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Claims  |
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What is claimed is:
1. A graphics generator for driving a display in response to plural groups
of pattern segment defining input signals to produce a visual pattern on
said display including a visual representation of said pattern segments,
said graphics generator providing first and second periodically varying
output signals from first and second output registers with each successive
pair of said output signals differing from a preceding pair by an amount
producing a stroke of constant length on said display, said graphics
generator comprising:
rotation control means responsive to one of said pattern segment defining
signals to store a quantity representative of at least initial pattern
segment orientation,
first means with sine and cosine outputs, producing respectively, sine and
cosine functions of an input, said first means coupled to said rotation
control means and receiving said input therefrom,
a first and second adder, each with a first input, said first input of said
first adder connected to said sine output and said first input of said
second adder connected to said cosine output,
said first and second output registers responsive, respectively, to outputs
of said first and second adders, second inputs of said first and second
adders connected, respectively, to outputs of said first and second output
registers,
clocking means for periodically strobing each of said output registers to
accept and store said first and second adder outputs,
whereby quantities stored in respective output registers, periodically
change in accordance with quantities provided to each respective adder by
said first means.
2. The apparatus of claim 1 further comprising:
(a) means for enabling said clocking means responsive to another of said
pattern segment defining signals,
(b) length register means responsive to a further pattern segment defining
signal for storing therein a pattern segment defining signal representing
pattern segment length and enabled by said another pattern segment
defining signal,
(c) decrementing means responsive to said clocking means, when enabled, for
decrementing said length register means and for disabling said clocking
means when incremented to a predetermined state, whereby said first and
second output registers produce output signals periodically varying from
reception of said another of said pattern segment defining signals until
disablement of said clocking means.
3. The apparatus of claim 2 wherein said at least one pattern segment
comprises a conic section to be approximated by a series of strokes of
varying orientation in which said pattern segment defining signals include
a first signal representing conic circumferential length, a second signal
representing radius of curvature and a third signal identifying said
pattern segment as a conic, said apparatus further including:
a direction register, adding means and rotation register in said rotation
control means, with said rotation register coupled to an output of said
adding means, inputs of said adding means coupled to said direction and
rotation registers,
means coupling said second signal to said direction register,
means coupling said first signal to said length register means enabled by
said third signal,
control means responsive to said third signal and to said clocking means
for producing a pulse stream at a rate related to said clocking means
output,
means responsive to said control means pulse stream for strobing said
rotation register to store an output of said adder,
whereby said periodically varying output signals represent said strokes
whose orientation changes at a rate determined by said pulse stream rate
and the quantity represented by said second signal.
4. The apparatus of claim 2 which includes means for generating
periodically varying output signals to represent a pattern segment
comprising one of a plurality of alpha numeric characters, said means
including:
a character address counter responsive to a one of said pattern segment
defining input signals,
second means for enabling said clocking means responsive to a character
defining input signal,
a memory device addressed by said character address counter with an output,
said memory device storing one or more character segment defining
quantities for each of said plurality of characters, said memory device
providing a character segment defining signal representative of a
character segment defining quantity stored at a location of said memory
device addressed by said character address counter,
a character segment length register responsive to a portion of a character
segment defining signal for storing therein a representation of said
signal portion,
means coupling another portion of a character segment defining signal to
said rotation control means,
character segment length register decrementing means for decrementing said
character length register responsive to said clocking means,
means to increment said character address counter when said character
length counter is decremented to a predetermined state, and means coupled
to said character length counter when in another predetermined state for
disabling said clocking means,
whereby said memory device produces a character segment defining signal for
each of one or more character segments to produce periodically varying
output signals from said graphics generator representative of a character
defined in said memory device wherein each character segment is comprised
of a plurality of said strokes.
5. The apparatus of claim 1 which includes apparatus for initializing said
graphics generator, comprising:
first and second multiplexers each with two inputs and an output, first
inputs of said multiplexers coupled respectively to outputs of said
adders, for coupling outputs of first and second adders to said output
registers when said clocking means is enabled,
bus means,
second inputs of said multiplexers coupled to said bus means,
control means responsive to first and second initializing control signals
for allowing an associated second multiplexer input to be effective,
whereby application of either a first or second initializing signal on said
bus means in time coincidence with first or second initializing control
signals is effective to initialize first or second output registers,
respectively.
