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GED: An Interactive Solid Modeling System
For
Vulnerability Assessments
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.AU
Michael J. Muuss
Keith A. Applin
J. Robert Suckling
Gary S. Moss
Earl P. Weaver
Charles A. Stanley

.AB

For many years the production and modification of target descriptions,
no matter what method used, has been a slow, labor-intensive
procedure.  In an effort to improve the response time of target
descriptions, the Ballistic Research Laboratory (BRL) has developed an
interactive solid modeling system.  This system is called the Graphics
Editor (GED) and is designed to replace the traditional manual method
of production and modification of target descriptions.  GED presently
applies to the combinatorial geometry (COMGEOM) solid modeling
technique.  Using GED, the target descriptions are interactively
viewed, modified, and constructed with immediate visual feedback at
each step.  The GED system virtually eliminates explicit numerical
input and opens a new dimension in the target description process.

.AE
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.PP
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I.  INTRODUCTION
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.PP
As the Army's lead laboratory for vulnerability technology, the
Ballistic Research Laboratory (BRL) constantly performs vulnerability
analyses for a wide variety of military systems.  Vital to these
vulnerability studies are three dimensional computer models of the
physical characteristics of these systems.  These computer models are
generally called target descriptions.  Since the mid 1960's, BRL has
used a solid modeling technique called combinatorial geometry
(COMGEOM) to model targets.  The COMGEOM technique uses logic
operations to combine basic geometric shapes or primitives to produce
the three-dimensional target description.  The COMGEOM target
descriptions are processed by the Geometric Information For Targets
(GIFT)\u1,2\d
.FS
\u1\d Lawrence W. Bain, Mathew J. Reisinger, "The GIFT Code User Manual; Vol I,
Introduction and Input Requirements," BRL Report No. 1802, July 1975. (AD# B0060371)
.FE
.FS
\u2\d Gary G. Kuehl, Lawrence W. Bain, Mathew J. Reisinger, "The GIFT Code User
Manual; Vol II, the Output Options," ARRADCOM Technical Report No. ARBRL-TR-02189,
September 1979. (AD# A078364)
.FE
code for use in follow-on vulnerability assessment codes.
.PP
Target descriptions are basically large collections of numerical data
which traditionally have been handled manually in a batch environment.
Hence, the production and modification of target descriptions has been
a slow, labor-intensive process.  In 1980, BRL initiated an effort to
improve the response time of the target description process by
applying interactive computer graphics techniques.  As a result of
this work unit, BRL has developed the Graphics EDitor (GED), an
interactive solid modeling system based on the COMGEOM technique.
Using GED an analyst can view, build, and modify target descriptions
interactively by manipulating the graphical representation, thus
providing immediate visual feedback on a vector display device.  The
GED system is designed to replace the manual method of production and
modification of target descriptions.
.PP
This report is intended to serve as a user manual for the GED system.
The process of viewing and editing a description using GED is covered
in detail.  The internal data structure is also covered, as it is an
important part in the overall design of the GED system.
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II.  COMGEOM BACKGROUND
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Since the GED system is presently based on the COMGEOM solid modeling
technique, a brief overview of the COMGEOM technique is required to
effectively use GED.  For more detailed information on the COMGEOM
technique see references 1 and 2.
.PP
The COMGEOM technique utilizes two basic entities - a solid and a
region.  A solid is defined as one of fifteen basic geometric shapes
or primitives.  Table I contains a list of these fifteen solids.  The
individual parameters of each solid define the solid's location, size,
and orientation.  A region is a combination of one or more solids and
is defined as the volume occupied by the resulting combination of
solids.  Solids are combined into regions using any of three logic
operations: union(OR), intersection(+), or difference(-).  The union
of two solids is defined as the volume in either of the solids.  The
difference of two solids is defined as the volume of the first solid
minus the volume of the second solid.  The intersection of two solids
is defined as the volume common to both solids.  Figure 1 presents a
graphical representation of these operations.  Any number of solids
may be combined to produce a region.  As far as the COMGEOM technique
is concerned, only a region can represent a component of the target.
The solids are only building blocks to be combined into regions.
Since regions represent the components of the target system, they are
further identified by code numbers.  These code numbers either
identify the region as a target component (nonzero item code) or as
air (nonzero air code).  Any volume not defined as a region is assumed
to be "universal air" and is given an air code of "01".  If it is
necessary to distinguish between universal "01" air and any other kind
of air, then that volume must be defined as a region and given an air
code other than "01".  Normally, regions cannot occupy the same volume
(overlap), but regions identified with air codes can overlap with any
region identified as a component (i.e. one that has a nonzero item
code).  Regions identified with different air codes however, can not
overlap.
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III.   COMPUTER GRAPHICS
.PP
Computer graphics is one of the fastest growing fields in the computer
industry.  Computer graphics has applications in many diverse areas,
from electronic games to medicine, from cartoons to the space
industry.  Just what is interactive computer graphics and why is it so
versatile?  Human visual perception is quite keen and communications
with a computer are generally faster and easier with images or
displays rather than with numbers. Furthermore, by driving the display
image with the computer, one can actually communicate with the
computer through the display itself.  This technology is called
interactive computer graphics.  The user converses with the computer
through the display using devices such as light pens, data tablets,
function switches, and control dials.  The response of the computer is
immediately reflected on the display.  Thus a fast channel of
communication is available between person and machine.
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IV.  APPROACH TO THE PROBLEM
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To speed up the model description process, a faster more efficient
method of manipulating the large masses of data involved in a target
description is needed.  Interactive computer graphics seems to provide
the capability of handling such data in an efficient, relatively fast
manner.  The key to the whole situation is SOFTWARE - the missing link
between the target description problem and a possible solution.
.PP
In view of this fact, existing software graphics packages were
evaluated with respect to their utility in the target description
process.  Quite an exhaustive search of commercially available systems
was conducted and none were found which completely satisfied our
requirements.  A study was then initiated to examine the feasibility
of producing the required graphics software in-house.  The result of
this study was a preliminary version of GED which, using existing
hardware, quite convincingly demonstrated the feasibility of such an
undertaking\u3\d.
.FS

