser disc player 20. Video images are 
extracted from the indicated laser disc locations and provided by the 
player 20 as video image data to the NTSC decoder 22 which processes the 
video image data into conventional NTSC composite video format and 
provides the formatted video image signal to the control and composition 
circuit 18. 
As is typical, the NTSC decoder also provides a composite NTSC 
synchronization signal. The NTSC synchronization signal is fed to the 
display 10 which responds to it and to the DISPLAY signal to display the 
selected images in the selected format according to the well-known, two 
dimensional scanning technique. 
The synchronized graphics image and composite video image signals are fed 
to the display format control and composition circuit 18 which combines 
them in a manner described hereinbelow to produce the DISPLAY signal. The 
DISPLAY signal comprises components for operating the red, green and blue 
(R, G, B) guns of the display device 10. 
There are a variety of available equipments and devices which can be used 
to perform after "certain" functions certain of the apparatus 12 
illustrated in FIG. 1. For example, the graphics generator 16 can include 
a high resolution color graphics generator available under the equipment 
number STD-C-GDC-1024 from Antares Technology Associates, Inc., San Diego, 
Calif. The laser disc player 18 can include a Pioneer LDV1000 layer disc 
player which operates using a laser disc having a video track which is 
divisible into separate, uniquely addressable sections, each of which can 
conventionally store video image data. 
When the apparatus of the invention operates in conjunction with a 
user-interactive setup such as the touchscreen display 10, the control 
processor 13 can be appropriately programmed to select various formats and 
images for display on the display device 10 in response to touch 
indications received from the touch screen apparatus 11. For example, the 
processor may be programmed as illustrated by the flow chart of FIG. 2 to 
output an initial screen showing a video image including a moving 
automobile. This display can be obtained, for example, from an 
appropriately addressed section of the laser disc included in the disc 
player 20. The processor also controls the graphics generator 16 to 
produce a graphics message directing a person to touch the screen if he 
wishes to rent an automobile. Both the graphics image comprising the 
message to the user and the video image comprising the scene of the moving 
automobile are provided to the display control and composition circuit 18 
which combines them to produce the DISPLAY signal. 
The initial scene is displayed until a customer, desiring to rent a car, 
touches the screen, which action is detected by the touch screen apparatus 
11 which provides an appropriate touch response signal to the processor 
13. 
When the touch response signal is received by the processor 13, its 
operational program selects a scene format which, for the purposes of 
illustration, can be characterized as format 1, and selects images to be 
displayed according to the selected format. 
The processor 13 then sends appropriate FORMAT and graphics select signals 
to the graphics generator 16 indicating a selected format and a graphics 
image, if one is included in the format, and a VIDEO SELECTION signal 
which is passed through the control and composition circuit 18 to the disc 
player 20, indicating a selected video image if the scene includes one. At 
the same time the FORMAT SELECT signal, indicating the format selected, is 
also provided to the display format and composition circuit 18, which 
causes the circuit to combine the selected graphics and video image 
signals in a manner determined by the format control signal and to provide 
the combined signals as the DISPLAY signal. 
The selected format 1 can include the scene illustrated on the display of 
FIG. 1. In this case, the selected video image would be of a moving 
automobile while the selected graphics image would comprise the 
alphanumeric characters enclosed in the respective rectangles. Now, the 
selection of another scene format and another image or images to be 
presented will depend upon which rectangle has been touched in the scene 
illustrated on the display 10 of FIG. 1. In case the NO image is touched, 
the touch screen apparatus 11 will provide an appropriate signal to the 
processor 13 indicating the customer's rejection of the displayed 
automobile. This may be characterized as TOUCH RESPONSE 1 in FIG. 2. 
In the case of TOUCH RESPONSE 1 the processor 13 will select another video 
image illustrating another automobile and will display that image together 
with the previously-selected graphics image in format 1. This process can 
be repeated until a YES response, characterized as TOUCH RESPONSE 2 in 
FIG. 2 and indicating selection of the displayed automobile, is received. 