6. The apparatus of claim 1 wherein said first means comprises a digital
memory device with a single addressing input and sine and cosine outputs,
said memory device providing at a sine output a digital representation of
1 sin 0 and at a cosine output a digital representation of 1 cos 0 in
response to an addressing input representing 0.
7. A graphics generator for producing periodically changing pairs of
digital output signals, changes in said pairs of output signals in each
period representing a unit length stroke when displayed to provide for
constant writing speed CRT display when said pair of output signals are
employed as deflection signals comprising:
an input data bus,
a pair of output registers for supplying said output signals and
selectively coupled to said bus for initializing said output registers,
a length register selectively responsive to said bus for initializing said
length register,
a pair of adders, each with two inputs and an output, an input of each said
adder coupled to a different one of said output registers, said adders
outputs coupled to a corresponding one of said output registers,
incrementing means coupled to another input of each said adder, said
incrementing means including a means for storing a quantity representative
of desired orientation for said unit length stroke, and further including
means for deriving sine and cosine functions of said quantity, and
clocking means for periodically strobing adder outputs into said output
registers and for decrementing said length register.
8. The apparatus of claim 7 wherein said incrementing mean includes:
a memory device storing sine and cosine functions for a plurality of
angular valves of orientation, and
at least one register provides an input to said memory device for
addressing a particular sine and cosine function stored therein.
9. The apparatus of claim 7 in which said graphics generator produces
digital output signals representing a straight line starting at X.sub.O
Y.sub.L of length L and orientation 0 which further includes,
means responsive to first and second control signals for coupling one and
another of said output register to said bus to store in said registers
quantities representing X.sub.O and Y.sub.O, respectively.
an orientation register in said incrementing means,
means reponsive to a third control signal for storing a quantity
representative of 0 on said data bus in said orientation register,
means responsive to a fourth control signal for coupling said length
register to said data bus for storing a quantity representative of L and
for enabling said clocking means,
said incrementing means further including a memory device addressed by said
orientation register and storing sine and cosine functions, said memory
device coupling said sine and cosine functions respectively to said
adders.
means for decrementing said length register responsive to said clocking
means, and
means for disabling said clocking means when said length register is
decremented to a predetermined state.
10. The apparatus of claim 7 in which said graphic generator produces
digital output signals representing a conic starting at X.sub.O, Y.sub.O
of circumferential length L and radius of curvature r which further
includes:
means responsive to first and second control signals for coupling one and
another of said output registers to said bus to store quantities
representing X.sub.O and Y.sub.O, respectively,
a direction register, rotation register and rotation adder in said
incrementing means, said direction register coupled to said input bus,
said rotation adder having an input coupled to both direction and rotation
registers and an output coupled to an input of said rotation register,
means responsive to a third control signal for storing a quantity
representing r in said direction register,
means responsive to a fourth control signal for coupling said length
register to said data bus to store a quantity representative of L and for
enabling said clocking means,
means responsive to said clocking means for enabling said rotation register
to periodically store a quantity provided by said rotation adder,
said incrementing means further including a memory device addressed by said
rotation adder output and storing sine and cosine functions, said memory
device coupling said sine and cosine functions respectively to said
adders.
means responsive to said clocking means for decrementing said length
register, and
means for disabling said clocking means when said length register is
decremented to a predetermined state.
11. The apparatus of claim 7 in which said graphic generator produces
digital output signals representing an alpha numeric character which
further includes:
an alpha numeric character definition memory and an addressing counter,
selectively responsive to said bus, for addressing said character
definition memory,
a character segment length counter responsive to said character definition
memory for storing a quantity representative of length of a character
segment,
means coupling said character definition memory to said incrementing means
for storing a quantity representative of character segment orientation,
means coupling said clocking means to decrement said character segment
length counter and for incrementing said addressing counter when said
character segment length counter has been decremented to a predetermined
state, and decoding means responsive to another predetermined state of
said character segment length counter for disabling said clocking means.
12. The apparatus of claim 11 which further includes:
means responsive to a first control signal for coupling said addressing
counter to said bus and for coupling said clocking means to said character
segment length counter.
