An N-body Simulation in a Virtual Universe User`s Manual
Contents
1. Body positions Initial body velocities Body mass Body charge Apparent size Body color Attached to a body This defines what body if any the user is attached to Simulation model choice Parameters associated with that model vov ov ov v v v v Initial set up This defines the position scaling and orientation of the simulation in the virtual world furthermore it may also define the position of the user in the virtual world Parameters may be amp Simulation scale Simulation orientation Simulation translation amp User translation Default body info These fields define default parameters for newly added bodies created while the application is running Furthermore these default parameters will be applied to any body defined in the save file which does not have that particular parameter defined i e any undefined field in a body declaration will attempt to take its value from the appropriate default body field The default body declaration may contain any subset of the fields possible in the regular body field 23 An N body Simulation in a Virtual Universe 42 I Specifi This section describes the actual language of the state file format It contains a list of keywords and their respective arguments and describes each keyword s function as well as the argument s type The units for all values are defined relative to the units of the function parameters specified in the declaration MODEL This k2. User s Manual The user has the ability to edit the simulation characteristics as well as display of the simulation universe To do this a collection of tools are available The way the tools are presented to the user as well as their function is what constitutes the rest of this chapter 3 5 Tools 3 5 1 Global Tools These tools are selected from the tool panel which is invoked by pressing the right mouse button The tool itself is selected by pressing the left mouse button when the mouse pointer is in the proper button s bounding volume A representation of the editing mode tool selection panel appears below Flying Tool When the user has the flying tool in hand the user s hand appears to be an arrow When the left button is pressed the user flies through the virtual world in the direction pointed by the arrow The user may adjust the speed of flight by using the hand as a throttle The further away the hand is from the head the faster the user flies through the virtual world The speed is signified by the color of the arrow the arrow changes from blue to red as the user s speed increases Grabbing Too The grabbing tool attaches the universe to the user s hand This allows the user to reorient the display of the universe so that the interactions between the bodies may be viewed from any angle without the user having to perform contortions to position himself appropriately Holding the left button keeps the universe
3. the button will be highlighted Clicking the right mouse button cancels the tool selection panel and the user is returned to his previous state and previous current tool Clicking the left mouse button selects the currently highlighted tool The only time that pressing the right button won t bring up the tool selection panel is when a body editing tool is currently in use At this point the right mouse button simply cancels the body editing tool and the user is returned to his previous tool Example tool selection panel Edit Mode Tool Selection Panel 3 3 Mode Features The following functions are available in the various modes Any Editing Modes Select a body by using the selection slider Go to the selected body amp Attach detach user from a selected body World Editing Mode Add new bodies Delete bodies Show hide velocity vectors Show hide disabled bodies Select a different interaction function force law Change parameters to the interaction function Change the time step of the simulation Start the simulation go to Running Mode Load or save a scenario from or to a file Load a new simulation Go to Body Editing Mode Exit the system quit ow o w w An N body Simulation in a Virtual Universe Body Editing Mode Adjust Position tool Set the default body Adjust Radius tool Disable enable a body Adjust Velocity tool amp Establish the origin body Adjust Mass
4. complex interactions might prove interesting A Solar System simulation This involves researching some orbital data and making the simulation more accurate or more tailored to such a simulation An audio cue to the user when he strays from under the tracker s effective range Possibly include both fine and coarse control for changing a bodies velocity Any operations which involve deleting data should have appropriate warning sounds and confirmation actions to be initiated 34 User s Manual E References for Astronomy Information In writing this simulation we gathered several references to astronomical data and methods which could be used for writing an orbital simulation They are provided below as a reference for writing your own simulation Texts on Celestial Mechanics Roy A E Orbital Motion Danby J M A Fundamentals of Celestial Mechanics Analytic expressions for orbits Van Flandern T C and Pulkkinen K F Low Precision Formulae for Planetary Positions Astrophysical Jounal Supplement Series vol 41 pp 391 411 November 1979 Numerically Tabulated Ephemerides at JPL Contact Dr E Myles Standish ems smyles jpl nasa gov Initial Conditions Quinn Tremaine and Duncan Astron J 101 2287 1989 Algorithms for computing celestial mechanics Wisdom and Holman Astron J 102 1528 35 An N body Simulation in a Virtual Universe Index 3 D mouse device 3 Running Mode Tools 21 Add ne
5. a body s charge COLOR rgb Designates a body s color in terms of red green and blue values in the range of 0 255 Note that this is not HSV RADIUS r Designates a body s radius apparent size in virtual world ATTACHED Define this body as the one the user is attached to This command must be specified inside a BODY declaration indicating that the user is attached to that particular body 24 User s Manual ORIGIN BODY Designates this body as the origin of the universe In run mode all other bodies will move relative to it Also must be specified inside a BODY declaration DEFAULT BODY Similar to the BODY declaration except this command defines the default parameters of a newly generated body created by the user while the application is running This makes it simple to have a consistent manner of body generation This statement is not required If it does not exist a default default will be imposed by the user interface These fields may occur in any order after the DEFAULT BODY statement Again all fields must be inside braces f Possible fields 1 MASS 3 COLOR 5 VELOCITY 2 CHARGE 4 RADIUS 6 POSITION NOTE DEFAULT BODY will also be used by the application to fill in any blanks left in a BODY declaration which follows the default body specification in the save file The final default body specification will be the one used as the initial default in the user interface Thus if it is desired t
6. grabbed When the user lets go the universe is left at its current location Resizing Too This tool allow the user to change the size of the universe with respect to the user The universe may be shrunk down to fit in the palm of the user s hand or enlarged to fill a football stadium It functions like a vertical slider moving down to shrink and up to expand eu po Holding the button down will continue scaling in that direction One important note is that the center of the shrinking or enlarging is at the tool itself As the universe is scaled up it scales away from the tool and thus the increase in size may not always be very apparent Select Tool This tool allows for the selection of a body With this tool when the user clicks the left button the body that is nearest to the user s hand the select tool will be selected Selection is indicated by a white highlighter box surrounding the selected body A body must be selected in order to use the tools in Body Editing mode 15 An N body Simulation in a Virtual Universe Edit Mode Too Selection Panel 3 5 2 Editing Mode Tools When the user is in editing mode a control panel is presented to the user This is an object that is displayed in the virtual world and may be manipulated by the user Currently the control panels consist of a set of buttons sliders and dials The user activates a control panel widget by putting his hand inside the widget s active