\u3\d Michael John Muuss, Earl P. Weaver, "Interactive Construction of
COMGEOM Targets - A Feasibility Study", report to be published.


.FE
It was then decided to develop GED into a full production code and to
acquire the hardware necessary for full implementation of the system.
The production level GED code has been used since January 1982 to
build target descriptions interactively at BRL.
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V.  GED OVERVIEW
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The main function of the GED system is to edit target descriptions
with immediate visual feedback.  Hence, GED produces images both
suitable for editing and capable of being displayed in real time.
When an object is displayed, the solids combined to model the object
are drawn.  No attempt is made to produce edge representations of
regions or to remove hidden lines.  At the present, such an
undertaking would take away from the "real time" displays now enjoyed.
.PP
The GED is a modular code written in the C programming language which
presently runs under the BRL UNIX Networking System\u4\d.
.FS
\u4\d Michael John Muuss, "BRLNET - The Implementation of a
Multi-processor UNIX System", a final report of work done under ARO
Project Number 1127, August 27, 1980.

.FE
The workstations presently used are Vector General (VG) 3DI vector
refresh systems driven by DEC PDP-11/34 or PDP-11/70 computers.
Peripheral devices include a joystick, a pen and data tablet, function
switches, and control dials.  These peripheral devices and functions
of each are discussed in later sections.
.PP
Overall control of GED is maintained via the keyboard.  The keyboard
commands perform general housekeeping functions and allow one to
control what is displayed, create/modify combinations, list solid
parameters, etc.  The keyboard commands are discussed in a later
section.
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VI.  GED INTERNAL DATA STRUCTURE
.PP
One of the critical aspects of a graphics software package is its
internal data structure.  Since the target description problem deals
with such large volumes of data, the importance of the data structure
here is emphasized.  Thus it is felt that an introduction to the
internal data structure of GED is important for any potential user.
.PP
The GED internal data structure is hierarchical in nature with each
node or position in the hierarchy being occupied by an OBJECT.  An
OBJECT is the GED basic data unit and is defined as either a SOLID or
a COMBINATION.  A SOLID is one of the generalized COMGEOM primitive
types while a COMBINATION is a group of objects.  Each member object
of a combination has a transformation associated with it.  Any object
not at the top of an hierarchy is referenced by (is a member of) a
higher combination and each such reference has an associated
transformation.  The bottom object of every hierarchy path is a solid.
This hierarchical data structure allows actual subsystems of a target
to be grouped together and edited as a unit without worrying about
individual parts.  To display any object, each path in the object's
hierarchy is traveled to its bottom position where a solid is located.
The transformations are accumulated for each path traversed and then
applied to the solid at the bottom of the path.  The solid is then
drawn.  The hierarchy is traversed recursively until all objects are
drawn.  Let's use a high explosive projectile as an example.  Figure 2
depicts the hierarchy involved.  If one were to display the
combination "heround", six solids would be drawn on the screen.  Table
II lists each solid name, the transformation applied to the solid's
parameters, and the path used to reach the solid.
.PP
One very useful type of combination is the INSTANCE.  An instance is a
referral to an object (solid or combination) without the object itself
being duplicated.  To produce an instance of an object, a combination
is created with the object being instanced as a member object.  As
with any other combination, this member has an associated
transformation, hence it can be edited as a unit.  The instance is
most useful when an object appears many times in a description.  The
desired number of instances are made and positioned in the
description.  Any future changes in the object require that only the
original (prototype) be changed.  The modifications of the prototype
object will then be automatically reflected in all the instances of
that object.
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VII.  KEYBOARD COMMANDS
.PP
To execute GED, one simply types "ged file", where "file" is the data
file to be edited.  The GED keyboard commands are used to maintain
overall control of the system and to perform general housekeeping
functions.  Each command is invoked by entering a single character
followed by other parameters (if necessary) separated by spaces.  In
most cases, the response on the screen will be the word "done" once
the command is completed.  This response is not printed for the
commands where the results of the command are readily visible on the
screen.
.PP
In the following pages, the GED keyboard commands will be presented
and discussed.  Examples will be given in appropriate cases.  Table
III, at the end of this section, presents a summary of the GED
keyboard commands and the function of each.
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c oldsolid newsolid
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This command is used to produce and display a copy of a solid.  In
this case, the solid "oldsolid" will be copied into a solid called
"newsolid".  A new identical solid record is added to the data file.
The solid parameters are copied as they appear in the solid record.
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Examples:     c arb8 hullbot.s
              c arb8 turrettop.s
              c tgc wheelrim.s
              c tor tire1.s
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n old new
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This command is used to rename objects in the data file.  In this
case, the object "old" will be renamed "new".  A note of caution: the
name is changed only in the object record itself, not in any member
records.  Thus if the object "old" appears as a member of any other
object, the name will not be changed there.
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Examples:     n test hull
              n g00 air
              n g1 turret
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Z
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This command clears (Zaps) the screen of all displayed objects.
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g group obj1 obj2 ..... objn
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This command creates or appends to a combination record and is used to
group objects together either for editing or displaying purposes.  In
this case, "obj1" through "objn" are added as members to the
combination "group".  If "group" does not exist, it is created and
"obj1" through "objn" are added as members.
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Examples:   g shell hull turret
            g tank wheels engine crew shell
            g tank track
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r region op1 sol1 op2 sol2 .... opn soln
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This command is used to create regions or append to regions.  If
"region" exists, then solids "sol1" through "soln" are added as
members with "op1" through "opn" as the defining operations.  If
"region" does not exist, then it is created and solids "sol1" through
"soln" are added as members with "op1" through "opn" as the defining
operations.  A region is merely a combination record with a flag set
and is distinguished from other combinations (groups) since it has
meaning to the COMGEOM solid modeling system.
     When a region is created, the item and air codes are set equal to
zero.  To change the item and air codes use the "I" command.  Note: In
the past all members of a region had to be solids, but recently
regions have been allowed as members of regions.  Hence, the names
"soli" can also be regions now.
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Examples:    r hulltop.r + hulltop.s - hullleft.s - hullright.s
             r gun + gun.s - gunin.s
             r gunair + gunin.s
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i object combname brname
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This command is used to make an instance of an object.  An instance of
an object is produced by creating a combination and making the object
a member.  In this case, an instance of "object" is made by creating
the combination record "combname" (if "combname" does not already
exist) and adding "object" as a member.  This member also has a second
name "brname" added to the member record, which can be thought of as
the name of this branch in the hierarchical data structure.  If
"combname" already exists, then "object" is added as the next member
and "brname" is added as the branch name.