13. The apparatus of claim 12 in which said character can be displayed at
any selected orientation in which said incrementing means includes, a
direction register, a rotation register and a rotation adder connected to
sum outputs of said rotation register and said direction register, with
said rotation adder output coupled to said means for deriving sine and
cosine functions,
said character definition memory coupled to said direction register of said
incrementing means, said direction register selectively coupled to said
bus.
means responsive to a second control signal for coupling said direction
register to said bus, and further means responsive to a third control
signal to couple said rotation register to said direction register through
said adder to store in said rotation register a quantity from said
direction register,
whereby said character is displayed at an orientation determined by said
quantity stored in said rotation register.
14. A graphics generator for driving a CRT display with constant writing
speed from a pair of output registers which supply signals changing
periodically to represent a vector, conic or alpha numeric character
comprising:
mode selection control means responsive to predetermined control signals
for operating in vector, conic or alpha numeric modes,
a data bus,
a pair of adders coupled to corresponding output registers to store adder
output when said output registers are clocked, each adder having an input
connected to a corresponding output register, each adder having a further
input,
incrementing means connected to said further input of each adder, said
incrementing means supplying a digital output representing sine or cosine
functions of a variable to a different further input of said adders,
a rotation register and a direction register, said direction register
selectively coupled to said data bus under control of said mode selection
control means,
a rotation adder included in said incrementing means supplying said
variable, said adder having inputs connected respectively to said rotation
register and said direction register,
a length counter for storing a quantity representative of pattern length,
a multiplexer selectively connecting said output registers to said data bus
or to said pair of adders under control of said mode selection control
means,
a character address counter selectively connected to said data bus under
control of said mode selection control means,
a character segment length counter,
a character definition memory addressed by said character address counter
and providing outputs to said character segment length counter and to said
direction register,
and clocking means for clocking said output registers and for selectively
decrementing said length counter or said character segment length counter
under control of said mode selection control means, and
disabling means for disabling said clocking means when either said length
counter or said character segment length counter are in predetermined
states.
15. The apparatus of claim 14 in which said incrementing means includes a
memory device addressed by said rotation adder storing sine and cosine
functions.
16. The apparatus of claim 14 in which said rotation register is
selectively coupled to said rotation adder to store said adder output
under control of said mode selection control means and said mode selection
control means periodically couples said adder output to said rotation
register during conic mode operations.
17. The apparatus of claim 14 in which said character definition memory
stores plural words for each of several characters, each word defining
length and orientation for each of plural character segments.
means for incrementing said character address counter under control of said
mode selection control means when said character segment length counter is
decremented to a predetermined state. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates to apparatus for the display of patterns, and
more particularly, relates to apparatus which can be driven by digital
signals provided by a computer or the like, and which produces signals
capable of driving a display such as a cathode ray tube.
BACKGROUND OF THE INVENTION
For many years the art has been aware of the benefits to be gained by
driving a display such as a cathode ray tube with signals derived from a
computer, such as a digital computer. The computer can be arranged to
provide signals in a variety of formats depending upon the particular
application, and usually some type of graphics generator is employed so as
to produce signals of the type required to drive the display. In the case
of a cathode ray tube, for example, the signals required to drive the
display include beam deflection signals and unblanking signals. Deflection
signals determine beam position in, for example, an x-y or an r-.theta.
coordinate system. The pattern defining signals derived from the computer
can be in a variety of forms.
At one extreme, the signals provided by the computer can actually define
the movement pattern of the beam and may be connected directly, or through
a digital/analog converter to the beam deflection system. Such an
arrangement, however, can needlessly tie up a computer in generating beam
position signals preventing the computer from doing more useful
operations. It has therefore been appreciated that a simpler device can be
driven by coded signals and the simpler device, i.e., the graphics
generator, can actually produce the beam position signals necessary to
produce the pattern or pattern segment defined by the input coded signal.
For example, a vector can be defined by three quantities, a beginning
point, a length and an orientation or angle and a graphics generator can
be employed to convert these three quantities into beam position signals
to cause the beam to traverse the desired pattern. In addition, an
unblanking signal is required to illuminate the vector or line by
unblanking the cathode of the cathode ray tube at the time the beam
reaches the initial position, maintaining the cathode in an unblanked
condition during the time the beam traverses the desired vector, and then
blanking the cathode after the beam has passed the end of the vector.
Graphics generators capable of substantially performing the foregoing
functions are well known in the art. With the advent of such graphics
generators, it became apparent that special attention was required to
control the writing speed of the beam at a constant rate, otherwise,
pattern segments written at a slow rate would be over-illuminated, and
other pattern segments written at a faster rate would be
under-illuminated.