When the YES response to received by the processor 13, it can select a new 
scene format, different from format 1, and a new set of images to be 
displayed according to the new format. A representative new format would 
include a written verbal representation of leasing rate plans which would 
be available to the customer for the selected automobile. This could 
comprise, for example, a list of time periods for which the car could be 
rented, each displayed with the current rental rate applying to that 
period. The listing can be displayed as a solely graphics image including 
the graphics symbols necessary to present the list. The display of a 
graphics image according to format 2 is illustrated in FIG. 3. 
Returning to FIG. 2, after the format 2 list has been displayed for a 
predetermined period of time, the processor 13 returns to its initial 
state, reselects the initial state images and format, and the apparatus of 
the invention operates the display 10 to provide the initial display. 
The operation of the display format control and composition circuit 18, 
which is central to the operation of the apparatus 12 of the invention, 
can be understood with reference to FIGS. 4-6. As is conventional, a scene 
is presented on the display 10 in composite, scanned NTSC video format 
which is a time sequence of analog traces, each trace representing the 
three-color RGB (red, green, blue) signal along a horizontal sweep line of 
the scene. Each horizontal sweep line is composed of a sequential array of 
individual picture elements or pixels, each representing a small portion 
of the displayed scene which is scanned by the horizontal line. 
Subdividing the scanned scene in this manner produces a set of pixels in a 
two-dimensional array with each pixel individually identified by its 
coordinates in the array. 
FIG. 4 illustrates a 3.times.3 matrix of pixels from the location 
indication by 30 in the scene presented by the display 10 in FIG. 1. The 
pixels in the matrix of FIG. 4 are arbitrarily numbered for purposes of 
illustration only. In the format of the FIG. 1 scene, a portion of the 
graphics image generated and provided by the generator 16 and comprising a 
portion of the right hand vertical side of the box containing the message 
YES includes pixels P.sub.2, P.sub.5, and P.sub.8. The other pixels of the 
matrix represents portions of the road included in the video image 
extracted from the laser disc player 20. The control and compositions 
circuit 18 controls the characteristics of the DISPLAY signal so that, 
while it is tracing the segment of the horizontal line containing pixels 
P.sub.1 -P.sub.3 those characteristics are determined by the composite 
video image signal while the DISPLAY signal is tracing the portion of the 
scene occupied by the pixels P.sub.1 and P.sub.3. Similarly, while the 
DISPLAY signal is providing the park of the scene occupied by the pixel 
P.sub.2, its characteristics are determined by the portion of the graphics 
image signal supplied by the generator 16 which defines that portion of 
the YES box. 
A portion 32 of the FIG. 3 scene is magnified in FIG. 5 where the 3.times.3 
pixel array now spans the symbol numeral 1 which is presented against a 
uniform background. As with the scene portion illustrated in FIG. 4, that 
part of the symbol including pixels P.sub.2, P.sub.5, and P.sub.8 are 
drawn by portions of the DISPLAY signal whose characteristics are 
determined by the graphics image signal from the generator 16. The 
background portions including the other pixels are determined by the 
FORMAT signal supplied to the control and composition circuit 18 in a 
manner described hereinbelow. 
The formats among which the processor selects to specify the general form 
of the scene presented by the display 10 can be understood with reference 
to FIG. 6. In FIG. 6 three planes are shown which are selectively 
activated by the apparatus of the invention in order to establish a scene 
format. When a video image is to be included in the format, it is 
displayed on what may be considered a video plane. When a graphics image 
is to be displayed embedded in a selected background, the video plane is 
blocked by the activation of background/graphics plane, with the selected 
image being presented on the background. In the case where a graphics 
image is to be superimposed onto a video image, the background/graphics 
plane is made transparent, everywhere except where the graphics image is 
to be displayed. With this arrangement, the graphics image overlays the 
video plane to produce the effect illustrated in the FIG. 1 scene. 
In the case where the control processor 13 selects a large or complex 
graphics image for display, it may be that the graphics generator 16 
requires a measurable amount of time to reconfigure itself for generation 
of the image. In this case it would be desirable to block the graphics 
plane while the image is being generated until such time as the graphics 
generator 16 has configured itself to generate the image. A foreground 
format is provided for this purpose which essentially consists of 
transferring the background/graphics plane from its position to a point in 
front of where the graphics image is to be displayed so that it is placed 
between the viewer and the asembling graphics image. Then, activation of 
the foreground plane will block the display of any of the planes behind it 
until such time as an image is ready for display on its respective plane. 