7. is used p config file name Specifies the name of a process configuration file to use f scenario file name Specifies the name of a scenario file to use If no file is specified a default simulation sim1 is used r Enables the refresh system The refresh system is designed to allow graphic updates while the system is loading or saving a file to disk This prevents the display from freezing while this occurs Unfortunately the system is not yet very stable and may crash unexpectedly 31 An N body Simulation in a Virtual Universe C Writing Your Own Simulator Mark Parris In order to use a different simulation with the user interface the user simply needs to provide a simulation that can fulfill the requirements of the communication protocol see the Protocol section of the Implementation Manual In particular given the handshaking protocol that occurs at initialization a simple orbital tracker can inform the user interface that it doesn t support any of the body editing functions but only the modification of the time step and the sending of body positions Using this simpie simulation a user may attach to a body and ride it fly among the bodies resize the bodies scale the simulation and any other functions that don t require modifying the computations of the simulation itself However the user could reset the time step to produce a faster or slower model Other simulations which support different sets of the function
8. maintain command scripts which look up the current configuration and set the necessary environment variables for the desired device To start an n body session the user must be logged into either jason or hugin depending on how Pixel Planes 5 is configured Currently hugin is the host for the head mounted display in the Graphics Lab while jason is the host for the HMD Lab Logging into one of these hosts can be accomplished via a simple rlogin rlogin jason To set up the environment variables for the head mounted display in the Graphics Lab execute the following command from the jason or hugin prompt source hmd bin glab To use the unit in the HMD Lab execute instead Source hmd bin hmdlab If executing one of these scripts yields an error message an incorrect host has been chosen In this case the script describes which host to log in to The user then must then re source the appropriate shell script 23 A le Session Carl Mueller To run the program with the default setup execute the following command nbody See Appendix B for possible command line options Several startup messages will be displayed If you see the message Waiting for Pixel Planes then another user is currently using the machine You should wait until you see the Command q to quit message before putting on the head mounted display When putting on the VPL Eyephones simply slide them over your face and make
9. sure they fit fairly snugly The only trick is to make sure you press the plastic back flap back before putting the Eyephones on To wear the Virtual Research head mounted display twist the knob on the back of the helmet to loosen the interior halo so that the helmet slides on easily Then tighten the halo until it fits snugly and does not wobble User s Manual Use this knob to tighten to head Watch out for this part VPL Eyephones Virtual Research Eyephones Input to the simulation is generated by a 3D mouse one of which is positioned next to each of the head mounted displays in the department Tools described in details in Section 3 of this document can be selected and manipulated by using these input devices Both the pool balls and the Python3 devices may be used in this application For compatibility s sake both devices only use two buttons in the nbody system the Python3 uses the trigger as the left button and the top left button is the right button Start Button sx Top Buttons 2 Switch Left Button A Right Button Pool Ball Mouse Python 3 Mouse In general the left button is used to activate a tool or select a control panel item while the right button is used to change modes cancel a command or bring up control panels To select a general tool the right button can be pressed to call up the tool panel The left button can then be pressed when the appropriate icon is highlighted to select that
10. 80 front end host computer See the handout Pixel Planes 5 A Heterogeneous Multicomputer for Graphics provided with this document for more information Both VPL Research and Virtual Research produce HMD devices The screens in both are color LCD displays A light baffle around the user s eyes keeps out stray light and thus all the user sees is the environment displayed on the screens Headphones provide for audio feedback Attached to the top of the unit is a Polhemus magnetic tracker sensor Another sensor and a pair of push buttons are located in a 3 D mouse device Suspended over the user is a tracker emitter which provides a one meter radius area in which the user may operate See the handout Head Mounted Display Research provided with this document for more information The tracker control unit is attached via a serial cable to the Pixel Planes host Another serial cable connects the host to a sound server machine currently an Apple Macintosh Ilci This computer can play back digitized sounds under program control to provide for audio feedback n N body Simulation in a Virtual Universe 13 How does it work As the name states there are two main activities occurring in this system An n body simulation is being computed on the Maspar and a virtual universe interface is running on Pixel Planes 5 These two machines are communicating with each other over a semi public ethernet network using the protocol specified in the Prot
11. An N body Simulation in a Virtual Universe User s Manual TR92 045 Spring 1992 l d Adam Duggan Erik Erikson Carl Mueller Mark Parris Quan Zhou Department of Computer Science University of North Carolina at Chapel Hill Chapel Hill NC 27599 3175 UNC is an Equal Opportunity Affirmative Action Institution An N body Simulation in a Virtual Universe User s Manual Project Members Adam Duggan Erik Erikson Carl Mueller Mark Parris Quan Zhou COMP 145 John McHugh Spring 1992 UNC Chapel Hill An N body Simulation in a Virtual Universe T Introduction ote ie m eite lois tee irte Ee io rosse tU Eee ve ncs eges bea p aG 2 SES US Qc taut tend tierie aa eer ia citead CTAA Ra oor RSS eaaa ESR E Ea i 2 12 What hardware does It use anono edt wed xke a cores Pra axe SEN YN adane Eindir EAR ise 3 IE Mound t 3 IN uu m E 5 2 HOW TO Get Started ion ide cea tr odisti visceral iaa aS or adeb PA UN va arl dai ee wah IR DU anton tie qais 7 21 Hardware etquetfescaconi ub eiobyicceitula io irit EUR ERI Cielo ite EG hir vd p ia Dart A De oak 7 22 How torun the program iio ikiidee cer nodi illas erre R ond ants e RR TU gauses UL Ensa eei db r Sesona 7 2 3 A Sample Sessio o o eee eer a ereua E out Eus EEEa eade FI S eM at pa rand 8 3 Ihe User Interfaceo darc genstestatcea teres sod dM Pav tend Svat aswadlta cicuints ou EG Govan Vue M E Rocca ku pa 11 3 1 Overview of th