The desired goal of constant writing speed can be achieved, in a beam
deflection system operating in an x-y coordinate arrangement, by
controlling the incremental beam displacement per unit time such that the
square root of the sum of the resolved incremental component displacements
squared is equal to a constant. Such an arrangement is shown, for example,
in U.S. Pat. Nos. 3,576,461; 3,725,723; 3,761,765; 3,869,085, and
3,818,475. However, in each of these systems, the resolved incremental
offsets are derived from the computer, and thus to at least this extent,
the computer is tied up and prevented from performing other tasks.
It has also been recognized in the prior art that the computer can be freed
from unnecessary operations if predetermined patterns are defined by
selected codes, and a graphics generator employed to generate beam
position and unblanking signals responsive to the codes to produce the
desired pattern. For example, alpha numeric patterns can readily be
defined by the computer in shorthand (or code) form if the graphics
generator can be provided to respond to the shorthand (or code) form to
produce the necessary beam deflection and unblanking signals to produce
the pattern. Such a device is disclosed, for example, in U.S. Pat. No.
3,952,297. As disclosed in that patent, a character code selects a memory
location in which is stored data definitive of a series of micro strokes
which can be employed to produce the desired character on a CRT or the
like. While the patent alleges that it discloses an arrangement having
minimal data storage requirements, nevertheless, it requires storage of a
plurality of micro strokes, that is, the x and y coordinates of the end of
each micro stroke for each character. In addition, it is not at all
apparent that the apparatus disclosed can be employed to generate anything
but the preselected characters whose definition is stored. Furthermore,
since the storage includes the resolved increments in x and y directions,
it is not seen that the arrangement is adaptable to provide the character
at any selected orientation.
It is therefore one object of the present invention to provide a graphics
generator which is capable of accepting coded signals from, for example, a
digital computer definitive of a desired pattern, in which each of the
signals represents a different pattern segment, and which is capable of
operating on those pattern segment defining signals to produce digital
signals for controlling beam deflection and unblanking of a display such
as a cathode ray tube display to produce the desired pattern segment. It
is another object of the present invention to provide apparatus such as
apparatus of the type mentioned in which the beam deflection varies at a
constant rate so as to produce constant brightness displays, regardless of
the pattern segment, the size thereof or its orientation.
It is yet another object of the present invention to provide a graphics
generator which is capable of producing signals for deflecting the beam of
a display such as a cathode ray tube to display patterns comprising
pattern segments in the form of vectors, conics or characters. It is
another object of the invention to provide a graphics generator which
includes a storage device defining each character to be displayed which
further includes further apparatus so that such character can be displayed
at any desired position or orientation in the display.
SUMMARY OF THE INVENTION
These and other objects of the invention are met by providing a graphics
generator responsive to pattern segment defining signals in which each
pattern segment comprises a vector, conic or character. A vector defining
signal specifies at least vector length and orientation and the apparatus
includes a counter for storing the vector length and a register for
storing orientation. Each vector is broken up by the generator into a
series of strokes of uniform length regardless of orientation. To effect
this, the vector orientation is employed as an address into a memory to
provide resolved components of a unit length stroke at the proper
orientation, the respective outputs of the memory are coupled to adders
where the components are added to the previous beam position to produce a
final beam position signal for the stroke. At the same time, the counter
storing a quantity related to vector length is decremented once for each
stroke produced. When the counter is decremented to zero, a plurality of
strokes have been produced at the proper orientation to represent the
desired vector. Conics are achieved in much the same fashion, except that
the orientation address is varied at a constant rate, such as, for
example, by adding a constant to the orientation register after each
memory reference.
The apparatus for displaying characters employs the previously discussed
apparatus and, in addition, a character definition storage device which
defines a plurality of vectors making up the desired character.
By initializing the orientation register, the desired pattern segment,
whether it be vector, conic or character, can be written at the desired
orientation. By initializing output registers which receive the adder
outputs, the pattern segment is written beginning at the starting point
represented by the initialized values of the output registers.