An instance is used to refer to an object, without making actual
copies of the object.  Instances are useful when one is adding a
certain component to a target description many times.  Furthermore,
any modifications to an object which has been instanced need only be
done in the original (prototype) object.  These modifications will
then be automatically reflected in all the instances of the object.
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Examples:       i heround ammo he1
                i heround ammo he2
                i heat ammo heat1
                i heat ammo heat2
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f face distance
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This command allows the user to project a face of an arb (arbitrary
polyhedron) being edited a normal distance to create a new arb.  The
value of "face" is 4 digits such as 1256. If the face is projected in
the wrong direction use a negative "distance".  One use for this
command is for producing armor plates of a desired thickness.
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Examples:     f 1234 20
              f 2367 34.75
              f 2367 -34.75
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l object
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This command is used to list information about objects in the data
file.  The information listed depends on what type of record "object"
is.  If "object" is a combination record, then the members are listed.
If "object" is a solid record, then the GED general solid type and the
parameters as presently in the data file are listed.  Note: only the
solid parameters as they exist in the solid record are listed, no
transformation matrix is applied.  Hence, if the solid was edited as a
member of a combination, the "l" command will not reflect the editing
in the listed parameters.
.sp 2
Examples:      l hull
               l turret
               l turtop.s
               l arb8
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mirror -[axis] oldsolid newsolid
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This command is used to create and display a new solid record which is
the mirror image of an existing solid.  The mirror image is about an
axis and is created by changing the signs on the solid's parameters
depending on which axis the solid is mirrored about.  In this case, a
mirror image of the solid "oldsolid" will created about the axis
indicated by "axis" and the new solid record will be called
"newsolid".  The only acceptable values for the parameter "axis" are
"x", "y", and "z".
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Examples:     mirror -y tur.left.s tur.right.s
              mirror -z tur.top.s tur.bot.s
              mirror -x tur.front.s tur.back.s
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p dx [dy] [dz]
.sp 2
This command allows a user in the SOLID EDITING mode to input exact
parameter modifications by the keyboard.  The meaning of the values
typed after the p command varies depending on what solid editing
feature is being used.  Examples of this command will be given in the
discussion on solid editing.
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D comb mem1 mem2 .... memn
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This command allows one to delete members from a combination record.
In this case, members "mem1" through "memn" will be deleted from the
combination "comb".
.sp
Examples:     D tank hull wheels
              D region1 solid8 solid112
              D turtop.r tursidel.s tursider.s turback.s
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I region item air
.sp 2
This command allows one to change the item or air code numbers of a
region.  If the air code ("air") is not included, a zero is assumed.
To change the air code, a zero item code should be used (see second
example below).
.sp 2
Examples:
            I region1 105
            I region7 0 2
            I region11 129 0
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e object
.sp 2
This command is used to display (draw) objects on the screen.  In this
case, "object" will be displayed on the screen.  Note that "object"
must be found in the table of contents.
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d object
.sp 2
This command is used to drop (delete) objects from the display.  In
this case, "object" will be deleted from the screen display.  This
command is opposite of the "e" command.
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k object
.sp 2
This command is used to remove (kill) objects from the data file.  In
this case, "object" will be removed from the data file.  Note the
distinction between the "d" command which deletes objects from the
display and the "k" command which removes objects from the data file.
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t
.sp 2
This command produces a list of the table of contents of all objects
in the data file.  The objects are listed a screenfull at a time.  A
carriage return will produce the next screenfull.
.sp 3
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a arbname rot fb
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This command is used to create and display a new arb8 solid record.
This new arb8 has two square (5 units x 5 units) parallel faces which
are 0.5 units apart.  These parallel faces are in planes whose
orientations are defined by the two input angles - the rotation (rot)
angle and the fallback (fb) angle.  The orientation of armor plates
are frequently defined using rotation and fallback angles.  The vertex
of this new arb8 is located in the center of the screen.