When the image is ready for display, the foreground plane can be shifted 
back to the position of the background plane. In this position it can 
either be made transparent if video and graphics images are to be 
displayed together or it can be activated with a selected background if 
only graphics images are to be displayed. 
The apparatus of the invention therefore has the ability to select one from 
a plurality of display formats. In one format a video image extracted from 
the disc player 20 is displayed alone. In the second format, a graphics 
image generated by the graphics generator 16 is superimposed on a video 
image from the disc player. Another format presents a graphics image 
against a selected background. Finally, in the foreground format, the 
display of any image is curtained by a uniform foreground. 
With reference now to FIGS. 7 and 8, the detailed structure and operation 
of the apparatus of the invention can be more fully appreciated. The 
format control and composition circuit 18 includes an information staging 
and synchronization section 40. This section 40 includes a subaddressable 
section SUBADDR1 for selection of a video image in the laser disc player 
20. A second subaddressable section SUBADDR2 stages format selection data 
from the control processor 13. 
For the selection of video imagery, the control processor 13 address 
SUBADDR1 on the address portion of the bus 14. At the same time it 
provides data on the data section of the bus which indicates the storage 
section of the laser disc containing the desired video image. The data is 
forwarded as a VIDEO SELECT signal by section SUBADDR1 to the laser disc 
player 20. The disc player then accesses the selected section on the disc 
and forwards the stored information to the decoder 22 which renders it 
into a conventional composite video format including red, green and blue 
(R, G, B) signals. The composite signals are forwarded as the VIDEO IMAGE 
signal. As stated above, the decoder 22 also provides a standard composite 
SYNCHRONIZATION signal comprising horizontal and vertical synchronization 
segments which are used to synchronize the operation of the display 10 in 
a conventional manner. 
The SYNCHRONIZATION signal is also provided to a synchronization strip 
circuit 42 which operates in a well-known manner to strip the horizontal 
synchronization (HSYNCH) signal. As is known, the HSYNCH signal defines 
the beginning of the application of a horizontal sweep of the DISPLAY 
signal to the display 10 is displaying a line of a presented scene. The 
HSYNCH signal is fed to a standard phase comparator 44 which compares it 
with an oscillating signal having the same frequency as HSYNCH. The 
comparator circuit 44 develops an error signal indicative of the phase 
error between HSYNCH and the oscillatory signal which is provided to a 
pixel clock circuit 46. The pixel clock circuit provides a clock signal 
which oscillates at the frequency with which the pixels are swept in a 
horizontal sweep of the DISPLAY signal. The error signal from the 
comparator circuit 44 therefore synchronizes the phase of the PIXEL CLOCK 
signal with the occurrence of pixels during the sweep of a horizontal line 
on the display 10. 
The PIXEL CLOCK signal is provided to a synchronization section 50 of the 
graphics generator 16. The synchronization section 50 responds to the 
PIXEL CLOCK signal by clocking the operations of an image generation 
section 52 of the graphics generator 16. The STD-C-GDC-1024 device which 
forms the graphics generator 16 comprises a preprogrammed microprocessor 
which responds to high level commands from the control processor 13 to 
generate the selected graphics image and provide it pixel-by-pixel at the 
rate of the PIXEL CLOCK signal. The microprocessor of the generator 16 
receives the GRAPHICS SELECT signal from the control processor 13 which 
indicates which image has been selected, the color the image is to be 
displayed in, and the location in pixel coordinates which the image is to 
occupy on the screen. The microprocessor of the generator 16 then employs 
conventional graphics generation algorithms to generate the graphics image 
at the PIXEL CLOCK rate and to synchronize its output with the operation 
of the display 10 so that the image is provided at the time the DISPLAY 
signal is sweeping the display locations which the image is to occupy. 
When the graphics generator 16 has responded to the GRAPHICS SELECT signal 
provided by the control processor 13, and configured itself for provision 
of the graphics image signal, it sends a signal GDONE back to the control 
processor 13 indicating that the graphics image selected can now be 
generated. The signal GDONE is used by the control processor 13 to shift a 
foreground plane curtain to its normal background position so that the 
graphics image can be displayed. 