Border control apparatus and speed control apparatus are also included so
that the pattern display is written only in predetermined areas. While the
foregoing provides for constant writing speed, throughout the display, the
writing speed may be selected as one of a number of available writing
speeds.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the invention will now be described in further
detail in the remaining portion of this specification when taken in
conjunction with the attached drawings in which identical reference
characters identify identical apparatus and in which:
FIG. 1 is a block diagram of a typical manner in which the graphics
generator of the invention finds particular utility;
FIGS. 2A through 2D are useful in explaining how the output signals of the
graphics generator can be employed to produce displays of various kinds;
FIG. 3 is a block diagram of a preferred embodiment of a graphics generator
in accordance with the principles of the invention;
FIGS. 4A, 4B, 4C, 4D and 4E are a detailed block diagram of a preferred
embodiment of the graphics generator; and,
FIGS. 5, 6 and 7 are flow diagrams which illustrate typical operations
performed in the preferred embodiment.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
As shown in FIG. 1, a pattern selection and positioning device 10 is
coupled to a graphics generator 14 via a data bus 11 and a control bus 12.
The graphics generator 14 has three output lines, one for Z axis control,
15; one for X deflection control, 16 and one for Y deflection control, 17.
The latter two lines are provided, respectively, to digital to analog
converters 18 and 19, the outputs of which are connected to the deflection
system 21 of a conventional display such as a CRT 20. The Z axis line 15
provided by the graphics generator 14 is coupled to the control grid 22 of
the conventional display 20.
The pattern selection and positioning device 10 can actually comprise a
wide variety of apparatus, although typically the functions required to be
performed by this apparatus will be performed by a conventional digital
computer. The graphics generator 14 is capable of responding to selected
sets of signals on data bus 11, when provided in association with one or
more control signals on control bus 12, in order to produce a pattern on a
conventional display 20 in accordance with the signals provided to it. The
patterns which are capable of being displayed are broken up into one or
more pattern segments and signals defining each pattern segment,
separately, are provided to the graphics generator 14. Pattern segments
that can be displayed include a line, the length of which is variable from
some minimum length which will be specified below, to a line extending
across the full display area at an angle which can be parallel to the
horizontal or vertical coordinate, or any angle in between. In specifying
a pattern segment which comprises a line, normally only the line length,
and direction of the line are specified. The beginning point of the line
is usually the end point of a previously written segment, although, if
not, the generator 14 can be initialized to begin writing at any selected
point.
Another pattern segment which can be employed is a conic pattern segment,
and specification of the conic pattern segment normally requires
specification of a circumferential length and a quantity related to the
radius of curvature.
The third and final type of pattern segment which can be displayed is a
character within a repertoire of characters which are internally defined
in the graphics generator 14. In addition to specifying the particular
character in the repertoire that is to be displayed, angular positioning
of the character may be specified.
A particular pattern can be made up of any combination of such pattern
segments.
In order to display any one of the aforementioned pattern segments, the
graphics generator produces an initial deflection signal so as to position
the display at the starting point for the pattern segment. For example, in
a display based on an X/Y coordinate system, such as the one which will be
disclosed in connection with the preferred embodiment, the initial
deflection signal includes an X and Y deflection component. Typically, one
segment will begin at the end of a previous segment so initializing may
only be required once per pattern display. In a timed sequence,
thereafter, the deflection signals (both X and Y) are altered by
incrementing each deflection signal (by incrementing, we intend to include
either positive or negative incrementing) such that the result of
incrementing the X and Y deflection signals produces an overall deflection
signal which varies at a constant rate. Since the relationship between the
components of the incremental deflection signal depends upon the
direction, the components of the incremental deflection need not (and
usually do not) vary at a constant rate.
More particularly, the graphics generator produces deflection signals which
produce, on the display, the desired pattern segment which is comprised of
a series of short straight lines or strokes which approximate the desired
pattern segments. Each stroke, regardless of direction, is of unit length
and since the number of strokes per unit time is constant, the overall
writing speed of the display is constant.
The foregoing is achieved by producing, at the output of the graphics
generator 14, X deflection and Y deflection signals 16 and 17,
respectively, in timed succession, which, when converted to analog form
and applied to the display 20, will produce the desired pattern segment.
Each pattern segment is in turn composed of a plurality of strokes, each
stroke of unit length and of a direction determined by the particular
pattern being displayed.