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Examples:     a hullsec3.s 45 30
              a tursec2.s 90 20
              a topglacis.s 0 60
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x
.sp 2
This command is the display debug command.  It will produce a list of
all objects that have been displayed , the paths traversed to draw the
objects, and whether or not the objects are displayed in the present
view.
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q
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This command is used to quit the graphics editor code.  This is the
normal halt.
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VIII.  PERIPHERAL DEVICES
.PP
Before we discuss the features of GED, we will introduce the hardware
devices used to implement them.  These devices are the "tools of the
trade" for the GED user.  We will discuss only basic operational
characteristics here.  Specific use of these devices will be covered
in the later sections on the viewing and editing features of GED.
.PP
The JOYSTICK is a mechanical device used to do the rotations in GED.
Any movement of the stick left or right rotates the display about the
x-axis.  Any movement of the stick up or down rotates the display
about the y-axis.  When the joystick is twisted in a clockwise or
counterclockwise direction, the display rotates about the z-axis.  Any
combination motion of the stick will produce a "combined" rotation
about the appropriate axes.  All of these motions have a spring return
to a null center position.
.PP
The FUNCTION SWITCH box contains thirty-two buttons.  When any of the
buttons is pressed, an action occurs or condition is set.  Figure 3
depicts the functions programmed for each button.  The buttons in the
shaded area are used for editing while the rest are used for viewing
the display.  The exact functions assigned to these buttons will be
discussed in the sections on viewing the display and on editing.
.PP
The CONTROL DIALS (knobs) are used to send digital information to the
computer.  As a knob is turned, a succession of numbers are available
for use by the computer.  Figure 4 depicts the functions assigned to
each of the ten knobs.  The exact functions of each of these knobs
will be discussed in the angle distance cursor section and in the
viewing features section.
.PP
The DATA TABLET is a graphics input device containing an X-Y
coordinate grid which corresponds to the grid on the screen.
Information is entered using a pen-like stylus.  The distance this pen
is from the tablet is important.  If the pen tip is within one half
inch of the tablet surface, the cursor location on the screen
corresponds to the X,Y location of the pen on the tablet.  This
condition is called the "near" position.  If the pen is more than one
half inch from the tablet surface, the cursor remains located in the
center of the screen.  When the pen is pressed against the tablet
surface, the pressure switch is activated and a bit is set which is a
signal used by GED.  The exact meaning of the pen near and pen press
depends on what is being done and will be covered in the appropriate
sections of this report.
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IX.  ANGLE DISTANCE CURSOR (ADC)
.PP
The angle distance cursor is a construction aid used to measure angles
and distances. It should be noted that all measurements are made in
the projected space of the screen, so one should measure only in a
view normal to the surface where the measurement is to take place.
The ADC is placed on (or removed from) the display by pushing the
"ADC" function switch button (see Figure 3).  The ADC consists of
three cursors which cover the entire screen.  Figure 5 depicts the ADC
as it appears on the screen.  All the cursors are centered at the same
point and can be moved to any location on the screen.  Two of these
cursors rotate for angle measuring purposes. Angle cursor 1 is solid
while angle cursor 2 is dashed.  Angle cursor 1 has movable tic marks
for measuring distances on the screen.  The two angle cursors move
with the horizontal and vertical lines of the main cursor.  The
resulting effect is the moving of the center point horizontally or
vertically.  The ADC is controlled by the bottom row of the control
dial knobs (see Figure 4) :

     Knob 6   moves the center in the horizontal direction
     Knob 7   moves the center in the vertical direction
     Knob 8   rotates angle cursor 1  (alpha)
     Knob 9   rotates angle cursor 2  (beta)
     Knob 10  moves the tic marks