The format selected by the control processor 13 is provided s the FORMAT 
SELECT signal to both the generator 16 and the subaddressable format 
section SUBADDR2 of the circuit 40, which forwards the signal as a FORMAT 
signal. 
A select logic circuit 60 receives the GRAPHICS IMAGE signal from the 
generator 16, the FORMAT signal from the subaddressable section SUBADDR2, 
and the VIDEO IMAGE signal from the NTSC decoder 22. The select logic 
circuit 60 responds to the FORMAT signal by processing the GRAPHICS IMAGE 
and VIDEO IMAGE signals to produce a DISPLAY signal which presents a 
selected image or selected images in the selected format. The control 
logic circuit 60 also responds to the FORMAT signal by selectively 
activating the foreground and background planes and implementing the 
selected background or foreground. The select logic circuit 60 further 
responds to the FORMAT signal by making the background/graphics plane 
transparent when the video display format is selected. 
As shown in FIG. 8, the select logic circuit 60 comprises a conventional 
programmable logic array (PLA) which, in the preferred embodiment, 
comprises an 82S153 device. An array of three decoders 64-68 are connected 
to the outputs of the PLA 62. In turn, the outputs of the decoders 64-68 
are fed through a bank of standard buffer circuits, one of which is 
indicated by 70, to the inputs of three bilateral switches 80, 82, and 84. 
Preferably, the decoder circuits comprise 7445 devices, and the switches, 
4066 circuits. 
The inputs to the PLA 62 include the GRAPHICS IMAGE signal which is a 
four-bit digital word representative of a current pixel portion of a 
graphics image and output by the graphics generator 16. The GRAPHICS IMAGE 
word is updated at the PIXEL CLOCK rate. The four bits of the GRAPHICS 
IMAGE signal are provided by the graphics generator on separate, parallel 
lines G.sub.0 -G.sub.3. The FORMAT signal forwarded from the control 
processor 13 also comprises four separate, parallel bits F.sub.0 -F.sub.3. 
The FORMAT and GRAPHICS IMAGE signals are connected to respective input 
nodes of the PLA 62 as shown in FIG. 8. 
The output nodes of the PLA 62 comprise three groups of two nodes A.sub.i 
and B.sub.i. Each group of two nodes is assigned to be determinative of 
the characteristics of a respective portion of the display signal which 
controls one of the guns on the display 10. Thus, for example, the output 
nodes A.sub.0 and B.sub.0 are assigned to control the red gun, and the 
signals which they provide are denoted as A.sub.R and B.sub.R, 
respectively. 
In the explanation which follows, control of the portion of the DISPLAY 
signal which is fed to the red gun of the display 10 is described, with 
the understanding that the description also characterizes the respective 
portions of the DISPLAY signal controlling the green and blue guns as 
well. Thus, the A.sub.R and B.sub.R signals are fed to the decoder 64 
which decodes them by activating a respective one of its output ports. The 
output ports of the decoder 64 are connected through their respective 
buffering circuits to the control nodes C.sub.0 -C.sub.3 of the switch 80. 
The switch 80 also receives, as illustrated, at its input nodes I.sub.0 
-I.sub.3 three input voltages V.sub.1 -V.sub.3 and the red portion R of 
the composite video signal from the NTSC decoder 22. The output nodes of 
the switch 80, O.sub.0 -O.sub.3, are connected together with the common 
connection being fed as the red R display portion of the DISPLAY signal. 
The R portion of the DISPLAY signal is conventionally connected to drive 
the red gun of the display 10. 
Returning to the switches 80-84, the voltages V.sub.1 -V.sub.3 are set at 
levels corresponding to white (WH), gray (G) and black (B) voltage levels 
respectively. Thus, if, for example, voltage V.sub.1 were passed by the 
switches 80-84 and connected therethrough to the red, green and blue guns 
of the display 10, the composite display would be white. 
The operation of the select logic circuit, illustrated in FIG. 8, is 
determined by the combination of the instantaneous states of the GRAPHICS 
IMAGE and FORMAT signals input to the PLA 62. The input signal 
combinations define the output states of the PLA 62 according to the table 
illustrated in FIG. 9. The output of the PLA 62 establishes the scene 
format, determines the pattern of the background and foreground, maps the 
GRAPHICS IMAGE signal to a color contained on a color look-up table, and 
implements the display of the images represented by the VIDEO and GRAPHICS 
IMAGE signals, if required. 