12. R or SIZE specified so the system obtains these characteristics from the DEFAULT BODY declaration POSITION 1000 0 0 1000 km out on x axis VELOCITY 0 3000 0 3000 km sec in y direction ATTACHED TO indicates that user is attached to this body 27 An N body Simulation in a Virtual Universe 5 Thanks Thanks to the following people for their assistance Russ Taylor libsdi a libconvert a Jim Chung Use of text in a virtual world Mark Wetzel UT Astronomy information Jack Wisdom MIT Astronomy information HMD team vlib head mount support Pixel Planes team Pixel Planes libraries Clients Support in getting access defining project Providing creative freedom Boss Pointers to resources disk space Facilities David Harrison Video Crew 28 User s Manual Appendices A Pr igurati ification Mark Parris There are several ways to specify the name of the program to run as the simulation process as well as the machine to run it on This was desired in order to allow the user the flexibility of running the simulation on different machines as well as providing the ability to run different simulation programs Thus multiple simulations may exist each utilizing different numerical methods to generate the simulation data or even to run a different kind of simulation as long as it follows the specified protocol These differences will be invisible to the user interface portion of the system as l
13. ain gets the Maspar The time slice is currently ten seconds Thus with multiple users every ten seconds the simulation will pause for ten seconds Pixel Planes will continue to do display updates so the user may still look or move around but the simulation pause becomes annoying As a result it is best to use the Maspar when others are not It is obvious that a dedicated user of the n body system will need to select time slots to run the programs such that both the Maspar and Pixel Planes 5 are relatively free The head mounted displays are located in both the Graphics Lab and in the HMD Lab room 262 The VPL Eyephones in the HMD Lab need to be turned on with the power strip near the north wall The Virtual Research helmet should always be powered on and only needs to have its displays turned on via the switch on the back of the helmet Always remember to turn off the head mounted displays after you are done using them 2 2 r When running the n body simulator with the head mounted display it is best to be logged onto one of the workstations in the Graphics Lab or HMD Lab to be near the head mounted display equipment An N body Simulation in a Virtual Universe Pixel Planes 5 has several configurations and may be reconfigured without advance notice Thus the exact unit which is attached to the head mounted display in the Graphics Lab or HMD Lab on any particular day is somewhat unpredictable Fortunately the members of the HMD group
14. and provide a three dimensional total immersion view To accomplish the first part we utilized a highly parallel computer a machine with a good deal more computational power than the conventional workstation This enables us to calculate the positions of on the order of 100 bodies at once For the second extension we use a head mounted display apparatus HMD This consists of a set of video screens and optics held closely in front of the user s eyes and in addition a tracking system to find out the position and orientation of the user s head Add a high performance graphics engine and you can now simulate a virtual universe A 3 D mouse input device allows the user to interact with the simulation Henceforth the portion of the system devoted to calculation of the bodies positions will be referred to as the simulation The portion devoted to the display of the bodies and the user s interaction with them will be referred to as the user interface Each portion is a separate process These processes communicate with each other through a specialized protocol A note to keep in mind while reading this manual while the simulation described here is an n body simulation running on a Maspar MP 1 the user interface is stand alone and may be used with any machine running any moving body simulation that follows the specified protocols See Protocol specification in the Implementation Manual In particular note that the simulation is only req
15. basic color saturation amount of color and brightness overall intensity The combination of these dials will give a final color for the object Set Defaults Any new bodies created using the Add Body function will have default parameters If the Set Defaults button is pressed the currently highlighted body s parameters become the default values for any new bodies With the Add Body tool this allows the user to create a whole set of bodies with the same user specified parameters Disable enable body Pressing this button changes the enabled status of the current body Once the body is disabled it may no longer be edited until it is re enabled Disabled bodies are not 20 User s Manual displayed during run mode and do not figure into the simulation s calculations If the showing of disabled bodies is turned off then the only function available to the user is the re enabling of the body When disabled bodies are shown they are indicated by a blue highlighter box Origin Body This button allows you to set the origin body When this mode is turned on everything will move relative to the origin body when run mode is invoked This makes it simple to create a center for the system and view the simulation relative to that body The origin body is indicated by colored orthogonal axis lines centered on the body Start the simulation This button is identical to the one in the world edit panel It starts the simulation runn
16. e Functions Available to the User sesssssesssreesrerrnnersrseeerenssreserreree 11 3 2 Interface Paradigmi idi aues ntn Aes Shee rch oa dci prece Lipa dapes e Even S raus 11 321 Control Panels er ree vete Panties ERE EU vn IRR CX rupe EVER AS EESE 12 3 2 2 Tool Selection Panels necie aeeaiei aerie ette eset tenent nenne 12 Bd Mode Features sei aenea esr E Grae etia lr V one taa e EORR IMEA VE Ve aen eeu up Gs 13 3 4 General Appenrante ocio te dee BE Fase vest ue EN aust HOO A RD ATHEN eee epe 14 a E M MA 15 So LF Gl bal TOONS 20 05 riesen hb Dx Drs olia P eee Dn RERBA REEL Bop 15 3 52 1 Common Tools for Editing Mode sss 16 3 5 2 2 World Editing To0lgs edis duisi s qa bi ri eran oe Fa OP PK RURSUS 17 23 5 2 3 Body Editing TOOLS iere rero tr eh tra ova Erro bi arx egg d 19 3 5 3 Running Mode Tools ricette Mti n teh Eee Ye SERRA eV a tai Eeri 21 4 Scenario Specification hime iet ee ee ho ge Deer r e XE aan ea ELE epe Heg ce eaa Peace ite Tn 23 4 1 Scenario Overview Je ovas Hcr ascii e ias Usa bate PD sun ra Fo RR Rd RE 23 4 2 Language Specifics MM 24 BT CODIFBEDS iiie nie ee EA aa ed tush eaten EE AA ET A peus ks bd 24 4 2 2 Body cuui i ETT 24 42 3 World Environment Specification eese recreate tenens 25 4 3 Sample State File ree ne eere tire eem RH EEEE Pone le X ER C EN V RS BARES E 27 5 TIRES ioi etes cust doy au ged Fuck anres bateau PI Dude E p timed
17. e dre ique r ie MEE bRE 28 Appendices ee T 29 A Process Configuration SDCCHTICRHOTEuS d oe etait altered vos Ubi Mr eq Men Ri 29 B Command Dine Options sao eon euet t der octies Era E DS ES A D beds 31 C Writing Your Own Simldfop dieute coena ree unas eere ee nrbe pF xen s lau k eR pad dn RENS RM ER i EE 32 D Using the Nbody Debug LID rary ccc ccccsis oiccscincvcessa vases ein Rl Rnegep det eqip e php Nace s mede 33 E Future Direclioriss tance meet e dede eias ane tee uoi epa TREE 34 F References for Astronomy InfortmatiOfiss ose tinsetereeid irre saves oes Uere EXE Pony pra eie vod 35 36 An N body Simulation in a Virtual Universe An N body Simulation in a Virtual Universe 1 Introduction Carl Mueller An n body simulation is a physically based calculation of the effects of gravitational or electrostatic forces on several bodies all of which may affect each other The resulting interactions may produce very orderly patterns of motion such as that of the bodies in our solar system or much more complicated motion patterns like those of atomic particles The large number of possible interactions makes the n body simulation a computationally intensive problem With a normal workstation one could perhaps view and manipulate a simple simulation using a small flat screen We wanted to go beyond this in two ways 1 to allow a more complicated simulation more bodies and 2 to actually place the user inside the simulation