Whenever the ADC is on the screen, there is a readout at the bottom of
the screen listing pertinent information about the ADC.  This
information includes the angles that angle cursors 1 and 2 have been
rotated (alpha and beta), the distance the tic marks are from the
center of the ADC, and the location of the center of the ADC.  This
information is continually updated on the screen.
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X.  VIEWING FUNCTIONS
.PP
The GED viewing features are designed to allow one to examine a target
description in close detail.  Any of the viewing features can be
invoked at any time.  It should be noted, that these functions do not
change the actual data, only the way these data are displayed.
.PP
Six standard views (front, rear, top, bottom, left, and right) and one
oblique view (azimuth 35, elevation 25) are each assigned to the
function buttons (see Figure 3).  Hence, any of these views is
immediately available at the press of the appropriate function button.
The views available are not limited to these standard views however,
as the display can be rotated to any view by using the joystick.  By
pressing the function button labeled "save view" (see Figure 3), the
present viewing aspect angle of the display is saved.  At any time,
the saved view can be immediately returned to the screen by pressing
the "restore view" function button (see Figure 3).  The "restore view"
button will be lit whenever a view has been saved.  The function
button labeled "reset" (see Figure 3), restores the display to the
default view (front) when pressed.
.PP
The display can be panned or slewed on the screen in two ways - using
the data tablet and pen or by using the control knobs.  When one is
editing, the tablet and pen are not available for slewing, hence one
must use the control knobs to slew the display.  If one is NOT
editing, then whenever the pen is pressed, the display moves in the
direction of an imaginary vector drawn from the center of the screen
to the cursor location on the screen.  Recall that there is a
one-to-one correspondence between the pen location and the cursor
location.  The longer this vector, the faster the display will move.
To slew the display using the control knobs, one uses the knobs
labeled "slew x" or "slew y" (see Figure 4).  The null positions on
these knobs is in the center or straight up.  If the "slew x" knob is
turned clockwise of center, the display will move to the right. If it
is turned counterclockwise, the display will move to the left.  For
the "slew y" control knob, clockwise of the center moves the display
up and counterclockwise moves the display down.  The further these
knobs are turned from center, the faster the display moves.
.PP
One can zoom the display by using the control knob labeled "zoom" (see
Figure 4).  Again the null position of this knob is center or straight
up.  Turning this knob clockwise of center causes the display to
increase in size producing a zoom-in effect.  Turning this knob
counterclockwise of center causes the display to decrease in size or
zoom-out.  Again, the further the "zoom" knob is turned from center,
the faster the zooming will occur.
.PP
The viewing features of GED also include a slice mode.  To invoke the
slice feature, one presses the "slice mode" function button (see
Figure 3).  An imaginary slicing plane, parallel to the screen, can
then be moved about by turning the first control knob (see Figure 4).
Turning this knob clockwise moves the imaginary plane into the screen
and counterclockwise moves the plane out of the screen.  All portions
of the display in front of the plane are eliminated from the display.
The result of moving this plane into the screen is a slicing effect
with the parts of the display closest disappearing first.  To leave
the slice mode, just press the "slice mode" button again.
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XI.  SELECTING OBJECTS FOR EDITING
.PP
Before we discuss the editing features of GED, we will discuss how one
selects objects for editing.  To be edited, an object must be
displayed on the screen.  Since the object to be edited may be only a
small portion of the display, a procedure is needed to select an
object from the many objects being displayed.  This procedure relies
on the hierarchical structure of the GED data base.  As stated before,
when objects are displayed, every path of every object is traversed
and the bottom object, which is always a solid, is drawn.  Hence one
can think of each solid drawn not as one solid, but as the unique path
used to reach that solid.  As an example, the six paths (solids) for
the sample object "heround" were presented in Table II.
.PP
To select an object for editing, one first must enter the illuminate
mode by pressing the function button labeled "illuminate" (see Figure
3).  The data tablet and pen are then used to select the object.  The
surface of the data tablet is divided into imaginary horizontal
strips.  Each of these strips corresponds to a path (solid) drawn on
the screen.  For example, if the sample "heround" were displayed, the
tablet surface would be divided into six horizontal strips.  The pen
is used in the "near" mode (within one half inch of tablet surface) to
illuminate the paths.  The path corresponding to the pen location is
written in the upper right hand corner of the screen.  The solid at
the bottom of this path is illuminated (drawn brighter) on the screen.
In this manner one can "scan" the complete display by simply moving
the pen up and down the tablet in the near mode.  Hence, the
illuminate mode is also quite useful just to identify objects in a
description.  However, the primary purpose of the illumination mode is
to select objects for editing.
.PP
The actual selecting of an object for editing is a two-step process.
First, one selects the path desired, then one selects the particular
object of interest from that path.  One "selects" by pressing the pen
on the tablet surface.  When a path is selected, the word PATH
followed by the members of that path is written in the upper right of
the screen.  At that point, one is ready to select a specific member
(object) from that path for editing.  Again, the tablet is used for
this selection.  The tablet surface is now divided into as many rows
(zones) as there are members in the selected path.  When the pen, in
the near mode, is in a path member's assigned zone on the tablet, that
member's name will be illuminated in the path listing.  When the