In the table of FIG. 9, the input combinations of the FORMAT and GRAPHICS 
IMAGE signals are listed vertically. Since these signals are provided on a 
conventional tri-state databus, each can have one of three possible 
states. Thus, for example, the F.sub.0 bit of the FORMAT signal can assume 
a high voltage state indicated by H, a low voltage state indicated by L, 
or an intermediate state indicated by no entry, which is essentially 
equivalent to placing the F.sub.0 line driver in a high input impedance 
condition. As is conventional, a dash - in the input portion of the table 
represents a don't-care input condition. In the output portion of the 
table, the designations H and L refer to high and lower states, 
respectively. 
In the table of FIG. 9 the possible combinations of the GRAPHICS IMAGE and 
FORMAT signals input to the PLA62 are numbered in the column 90 labelled 
"Input Signal Combination" and specified in the eight columns to the right 
of the column 90. Each input signal combination determines a respective 
combination of the outputs A.sub.i B.sub.i of the PLA; this is shown in 
the columns labelled "Display Selection Outputs". In turn, the outputs 
A.sub.i B.sub.i, which are input to the decoders 64-68, determine the 
outputs of the decoders according to the table of FIG. 10. Thus, for 
example, when A.sub.R B.sub.R are both low, the 0 output of the decoder 64 
is selected which causes the switch 80 to provide voltage V.sub.1 as the R 
portion of the DISPLAY signal. Similarly, when A.sub.R is high and B.sub.R 
is low, the red portion of the VIDEO IMAGE signal is provided as the R 
portion of the DISPLAY signal. 
It should be evident, then, that each input signal combination of FIG. 9 
selects either a respective signal combination of three voltages, or the 
red, green and blue portions of the VIDEO IMAGE signal, with the selected 
signals provided through the switches 80-84 as the DISPLAY signal. This is 
shown in the three righthand columns of FIG. 9. 
When an input combination causes a respective associated combination of 
voltages to be selected by the switches 80-84, the voltage combination 
causes the display to assume a respective color. The colors associated 
with the input and voltage signal combinations are in the column of FIG. 9 
to the left of column 90. 
Finally, FIG. 9 illustrates how the selected scene m and image displays are 
implemented. When the F.sub.0 bit of the FORMAT signal is high, the 
display 10 is caused to operate in either the video only or foreground 
formats. In the video only format, the F.sub.0 bit is high, and the 
F.sub.1 -F.sub.3 drivers are in their switched-off or high-impedance 
states. This causes the A.sub.R signal to be high and the B.sub.R to be 
low, which selects output node 1 of the decoder 64 as illustrated in FIG. 
10. When the output node 1 of the decoder 64 is selected, it assumes a low 
voltage state which is inverted by the buffer circuit connected between 
decoder node 1 and input C.sub.1 of the switch 80. This activates the 
switch S.sub.1 of the switch circuit 80, connecting the input node I.sub.1 
to the output node O.sub.1. Since node I.sub.1 is connected to the R 
portion of the VIDEO IMAGE signal, the switch S.sub.1 connects the R 
portion to the red gun of the display 10. As illustrated in FIG. 9, the 
video only state similarly configures the decoders 66 and 68 as well as 
the switches 82 and 84 so that the DISPLAY signal comprises the composite 
VIDEO IMAGE signal from the decoder 22. 
When a foreground format is selected, the F.sub.0 bit is set high together 
with one or more of the other FORMAT signal bits F.sub.1 -F.sub.3. When 
this occurs, the outputs of the PLA 62 cause the switch circuits 80-84 to 
be set to select some predetermined combination of the voltages V.sub.1 
-V.sub.3. The voltage combination is passed through the switch circuits 
80-84 as the DISPLAY signal. In the preferred embodiment, a foreground 
configuration causes the display to present an unbroken, monochromatic 
scene which cannot be affected by either the GRAPHIC IMAGE or the VIDEO 
IMAGE signal. The unbroken, monochromatic scene will be displayed until 
the FORMAT signal is changed by the control processor 13. 