18. e illustrative only All other information should be passed via the protocol once the connection is established 29 An N body Simulation in a Virtual Universe To override the default command line parameters are used to select a different entry in the config file and or a different config file To choose a different entry use the e switch as follows nbody e nbodyO which would use the entry with the tag solar siminthefileconfig file To choose a different configuration file use the p switch as follows nbody p myconfig which would use the default entry in the config file myconfig Finally to select a specific entry in a different file use the switches together For instance nbody p this cnf e my sim would use the entry my sim in the configuration file this cnf in the current working directory Comments may be used in this document in the same manner that they can be used in the simulation state file ie Specifying a nonexistent tag or configuration file results in program termination with one of the following error messages No such configuration file file name gt No configuration entry in file name entry tag 30 User s Manual i Line Opti Mark Parris When starting the simulation from the command line the following options are available e tag Specifies which tagged entry from the configuration file to use If multiple entries with the same tag exist the first
19. e protocol section of the Nbody Implementation Manual The protocol is implemented in library libnbp a and a sample simulation is provided in nbody src protocol test sample sim c This sample simulation provides a good example of a basic simulation including the protocol interaction as well as examples of body and feature sets The user may simply insert his own function in this sample to perform calculation of body positions or he may use it as a starting point for creating his own simulation Scenarios While the simulation is an actual program implemented to calculate the positions of the bodies over time using a chosen numerical method scenarios are files which simply contain the state of a particular simulation These files may include any of the information accessible to the user in normal editing of the system e g time step sample ratio the user s position and the bodies orientation scaling body properties law parameter values etc They are basically the save file for the nbody system They save the current workspace in an ASCII format which may later be reloaded into the system However they are not tied to the simulation which the user is currently running Thus a user may save a scenario that represents the state of a particular simulation with a given set of input after some amount of time and then load that scenario into another simulation to see how the same bodies react under a different simulation User s Man
20. ection tool or using the body selection sliders Once a body has been selected the following widgets affect the individual parameters of the highlighted body Position Pressing this button will give the user the position tool This tool will function on the currently selected body When the user holds down the left mouse button the body will jump to the user s hand position similar to grab When the user releases the button the body will remain at its new position Radius Pressing this button will give the user the radius tool to create a new radius for the currently selected body This radius will be determined by the distance between the center of the sphere and the position of the mouse pointer Holding the button down will allow for dynamic resizing Velocity The velocity button turns on the velocity tool It operates in a manner similar to that of the radius tool except that the velocity of the body is changed instead Selecting this tool automatically turns on the display of velocity vectors Mass This button hooks to the body edit slider and turns on display of the currently selected body s mass The user may then change the mass of the body using the slider Charge This button hooks to the body edit slider and turns on display of the currently selected body s charge The user may then change the charge of the body using the slider Color Color selection is achieved through the use of dials A dial exists for hue
21. eeps things flexible by not restricting arguments to be in a specific unit However the user must remember to keep all of his units consistent across declarations or undesirable results may ensue All of the numeric parameters may be expressed in integers or real numbers Real numbers may be expressed in scientific notation i e 1 23E 45 4231 Comments The pound sign designates a comment Any characters after this character anywhere on a line will be ignored by the parser This feature should be used to place a descriptive header in the file in addition to comments on specific entries 4 2 2 Body Specification BODY Designates start of specific information for a particular body May have following fields for 1 Position 3 Mass 5 Color 7 User attached 2 Velocity 4 Charge 6 Radius 8 Origin body These fields may occur in any order after the BODY designation All fields pertaining to this specific body are declared inside a set of braces following the BODY declaration If any of these fields do not occur in the BODY declaration they will be read from the DEFAULT BODY declaration If they do not exist there then default defaults will be assigned by the n body system The following specifications are possible within a BODY declaration POSITION xyz Designates body position in x y and z VELOCITY x y z Designates body initial velocity in x y and z MASS m Designates a body s mass CHARGE q Designates
22. efault body which may be specified in the scenario file or changed via the body editing panel The right mouse button is used to cancel the Add Tool Delete bodies Clicking this button will simply delete the currently selected body Show hide velocity vectors This is simply a toggle the user has available to turn on off the velocity vectors When showing the velocity vectors appear as white lines extending from the surface of the bodies 17 An N body Simulation in a Virtual Universe Show hide disabled bodies This is also a toggle the user has available to turn on off the display of bodies marked as disabled When showing disabled bodies appear surrounded by a blue highlight box Disabled bodies may only be displayed in edit mode When run mode is invoked they disappear and have no further effect on the simulation although this may be determined by the simulation Select the interaction law A simulation can have several different interaction functions or laws which determine the behavior of the bodies in the simulator Pressing the Law button brings up a panel of laws to choose from A slider is provided to select the law Pressing the Ok button selects the highlighted law and pressing the world button returns to the world edit panel Change the parameters to the interaction function A B C It is assumed that the number of variable parameters to each of these simulation laws is three or less These three buttons c