FIG. 2A illustrates a pattern which can be displayed which is composed of
three pattern segments identified as I, II and III. Each of the pattern
segments in this example is composed of a straight line. The pattern can
be displayed by initializing at the point X.sub.0 Y.sub.0, specifying the
direction of the line from that point to the desired end point X.sub.1
Y.sub.1, and the length of the line. In order to produce the rest of the
pattern, the length of the line II is specified as well as the direction
from X.sub.1 Y.sub.1 to X.sub.0 Y.sub.1. Likewise, to produce the
remaining portion of the pattern specification of the pattern segment III
comprising the length of this line as well as its direction, from X.sub.0
to Y.sub.1 to X.sub.0 Y.sub.0 is required.
As will be disclosed in more detail in the portion of the specification
which follows, the pattern segment I is produced by providing incremental
deflection signals to deflect the beam across the face of the display to
trace out the pattern segment I. This is accomplished by providing a timed
succession of deflection signals, or varying the deflection signal as a
function of time, each change in the deflection signal produces a stroke
of unit length and since the rate of changes is constant, so is writing
speed. The direction of the stroke is equal to the direction of the line.
The separate strokes are illustrated in FIG. 2A as lying between the short
strokes perpendicular to the line I. Those skilled in the art will
understand, of course, that the drawing of FIG. 2A is not to scale and
that the actual stroke length produced at the display 20 can, and usually
will, be so small as not to be separately identified. The stroke length
limits the shortest line that can be displayed, although practically such
a short line would not be used.
FIG. 2B is another example of a plurality of pattern segments which can be
displayed. FIG. 2B differs from FIG. 2A in that at least one of the
pattern segments included is a conic or arcuate pattern segment. This is
generated by a series of linear strokes of equal length, but of varying
angular orientation so as to approximate the desired curve. A portion of
the pattern in FIG. 2B is shown dotted to indicate that open patterns can
be generated, i.e., by blanking the beam over the open portion of the
pattern.
FIG. 2C is similar to FIG. 2B in that the pattern displayed there includes
both straight line and arcuate segments made up, as in the two previous
examples of a series of equal length strokes. However, the pattern of FIG.
2C is different from the other two patterns in that it represents the
letter P and if this is within the repertoire of characters of the
graphics generator, it can be generated by merely identifying the
character, giving a starting location for the character and an angular
orientation, rather than identifying the six different pattern segments
which make up the pattern.
FIG. 2D shows, in an enlarged view, how the arcuate portion III is
generated by the display.
The graphics generator 14 is illustrated in block diagram form in FIG. 3.
FIG. 3 shows the data bus 11 as being coupled to a buffer 20.
Also coupled to the graphics generator 14 is the control bus 12, comprising
12 separate conductors, each coupled to a mode selection logic 22 as well
as a thirteenth conductor providing a control signal from the graphics
generator 14 to the pattern selection and positioning device 10.
The buffered data bus 21 at the output of buffer 20 carries, at different
points in time, representations for different data items used in creating
the pattern segment. This data is coupled under the control of mode
selection logic 22, for the purpose of initializing the graphics generator
14 to create the timed sequence of deflection signals necessary to create
the desired pattern segments. More particularly, buffered data bus 21 is
coupled to a speed control register 23, character address counter 24,
line/conic length counter 25, video blanking logic 26, multiplexer 37,
multiplexer 38, registers 41-44 associated with border detectors 30 and a
direction register 32 associated with a rotation control 28.
Before describing the manner in which the apparatus shown in FIG. 3
cooperates to produce the desired signals, a brief description of the
philosophy of the operation will be provided.
The incremental change in display deflection which is produced per unit
time is, as mentioned above, of predetermined and constant length. The
initial position for the stroke is defined by the data existing in the X
register 39 and Y register 40. The direction in which the incremental
display deflection is to be provided is determined by a quantity produced
by adder 31 included in a rotation control device 28. The output of adder
31 is employed as an address in sine/cosine lookup PROM 34. This
programmed read only memory has a cosine output coupled to an X adder 35
and a sine output coupled to Y adder 36. The memory is arranged to store
the sine and cosine trigonometric functions for all angles, i.e.,
360.degree.. PROM 34 is arranged so that the cosine output for any address
selected by the output of adder 31 is 1 cosine .theta. (where .theta.
represents the address) and the sine output is 1 sine .theta..