specific member to be edited is illuminated, it is selected by
pressing the pen.  The word EDIT will now appear in the upper right of
the screen ahead of the path listing.  One is now ready to edit this
selected object.
.PP
The only way out of the EDIT mode is to accept or reject any editing
performed.  This is done by pressing the function buttons labeled
"accept" or "reject" (see Figure 3).  If at any time in the selection
process an incorrect object is selected, just press the "reject"
button and begin the selection process again.
.sp 2
.ce
XII.  OBJECT EDITING
.PP
The heart of the GED system is its editing features.  The editing
features are divided into two classes: object editing and solid
editing.  Object editing is designed to allow one to change the
location, size, and orientation of an object.  Recall that an object
is defined as the basic data unit of the GED system and includes both
combinations and solids.  In the case of a solid, one needs to change
not only its location, size, and orientation, but also its "shape".
Changing the shape of a solid means changing any of its individual
parameters.  Hence, solid editing is handled separately.
.PP
Any object in the data file may be selected for object editing.  The
object is picked using the object selection procedure discussed
earlier.  Recall that in the GED data structure, all members of
combinations have transformation matrices associated with them,
allowing assemblies to be edited as units.  Object editing is the
vehicle for performing such tasks.  All object editing is done using
homogeneous transformation matrices.  When the editing is accepted,
new transformation matrices are created in all appropriate member
locations.  If the object happens to be a single-member path (solid)
then the matrix is applied to the solid's parameters.  Otherwise, all
object editing is stored in transformation matrices in the data base.
.PP
Once an object is selected for editing, the second row of function
buttons (see Figure 3) is used to select the editing to be done.  Then
either the data tablet and pen or the joystick are used to perform the
actual editing.
.PP
To SCALE a selected object, one first presses the function button
labeled "scale object".  The data tablet and pen are then used to
scale the object.  The object is scaled about the target origin, thus
unless the object is centered at the origin, some translation of the
object will occur.  The location of the pen on the tablet when pressed
determines the scale factor used.  If the pen is above the center of
the screen, the scale factor will be greater than one.  The further
the pen is located above the center, the larger the scale factor.
Likewise, if the pen is located below the vertical center, the scale
factor will be less than one and the further below the center, the
smaller the scale factor.  As long as the pen is pressed, the object
will continue to increase or decrease in size.
.PP
To ROTATE a selected object, one first presses the function button
labeled "rotate object".  The joystick is then used to rotate the
object.  The object is rotated about the center of the view (screen).
If one wishes to rotate the object about another point of interest,
the entire display can be slewed until the desired point is positioned
at the center of the screen.
.PP
To TRANSLATE a selected object, one has three function buttons from
which to select (see Figure 3).  The data tablet and pen are then used
to translate the object.  The center of the object "follows" the pen
(cursor) location whenever the pen is pressed.  The three function
buttons define the direction allowed for movement of the object.  The
button labeled with a horizontal arrow (<--->), allows the object to
follow only the horizontal location of the pen.  The vertical location
of the object does not change.  The function button labeled with a
vertical arrow ( ), allows the object to follow only the vertical
location of the pen.  The horizontal location of the object does not
change.  The function labeled with crossed arrows (<--->) allows the
object to follow the X,Y location of the pen.
.PP
The object editing features can be invoked in any order and at any
time once an object has been selected for editing.  During object
editing, any of the viewing features, such as changing views, zooming,
and slewing, can be used and in fact are usually quite useful.  Again,
the only way to exit the object editing mode is to accept or reject
the editing.  If the "reject" button is pressed, the object will
return to its pre-edit state.  If the "accept" button is pressed, the
data base will be changed to reflect the object editing performed.
.sp 2
.ce
XIII.  SOLID EDITING
.PP
The solid editing feature allows the user to interactively translate,
rotate, scale, and modify individual parameters of a solid.  Whenever
one is in the solid edit mode, the parameters of the solid being
edited are listed and continually updated at the bottom of the screen.
Certain parameters are also labeled on the solid being edited.  Solid
editing is generally used to "build" objects by producing solids of
the desired shape and size in the correct orientation and position.
Once the object is built, object editing is used to scale, orient, and
position the object in the description.  The general philosophy of
solid editing is to first create a copy of a prototype solid with the
desired name and then to edit this solid.  The prototype solids should
not be edited.  As an example, suppose one were to build the sample
object "heround" mentioned earlier.  To produce the base of the shell,
one would need a cylindrical shaped solid.  The prototype solid is the
TGC (see Table I), so one would type:
.sp
.ce
c tgc shellcase.s
.sp
A new solid record called shellcase.s would be created and displayed
on the screen.  This shellcase.s solid would then be edited using
solid editing to produce the exact solid parameters desired.  The
solid record tgc would be unchanged and available for copying the next
time a cylindrical solid is needed.
.PP
The procedure for solid editing is quite similar to that for object
editing.  First, the solid must be selected for editing.  A solid is
selected for editing using the illuminate mode, just as in object
editing, except a solid must be selected.  Recall that the bottom
object in every hierarchy path is a solid.  Second, one must push the
function button labeled "solid edit" (see Figure 3).  This button sets