23. es in detail the tools and functions available in the user interface for manipulation of the scenario setup and control of the simulation All the buttons sliders and dials discussed are part of the virtual interface that is seen in the display The exceptions to this are references to the physical 3 d mouse Again when this document refers to the 3 d mouse and its buttons it refers to the pool ball mice These mice have a left and a right button Since the nbody project began a new 3 d mouse has been added This device is called the Python3 and is a joystick with four buttons and a switch The mapping of our left button is to the Python3 s trigger Our right button is the top left button of the Python3 refer to the diagram in the previous section There are two major modes in the interface Editing mode in editing mode the user may adjust the parameters of the n body simulation During editing mode the simulation is halted so that bodies will remain stationary while their various attributes are altered Within this mode there are two submodes Body Editing and World Editing Running mode In running mode the simulation is being updated and user interaction is more limited Only tools affecting the display of the world may be used Nothing interactive may occur other than the use of the thrust tool 3 2 Interface Paradigm Adam Duggan The paradigm utilized in the design of the interface is the use of tools and cont
24. formation is generated for the bodies so that the user interface can display velocity vectors for the bodies from the appropriate set In addition to body features there are also world features including change sample ratio time step and interactive sample ratio These features are not assigned to a specific set of bodies but rather indicate that the simulation supports modification of these parameters and whether or not the features are supported interactively The simulation and user interface exchange information about body and An N body Simulation in a Virtual Universe feature sets at initialization as part of the protocol that allows them to communicate as described briefly below Note that determination of body sets as well as feature sets is specified in the simulation portion of the system If the user wishes to change the system at this level he will need to be able to modify a currently existing simulation or write one of his own For more information see Writing Your Own Simulator in the appendix Simulations There is no limitation to use only the Maspar as a simulation engine The system configuration file allows any networked computer to handle the simulation A program on any machine may be used instead as long as it follows the nbody communications protocol for interacting with the user interface Protocol The simulations provided with this package communicate using a specified protocol which is described in th
25. hat all bodies have a set of identical characteristics they need only be specified once This implies that an empty BODY declaration is allowed i e BODY is accepted 4 2 3 World Environment Specification MODEL expr abc Define the model to be used in this simulation as well as its corresponding parameters The value for expr is equal to the label defined for the desired model Thus possible arguments may be words such as gravity electro gravity electro gravity cubed etc depending upon what the available simulations expect a b and c are the model s constants There may be between zero and three of these constants depending on the model TIME STEP x This declaration defines the time step at which the simulation function performs its calculations SAMPLE RATIO x This declaration defines how many time steps to make per visual update of the system Thus with a value of 60 we get 60 time step iterations before we do a visual update SIMULATION SETUP v xform type Designate the simulation s initial orientation scaling and translation relative to the origin of the virtual world v xform type is a structure of the form translation x y z orientation x y z w quaternion 25 An N body Simulation in a Virtual Universe scale s These values will be explained further here See sample file for example USER SETUP x y z Specify the user s initial translation relative to the origi
26. hoose one of the three parameters to alter by using the slider Use of the slider is discussed below Change the time step of the simulation This is another parameter of the interaction function that may be changed It controls the size of the discrete time steps which are used to update the simulation Though this will change the apparent speed of the simulation it is different than the sample ratio adjustment available in Running Mode as changing it may affect the accuracy of the simulation Start the simulation This causes Running Mode to commence This button appears as the GO button Loading Saving a Scenario Pressing this button brings up another panel This panel contains several button text and a slider The text consists of valid scenario names which are in the current directory Valid name must have a nbody extension The provided slider allows one to choose a name from the list by scrolling through the list Pressing the load button signified by the disk with an arrow coming out will load a new scenario from the highlighted file name Pressing the save button signified by the disk with an arrow going in will overwrite the highlighted file with the current setup Pressing the world button signified by the globe returns to the world edit panel 18 User s Manual Loading a simulation This button is pressed to bring up a panel showing the names of the different simulations which are listed in the configurat
27. ifferent bodies are selected buttons on the control panels will appear and disappear depending on whether the feature associated with that button is turned on for the selected body 16 User s Manual Go to body This button causes the user to be transported to the location of the currently selected body Attach to Detach from a body The attach detach button may be clicked to indicate that the user will be attached to the currently selected body once the user enters Running mode This allows the user to see the simulation from that particular body s perspective When the user is attached to a body the button will have a full chain showing By clicking the button again the chain will break and the user is no longer attached to the body In edit mode the attached body is indicated by a pointy box 3 5 2 2 World Editing Tools The world editing control panel has the following appearance N22 pody kad load N N mode sconarn an N N add delete show show ttach goto N N body body vectors disabled body N N N N choose print Pa N law amount step N N 1000000 00 N NE Gp 8 T S V SNNSSSRSRSSASRESRSARSANSNSSNSNSSNSSSSSNSY Descriptions of each button and its function is as follows Add new bodies Pressing this button gives the user the Add Tool With this tool the left mouse button creates a new body at the pointer s location in the world The new body will have initial attributes specified by the d