Accordingly, incremental deflection will be of length 1, although in
general, the components will be some unequal portion thereof. Thus, for
every memory lookup the X and Y incremental deflection of the desired
angle are provided to X means 35 and Y adder 36. Also coupled to these
adders is a representation of the present display position from the X
register 39 and Y register 40. The sum is, therefore, the new display
deflection which differs from the previous display deflection by 1 display
unit. The sum is coupled through the associated multiplexer back into the
X register 39 and Y register 40.
Accordingly, to represent a line segment beginning at some initial position
which is initially loaded in the X register 39 and Y register 40, the line
direction is provided to the direction register 32 from the buffered data
bus 21. On the first memory lookup, the components of the required
incremental deflection, for this direction, are provided from the read
only memory 34 to the associated adders and the result is placed in the X
register 39 and Y register 40. At the same time, the line/conic length
counter, which had initially been set to represent the desired line
length, is decremented by a quantity corresponding to the stroke length.
After a number of cycles, equal in number to the length of the desired
line expressed as a multiple of the stroke length, the line/conic length
counter is decremented to zero. This terminates the display of the line
segment.
Arcuate or conic segments are displayed in much the same fashion except
that a quantity representing the incremental change in angle for adjacent
strokes is repeatedly provided by the direction register 32 and thus, the
resultant produced in adder 31 is continually changing. Accordingly, the
display is not a line of unchanging direction, but is a series of strokes
whose direction varies in a constant rate, related to the radius of
curvature of the desired arcuate segment.
In order to display a character, character definition PROM 27 is provided
which is loaded with data defining different characters that can be
displayed. Each character to be displayed has a reserved area in the
memory for storing data defining various segments making up the character,
and for each character segment a direction and length is stored. Thus,
when a particular character is selected, character address counter 24
addresses the appropriate reserved area in the memory 27 and the data
defining the first character segment is read out to the character length
counter 47 and the direction register. Each segment of the character is
then displayed in the same fashion, until all the segments of the
character have been displayed at which time an end of character signal is
provided to terminate the display of the character.
Before generating a line, conic or character, registers and status bits
must be initialized including the X and Y deflection registers 39 and 40,
respectively, the direction register 32, the rotation register 33, speed
control register 23, border limit registers 41-44, video status including
the video blanking logic 26 and inclusive/occlusive bordering. Once
initialized, in a line mode, all registers and status bits remain
unchanged except for the X and Y deflection registers 39 and 40. In the
conic mode, both the X and Y deflection registers as well as the rotation
register are varied, and in the character mode, the X and Y deflection
registers as well as the direction register are altered.
For line generation, the pattern selection and positioning device 10
provides signals to set the X and Y deflection registers representing an
initial value and provides the line length L and initial value for the
direction register 32. During actual line writing, the length counter is
decremented and the X and Y deflection registers are strobed to produce a
line starting at the initial point with the length L and an angular
direction equal to the sum of the value inserted in the direction register
plus any value remaining in the rotation register 33. The functional flow
diagram of FIG. 5 depicts the sequential operations performed in
generating a line. Step 50 performs the necessary set up operations as
outlined above except for initializing the line/conic length counter 25.
The line/conic length counter is initialized by energizing the control
line LVLD when the buffered data bus 21 carries a representation for the
length of the line. When the line/conic length counter is loaded, line
generation begins. The BUSY flag is set at step 52. Step 53 enables the
video, i.e., the video is unblanked. The angle, whose representation is
now produced in the adder 31, is employed as an address into the memory 34
producing cosine and sine outputs to the adders 35 and 36, respectively,
at function 54. These values cause the X and Y registers 39 and 40 to be
updated at function 55 (in response to a strobe signal applied at each of
these registers). Function 56 decrements the length counter 25. Function
57 determines if the remaining length (of the line/conic length counter)
is now zero, and assuming it is not, functions 55 and 56 are performed
repetitively until the length has been reduced to zero. At that time,
function 58 blanks the video and function 59 resets the BUSY flag enabling
the graphics generator 14 to process another pattern segment.
FIG. 6 is a flow diagram for generating a conic or arcuate display. The
timing and control logic, to be disclosed in more detail hereinafter,
activates the rotation register 33 at half the vector summation rate, that
is, at half the rate at which the X and Y strobes are produced.
Accordingly, constant direction change is achieved by accumulating new
values in the adder 31 to thereby draw an arc or | | |