up the solid edit mode: the solid parameters are listed at the bottom
of the screen, pertinent solid parameters are labeled on the display,
the solid editing function buttons are activated, and a menu header
depending on the solid type is written on the right side of the
screen.  The meaning of the menu will be discussed shortly.  Third,
one selects (by pressing the appropriate function buttons) and
performs the editing desired.  Finally, one exits the solid editing
mode by accepting or rejecting the editing performed, just as in
object editing.
.PP
Recall, in the section on object editing it was mentioned that since a
solid is indeed an object, a solid can be edited (translated, rotated,
and scaled) as an object.  Hence, the solid editing mode is really
only necessary to perform parameter modifications.  However, much of
the object editing is done about the target or screen center, which
can be annoying when editing a solid.  Also, precise parameter
modifications are possible (using the "p" keyboard command) in solid
editing.  Thus it was deemed desirable to include translation,
rotation, and scaling in solid editing also.
.PP
Solid TRANSLATION allows the user to place the solid being edited
anywhere in the description.  To invoke this option, one presses the
function button labeled "solid trans" (see Figure 3).  To move the
solid, use the data tablet and pen.  Whenever the pen is pressed, the
VERTEX of the solid moves to that location on the screen.  One can
read the actual coordinates of the vertex on the bottom of the screen,
along with the other parameters.  If the actual desired coordinates of
the vertex are known, one can place the solid exactly using the "p"
keyboard command.  For example, to place a solid's vertex at the
coordinates (x, y, z) one would type
.sp
.ce
p x y z
.sp
The solid would then jump to this location.
.PP
The solid SCALE feature allows the user to scale the solid being
edited to any desirable size.  The scaling is done about the vertex of
the solid, hence NO translation of the solid occurs.  The scaling is
performed using the data tablet and pen, just as in object scaling.
One can input an exact scale factor using the "p" keyboard command.
For example, typing
.sp
.ce
p factor
.sp
will scale the solid by an amount equal to "factor".  The value of
"factor" is absolute - the original solid is scaled.  The size of the
original solid is defined when the "solid scale" function button is
pressed.  By setting "factor" equal to one, the original size solid
will be displayed on the screen.
.PP
Solid ROTATION allows the user to rotate the solid being edited to any
desired orientation.  The rotation is performed about the vertex of
the solid.  To select this option, one presses the function button
labeled "solid rotate" (see Figure 3).  The actual rotation is done
using the joystick.  One can input exact angles to rotate the solid by
using the "p" keyboard command.  For example, typing
.sp
.ce
p alpha beta gamma
.sp
will rotate the solid "alpha" degrees about the x-axis, "beta" degrees
about the y-axis, and "gamma" degrees about the z-axis.  Alpha, beta,
and gamma are measured from the original "zero" orientation of the
solid, defined when the "solid edit" function button was pressed.
Hence, typing
.sp
.ce
p 0 0 0
.sp
will always return the solid to its original position (its position
when the current solid editing session began).
.PP
The PARAMETER editing feature allows the user to modify any of the
parameters which comprise the selected solid.  To invoke this feature,
one presses the function button labeled "menu" (see Figure 3).  A menu
(listing) of parameters to edit will then appear on the right side of
the screen under the menu header.  The contents of this menu of course
depend on the type of solid being edited.  To select an item
(parameter) from this menu, one uses the data tablet and pen.  The
immediate region (strip) surrounding each item of the menu "belongs"
to that item.  Whenever the pen is in one of these regions, that menu
item will get brighter in the menu listing on the screen.  A menu item
is picked by pressing the pen when that item name is illuminated.  It
should be noted that whenever the pen is in the "menu area", the code
assumes the user is attempting to pick a menu item.  Hence, the menu
area of the screen (the far right edge) can NOT be used by the pen for
editing while the menu is on the screen.
.PP
For parameter editing, the solid type determines the menu items.  The
GED code recognizes four general solid types for parameter editing.
All the specific solid types which fall into one of these general
classes are treated as the general type.  For example, all the
cylindrical solid types are treated as the general type - the TGC.  In
the following paragraphs, we will discuss parameter editing for each
of these general types of solids.
.sp 2
.nf
A.  ARB PARAMETER EDITING
.fi
.PP
The GENERAL ARB class of solids represents all the convex polyhedrons
(RPP, BOX, RAW, and ARBs).  The ARBs comprise five classes of
polyhedrons each with a characteristic number of vertices.  These are
the ARB8, ARB7, ARB6, ARB5, and ARB4, where the ARB8 has eight
vertices, etc.  GED handles all these ARBs as ARB8s, but with some
vertices equal.  All the eight vertices of the GENERAL ARB are labeled
on the screen, although in certain cases (RAW,ARB7,ARB6,ARB5,ARB4)
some vertices will be equal.  Figure 6 depicts typical ARBs as they
would be displayed and labeled during solid editing.
.PP
To edit the ARB vertices, EDGES are moved.
Hence, the GENERAL ARB menu is a list of edges to move:
.sp
.ce 13
GENERAL ARB MENU
  move edge 12
  move edge 23
  move edge 34
  move edge 14
  move edge 15
  move edge 26
  move edge 56
  move edge 67
  move edge 78
  move edge 58
  move edge 37
  move edge 48
.sp
The edge to be moved is selected from this menu and is then moved
using the data tablet and pen.  When the pen is pressed, the line
containing the selected edge is moved so it goes through the pen
location and is parallel to the original edge.  The endpoints of the
new edge are then calculated to remain in the "bounding" planes of
that edge.  In the case of the ARB8, for example, the bounding planes
for edge 58 are planes 4378 and 1265 (see Figure 6).
.PP
The information printed on the bottom of the screen is the x, y, z
coordinates of the eight vertices:
.ce 5
 vertex         coordinates
  1,2        x1 y1 z1  x2 y2 y3
  3,4        x3 y3 z3  x4 y4 z4
  5,6        x5 y5 z5  x6 y6 z6
  7,8        x7 y7 z7  x8 y8 z8