28. ing Return to World Editing mode This button is toggled to return to editing the world 3 5 3 Running Mode Tools When the user is in running mode only a select group of tools are available through the pop up tool menu The global tools mentioned before are accessible as usual In addition the tool menu now contains the following additional items Stop Selecting this item will send a stop command to the simulation When the simulation stops the user is returned to world edit mode and the world edit panel reappears Thrust tool Selecting this tool allows the user to apply an acceleration to the last selected body The tool operates in the same manner as the fly tool It is analogous to applying thrusters to the body as it interacts with the rest of the bodies in the simulation Sample Ratio Slider This slider allows the user to select the sampling ratio of the display of the simulation The middle position of the slider indicates that every calculated frame is displayed Moving the slider left will increase the number of times each calculated frame is shown thus slowing the display down Moving the slider right will increase the number of calculated steps per displayed step This may have the effect of speeding the display up depending upon how quickly the frames are calculated 21 An N body Simulation in a Virtual Universe Run Mode Tool Selection Panel Note on run mode Since running the simulation involves comm
29. ion file this file is discussed in another section of this manual Once again the slider may be used to choose a name the Ok button may be pressed to load the highlighted simulation and the world button may be pressed to return to the world edit panel Go to Body Editing mode This button may be pressed to enter body editing mode It is only available if a body is currently selected This button appears as a body Quit Pressing this button exits the system immediately This button appears as a skull and crossbones The time step thrust and parameter A B C buttons all use the same slider to change their values Pressing one of the buttons turns on the slider and its display and allows the user to change the value for the selected attribute of the simulation Along with the slider there are four buttons which change the range of values the slider represents Two buttons to the left of the slider move the slider s window of values to the right and left respectively The two buttons on the right of the slider double or halve the range of the slider By using these four buttons the slider provides both precision and varying value ranges 3 5 2 3 Body Editing Tools The control panel in this mode has the following arrangement e edit run mode set enable set goto SEE 1000000 00 19 An N body Simulation in a Virtual Universe The selection of a body to edit may be performed by either utilizing the body sel
30. ion headers follow for easy reference The first parameter of each is a file pointer to which the proper output will be written To print a body print_body FILE fp BodyType body To print a default body print default body FILE fp BodyType body To print a body list print body list FILE fp BodyListType bodies short num bodies ScenarioType scenario To print the scenario information time step laws etc print scenario FILE fp ScenarioType scenario To print the user setup and the simulation setup print xforms FILE fp v xform type sim xform q vec type user xform For the exact formats of the data types see nb types h and vlib 33 An N body Simulation in a Virtual Universe EF Directi Some features that would be nice to incorporate into this application are the following Include Alert panels that show up in front of the user when an error occurs The user would then have the opportunity to clean up and save the current scenario Additions to the protocol including the possibility of dynamic thrust updates having the two sides of the system synchronize when thrusts are being applied The ability for the user to change directories from within the head mounted display and perhaps enter new file names more accurate simulation on the Maspar and the possibility of more complicated and compute intensive simulations The current n body simulation was not taxing the Maspar as is and more
31. leted and manipulated in different ways from the alpha bodies Note however that there is no requirement that two sets of bodies be present A simulation can work entirely with alpha bodies or entirely with beta bodies Further realize that there may be simulations where the beta bodies can t be added or deleted or both The ability to manipulate the bodies is determined by feature sets described below Feature Sets Feature sets define the operations that can or must be performed on a set of bodies or the world There are three feature sets those for the alpha bodies beta bodies and the world Alpha bodies and beta bodies have the same features to choose from with the exception of add and delete Alpha bodies can not have either of these properties Features which a simulation may choose to enable include the following change position velocity mass radius color or charge send color or radius and show velocity The change features inform the user interface that the user can change those properties of a body from the specified set Alternatively the send features require the user interface to send information about those properties when they are changed Most properties are always sent but a simulation that is interested in color or radius is uncommon so the default is to not send that information unless the appropriate send feature is enabled The show velocity feature is used to let the user interface know whether or not any valid velocity in
32. n of the virtual world May be floats or integers 26 User s Manual 43 Sample State File This is a sample file to illustrate the n body save file language This file sets up two masses one orbiting the other along with a default for creating more masses which would be satellites of the larger one It also attaches the user to the orbiting mass dE dE tk db te MODEL gravity Set up for just a gravity model i e inverse square law 6 673E 20 Only parameter required for gravity model This is in 4 km 3 kg s 2 Thus all other definitions should be in km kg s etc TIME STEP 30 Set delta t to 30 second increments SAMPLE RATIO 2 In this case with 30 second increments display the results of every other calculation SIMULATION SETUP 0 0 0 At origin 0 0 0 i i Normal orientation 1 Normal scaling USER SETUP 0 0 0 At origin DEFAULT BODY The default is set as a satellite body This now makes it simple to create new bodies that are all satellites of the main body COLOR 0 0 200 solid blue MASS 10 kilograms RADIUS 10 kilometers BODY First body This is the massive center one POSITION 0 0 0 at center of sim VELOCITY 0 0 0 no velocity COLOR 200 0 0 solid red MASS 1 0E6 RADIUS 1000 BODY Second body This is the orbiting one With the position and velocity set as they are should remain in stable orbit around the center mass Does not have any MASS COLO
33. ocol section of the Implementation manual See the figure below MasPar Pixel Planes 5 AN A AAA A A AA A AA RAMS From other FBs video switch To other HMDs centurion ethernet ethernet VAX 3520 MasPar Terms polen ACU Array Control Unit DPU Data Parallel Unit Pixel Planes 5 Terms MGP Master Graphics Processor GP Graphics Processor HIF Host Interface oodd Bl RN Renderer tracker sensor FB Frame Buffer Other Terms MD HMD Head Mounted Display pq v 30 NR video screens inside User s Manual 14 Features Mark Parris This section discusses some of the features of the nbody system and explains how they may be useful to the user Body Sets This system supports simulations which have distinct sets of bodies each set having different features see Feature Sets Two sets of bodies are supported alpha or fixed bodies and beta bodies Alpha bodies represent a set from which bodies can be neither added nor deleted These bodies typically have an explicit ordering and a set of equations which are applied to the bodies based on that ordering An example of such bodies might be the planets in an orbital simulator where each planet has its own orbital equation In such a simulation there may be other bodies which are affected by the masses of the orbital bodies and each other These bodies could be beta bodies a second set of bodies where bodies can be added and de