When a graphics image is to be presented against either a selected 
background or a video image, the F.sub.0 bit of the FORMAT signal is set 
to a low L state. When the format combining a video and a graphics image 
is selected, all of the bits F.sub.0 -F.sub.3 are set low. This is 
detected by the graphics generator and causes it to set its tri-state 
logic drivers to provide bits G.sub.0 -G.sub.3 low when the display is 
sweeping pixels which do not contain portions of the graphics image. Thus, 
referring again to FIG. 4, when pixels P.sub.1 and P.sub.3 are being swept 
by the DISPLAY signal, the graphics generator 16 sets each of its output 
drivers providing bits G.sub.0 -G.sub.3 to a low state. As illustrated in 
FIGS. 9 and 10, when this occurs, the outputs of the PLA 62 assume states 
which cause the switch circuit 80-84 to pass the R, G, and B portions of 
the composite VIDEO IMAGE signal to the display 10. However, when pixel 
P.sub.2 in the same horizontal line is being swept, the graphics generator 
16 will set its GRAPHICS IMAGE signal drivers to produce one of the 
combinations of high voltage and high impedance signals which are 
specified in combinations 1-15 of the table in FIG. 9. For example, if the 
graphics image portion displayed at P.sub.2 in FIG. 4 is to be orange, 
bits G.sub.0 and G.sub.3 will be set to a high voltage level, while bits 
G.sub.1 and G.sub.2 will be set to the high impedance level. This 
corresponds to input combination 9, which will produce the combination of 
output signals necessary to cause the switching circuits 80-84 to provide 
the voltages V.sub.1, V.sub.2, and V.sub.3 to be red, green, and blue 
guns, respectively, of the display device 10. When the guns are activated 
by this combination of voltages, the resulting portion of the graphics 
image occupying the space defined by pixel P.sub.2 will be orange. As 
another example, if the portion of the graphics image occupying the pixel 
P.sub.5 area is to be black, then G.sub.0 is set high while bits G.sub.1 
-G.sub.3 are set to their high impedance levels. This corresponds to the 
graphic signal input at state 8 of the PLA table which will result in 
V.sub.3, the black level voltage, being fed to each of the color guns. 
When the background format is selected, the graphics generator 16 sets each 
of the bits G.sub.0 -G.sub.3 to the high impedance level when the display 
signal is sweeping horizontal line portions which do not contain portions 
of the graphics image being displayed. This permits the output of the PLA 
62 to be determined by the FORMAT signal bits F.sub.1 -F.sub.3. Thus, for 
example, if, in FIG. 5, the graphics image is to be displayed in a black 
background and the line containing the pixels P.sub.1 -P.sub.3 is being 
swept, the input signal bits for the PLA will assume the levels defined 
for them in input state 16. As the FIG. 9 table shows, input state 16 will 
cause the switch circuits 80-84 to provide V.sub.3, the black voltage 
level, to the red, green, and blue guns of the display 10. When the 
display signal is deflected to the pixel P.sub.2 location, the bits 
G.sub.0 -G.sub.3 will assume one of the input combinations 1-15, resulting 
in the display of the graphics image portion occupying that pixel 
location. 
In view of the operation of the select logic circuit 60, one can regard the 
DISPLAY signal which it produces as consisting of three parallel data 
streams, one for each gun of the display device 10. Each data stream 
consists of a succession of pixel-defining voltages which are synchronized 
by the GRAPHICS IMAGE signal inputs to the PIXEL CLOCK signal rate. Since 
PIXEL CLOCK is phased to the NTSC SYNCHRONIZATION signal, the data 
streams, and thereby the DISPLAY signal, are synchronized with the 
operation of the display 10 which receives the SYNCHRONIZATION signal from 
the decoder 22. 
Each pixel-defining voltage of a DISPLAY signal color stream is determined 
by a respective input state of the FIG. 9 table. Thus, the scene-producing 
characteristics of the DISPLAY signal can be said to be determined by the 
combination of the GRAPHICS IMAGE and FORMAT signals by the select logic 
circuit 60. 
Obviously, many modifications and variations of the described invention are 
possible in light of the above teachings. It is therefore to be understood 
that within the scope of the appended claims the invention may be 
practiced otherwise than as specifically described.