These values are continually updated during solid editing.
.PP
The user can force the line containing an edge through any point using
the "p" keyboard command.  Hence typing
.sp
.ce
p x y z
.sp
will force the line containing the edge being edited to pass through
the point (x, y, z).  No check is made for concave ARBs or for ARBs
with crossed edges.  Visual feedback from the display is considered
adequate to detect these situations.
.sp 2
.nf
B.  TGC PARAMETER EDITING
.fi
.PP
The TGC general class of solids includes all the cylindrical COMGEOM
solids.  The defining parameters of the TGC are two base vectors (A
and B), a height vector (H), two top vectors (C and D), and the vertex
(V).  Reference 1 contains more information on these parameters.  The
top vectors C and D are directed the same as the base vectors A and B
respectively, hence the top vectors are defined only by their lengths
(c and d).  Figure 7 depicts these parameters on typical TGCs.  When a
TGC is edited in the solid editing mode, only vectors A and B are
labeled on the display.
.PP
All the vectors (A,B,C,D,H) of the TGC can be scaled during parameter
editing.  In addition, the height vector (H) and the base (AxB) can be
rotated.  These parameter editing options are reflected in the TGC
menu:

.ce 8
TGC MENU
scale H
scale A
scale B
scale c
scale d
rotate H
rotate AxB

The scaling of the lengths of the vectors is done using the data
tablet and pen in the same manner as object editing.  Exact vector
lengths can be achieved easily using the "p" keyboard command.  If one
is scaling a vector, then typing
.sp
.ce
p x
.sp
will make the vector being scaled have a length equal to "x".
.PP
To rotate vector H or surface AxB, one uses the joystick.  Since
vectors C and D have the same direction as vectors A and B
respectively, the top and bottom surfaces of the TGC will remain
parallel.  Hence rotating the base surface (AxB) will also rotate the
top surface.  The "p" keyboard command is not used with the parameter
rotation of the TGC.
.PP
During solid editing, the parameters of the TGC are constantly updated
on the bottom of the screen:

.ce 7
V   x y z
H   x y z |H| a b g rot fb
A   x y z |A|
B   x y z |B|
c   |c|
d   |d|
AxB a b g rot fb

where "a", "b", "g" are the direction cosines and "rot" and "fb" are
the rotation and fallback angles of the listed vector.
.sp 2
.nf
C.  ELLG PARAMETER EDITING
.fi
.PP
The ELLG general class represents all the ellipsoidal solids,
including spheres and ellipsoids of revolution.  The defining
parameters of the ELLG are three mutually perpendicular vectors (A, B,
and C) and the vertex (V).  Reference 1 contains a more complete
definition of these parameters.  Figure 8 depicts typical ELLGs with
these parameters labeled.  When an ELLG is being edited, only vectors
A and B are labeled on the display.
.PP
The parameter editing of the ELLG consists of scaling the lengths of
the vectors A, B, C.  These parameter editing options are listed in
the menu:

.ce 4
ELLG MENU
 scale A
 scale B
 scale C

The scaling of these vectors is done using the data tablet and pen in
exactly the same manner as in object scaling.  The "p" keyboard
command again can be used to produce a vector of desired length.
.PP
The parameters of the ELLG are continually updated on the bottom of
the screen during solid editing:

.ce 4
V   x y z
A   x y z |A| a b g rot fb
B   x y z |B| a b g rot fb
C   x y z |C| a b g rot fb

where "a", "b", "g" are the direction cosines and "rot" and "fb" are
the rotation and fallback angles of the listed vector.
.sp 2
.nf
D.  TOR PARAMETER EDITING
.fi
.PP
The TOR general class of solids contains only one type of torus, one
with circular cross-sections.  The defining parameters of the TOR are
two radii (r1 and r2), a normal vector (N), and the vertex (V).  The
scalar r1 is the distance from the vertex to the midpoint of the
circular cross section.  The scalar r2 is the radius of the circular
cross-section.  The vector N is used to orient the torus.  Reference 1
contains a more detailed definition of these parameters.  Figure 9
depicts these parameters.  During solid editing, none of these
parameters are labeled on the screen.
.PP
The parameter editing of the TOR consists of scaling the radii, hence
the menu contains only two members:

.ce 3
  TORUS MENU
scale radius 1
scale radius 2

As before, the scaling of these radii are done using the data tablet
and pen and the "p" keyboard command can be used to produce a radius
of any desired length.
.PP
The parameters of the TOR are continually updated on the bottom of the
screen during solid editing:

.ce 4
V   x y z
r1  |r1|
r2  |r2|
N   x y z a b g rot fb

where "a", "b", "g" are the direction cosines and "rot" and "fb" are
the rotation and fallback angles of the vector N.
.PP
Table IV presents a summary of parameter editing for each of the
general solid classes.
.sp 2
.ce
XIV.  CONCLUSION
.PP
BRL has developed the Army's first interactive solid modeling system,
called GED.  GED performs two basic functions: viewing and editing.
The standard viewing capabilities of zooming, slewing, slicing, and
rotation are available.  Likewise, all the standard editing features
are also available.  The user easily traverses the hierarchical data
structure, applying the editing functions of rotation, translation,
and scaling to any position in the hierarchy.  The hierarchical
structure can be modified and regrouped and regions created and
modified.  Specific parameter editing can also be applied to the
solids to produce any shape solid desired.  GED has made great gains
in reducing the bottleneck the target description process has been
creating in the vulnerability process at BRL.