34. ong as all simulations use a consistent protocol to communicate with the user interface See the Protocol section of the Implementation Manual The default configuration is to use the Maspar front end machine to run the simulation program named sim1 an n body simulation This information is found in a file called config file The program uses the first entry in it to get the process to run and the machine to run it on An entry in the file is a single line consisting of a one word tag the name of the machine to run the process on and the full pathname of the program to run on the remote machine along with any arguments A sample file follows Each entry is one line Sample file This is the default simulation to run on the Maspar The current Maspar front end is called centurion siml centurion unc parris nbody parris net src proto psim a b This is the model of the solar system that uses standard elliptical orbits Solar sim degas unc parris nbody parris net src solar solar This is a simple model where bodies orbit in rings around one another rings centurion unc parris nbody parris net src rings rings The remote process should expect to be called with the arguments specified in the file appended to two arguments which are sent by the display program These are the name of the machine on which the display process is running and a port number which the display process is listening to The arguments here ar
35. rol panels Control panels provide a centralized means of executing specific actions or manipulating specific characteristics of the virtual universe Tools provide a more direct interaction with the universe and are used for changing the user s view or directly manipulating the visual characteristics of bodies The selection of tools is implemented through the use of the control panels When a tool has been selected the user s mouse pointer changes to represent that tool so the user knows that any action will be in terms of that tool s function 11 An N body Simulation in a Virtual Universe 3 21 Control Panels The main control panels are those for world editing and body editing modes Panels also exist for loading and saving scenarios simulations and selecting force laws These panels have buttons dials and sliders as means of selection and input Furthermore they have text displays to give the user feedback resulting from his actions Feedback examples are the display of a value as the user is dynamically changing it or display of filenames An example of a control panel is shown below for reference Note that in the virtual world the text in this example is replaced with graphic icons ITERORCRCRCR AH SCR RC RCRCRC RC EEE SCC CCS CASCOS NCC SC EEE mE show show vectors disabled i N E N N N N N N N N N N N SSS AN SSS SSS These control panels will be displayed automatically when the user enter
36. s could be created The user would then need only to place the pathname of this simulation and any necessary options in the process configuration file along with the name of the machine to run the process on as specified in the Process Configuration File appendix in order to get the system running Several references to data and methods of computing simulations are provided in appendix F References for Astronomy Information 32 User s Manual D Using the Nbody Debug Library for Scenario Creation Adam Duggan One of the libraries created by the nbody team for use in debugging and scenario file generation has possible uses for the end user as well Anyone wishing to create systems with large numbers of bodies and who does not wish to build them by hand in the virtual universe or even with a text editor may benefit from the use of this library A function call exists for every possible type of output to the scenario file and the output of the functions is in the correct file format recognized by the nbody system Thus the user may set up his own code to generate the bodies in some systematic fashion Then when he is ready to create the resulting scenario file he may simply call the routines with his appropriate data structures and the result will be a scenario file guaranteed to be in the proper format The team actually used this method in generation of many of its sample scenario files and it has proven to be a useful method The funct
37. s the appropriate mode They are defined in room space in the virtual world Thus they will translate with the user as he flies since flying moves the entire room space about in the virtual world In addition they will disappear during flight to allow easier viewing of the world Each panel has a grab area around its edge allowing the user to pick the panel up and orient it as he wishes 3 2 2 Tool Selection Panels In addition to the control panels the user interface also utilizes tool selection panels These are specialized control panels and as such function differently purpose is to allow the user a simple method for changing his current tool in all of the different modes of operation of the system Since many possible tools exist run mode has fly scale grab thrust sample rate and stop tools it is difficult to create a quick and easy method of switching between them The Vixen program utilizes the right mouse button to cycle between tools However having any number of tools makes this method difficult since one must cycle through all of the tools to obtain the tool previous to the current one Asa result we chose to implement selection of tools though a tool selection panel This is a control panel which pops up upon pressing the right mouse button The panel contains only buttons one for each of the possible tools Any time that the mouse pointer is inside the active volume of one of the buttons 12 User s Manual
38. slider amp Start the simulation Adjust Charge slider Go to World Editing Mode Adjust Color dials Running Mode Apply an acceleration to a selected body Go to Editing mode STOP Change the sampling ratio of the display Any Mode Flying Translating around the world Grab Rotate the simulation Shrink Expand the world 9 Select a body by pointing 34 General Appearance In this virtual world the nbody universe is defined as one collective object Thus the only objects in the virtual world are the user the universe and the control panels Since the universe is one object the user may easily perform positioning orientation and scaling operations on the entire universe at one time In the nbody system different sets of bodies may have certain features turned on or off see the Features section In order to make the user aware of what changes he can or cannot make and to make the control panel function smoothly buttons may be hidden if their feature is turned off by the simulation For example if a body is selected from a body set that does not allow the user to change the position or radius of the body those buttons will disappear from the control panel when the user selects that particular body This can cause an interesting effect when the user moves the body selection slider and looks at the control panel at the same time Different buttons appear and disappear at high rates of speed 14
39. tool If the right button is pressed no new tool is selected and the tool panel disappears To enact one of the widgets on a control panel the hand needs to be placed in the vicinity of the widget so that the item is highlighted When the left button is subsequently pressed that widget on the control panel is enacted The control panel widgets are described in detail in the next section After either loading a scenario or building one the simulation may be run by clicking on the Go button in either the World Editing or Body Editing control panels The bodies in the setup will obey the rules specified by the simulation unti the simulation stops The user may stop the simulation by calling up the tool panel with the right button and selecting the Stop button The scenario may then be edited or even reloaded before exercising the simulation again To quit the application either click the skull on the World Editing control panel or remove the head mounted display and type q at the keyboard An N body Simulation in a Virtual Universe Again if you are the last to use the head mounted display for the day you should power it off when you are finished using it The standard procedure is to either turn off the power strip provided for the head mount gear or in the case of the Virtual Research Flight Helmet use the switch on the back of the helmet 10 User s Manual 3 The User Interface Erik Erikson This section describ
40. ual 2 How to Get Started 24 Hardware etiquette Mark Parris Since Pixel Planes 5 is a one of a kind high performance graphics engine there is a high demand for its processor cycles Further because consistent performance is desired from the machine the front end processors currently machines named jason and hugin should only be used for executing processes that utilize Pixel Planes 5 Editing or other such activities should be done using a different machine Users should refer to the etc motd file on the Pixel Planes 5 host when they log in to be apprised of the current machine configuration as well as reservations for demos and the like Once one is up and running he should be considerate of others who may be waiting n occasional run of 5 10 minutes is probably okay but if a longer run is desired it is probably best to pick a time when a small number of people will be around The Maspar is also used heavily for research in the department Its front end centurion also will display an etc motd file upon logging in which will describe the current usage of the Maspar Unlike Pixel Planes the Maspar implements timesharing to distribute its processor cycles This creates a problem for the nbody system when multiple users are running After a certain time slice the user who has the Maspar is swapped for the next user in the queue This means that during run mode position updates will stop until the simulation process ag
41. uired to support a subset of the features specified here See Writing Your Own Simulator for more details Also any user interface that adheres to the specified protocols may also be used to view the n body simulation running on the Maspar MP 1 User s Manual L2 What hardware does it use The computer we use to calculate the simulation is a Maspar MP 1 highly parallel system called the Maspar from now on The particular model we have here at UNC has 4096 processors arranged in a toroidal mesh configuration Known as the Data Parallel Unit DPU this set of processors operates in lock step SIMD fashion under the control of an additional sequential processor known as the Array Control Unit ACU This assembly is attached to a Digital Equipment Corporation V AXstation 3520 host with the usual array of I O devices including ethernet See the MasPar manual MasPar System Overview for more details Attached elsewhere to the network is UNC s custom graphics multicomputer Pixel Planes 5 This system consists of a number of high speed microprocessors Intel i860 s and several specialized SIMD arrays of one bit processors all connected together with a high speed ring network Known as Graphics Processors GP s and Renderers these units cooperate to generate complex images in real time Also on the ring are several frame buffer devices to drive the video screens in the HMD and a host interface device which attaches to a Sun Microsystems 4 2
42. unication between multiple processes ensuring completely successful synchronization can be difficult Although the nbody system successfully handles this synchronization problems may occur For example the user might be testing a simulation of their own creation and a bug may occur which hangs the simulation process Normally this would cause the whole system to hang To prevent this problem an undocumented feature was added to the system Pressing the top button of the Python3 input device will cause the simulation process to be killed This will allow the user to return to editing mode in the user interface with no simulation running Thus the user need not exit the application to begin a new session He simply needs to load a new simulation 22 User s Manual 4 Scenario Specification Adam Duggan 11S ioQ The n body system allows the user to save the characteristics of his created universe in a file in order to allow for quick and easy retrieval at a later time The save file format utilized is an ASCII file containing English keywords and declarations This format was chosen in order to allow the user the option of generating a the universe and its corresponding environmental parameters without ever entering the virtual world as well as to make it simple to edit the saved configuration without starting the application itself The following is a list of the different pieces of information that may be stored in the save file
43. volume and pressing the left mouse button The user can tell when a button is selected by the highlight around it This highlight is displayed when the user s mouse pointer is within the active volume for the button Pressing the left mouse button while the button is selected either toggles the switch or simply activates the button depending on that button s function For sliders the left mouse button grabs the slider handle and allows the user to adjust that slider s parameter by moving the handle from side to side or top to bottom while holding down the left mouse button Editing mode has two sub modes World Editing mode and Body Editing mode World Editing mode controls parameters that affect the entire system such as the simulation s interaction function constants time step thrust etc In Body Editing mode individual characteristics of the simulation bodies may be altered Each mode has its own unique control panel 3 52 1 Common Tools for Editing Mode These tools are available on both the world editing and the body editing panels See the illustrations in those sections for locations of these tools on the panels Body selection slider Moving this slider changes which body is currently selected In conjunction with the body selection slider the up and down buttons associated with the slider may be used to cycle through all of the bodies highlighting them sequentially so that other editing commands may be used Note that as d
44. w bodies 17 Sample Ratio Slider 21 Attach to Detach 17 Sample State File 27 ATTACHED 24 SAMPLE RATIO 25 BODY 24 Scenario Specification 23 Body Editing Tools 19 Body selection slider 16 Change the parameters 18 Change the time step 18 Charge 20 24 Color 20 24 Command Line Options 31 config file 29 control panel 16 Debug Library 33 default configuration 29 DEFAULT BODY 25 Delete bodies 17 Disable enable body 20 display program 29 Editing Mode Tools 16 ethernet 4 Flying Tool 15 Future Directions 34 Global Tools 15 Go to body 17 Grabbing Tool 15 head mounted display 2 HMD3 hugin 7 jason 7 Loading a simulation 19 Loading Saving a Scenario 18 Maspar 3 Mass 20 24 MODEL 25 n body simulation 2 Origin Body 21 ORIGIN BODY 25 Pixel Planes 5 3 port number 29 Position 20 24 print body 33 print body list 33 print default bod 33 print scenario 33 print xforms 33 Radius 20 24 Resizing Tool 15 Return to World Editing mode 21 36 Select the interaction law 18 Select Tool 15 Set Defaults 20 Show hide disabled bodies 18 Show hide velocity vectors 17 simulation process 29 SIMULATION SETUP 25 Start the simulation 18 Stop 21 Thrust tool 21 TIME STEP 25 USER SETUP 26 Velocity 20 24 World Editing Tools 17 Writing Your Own Simulator 32
Download Pdf Manuals
Related Search