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Raster Graphics for Interactive Programming

1. Ito Research Center gt Raster Graphics for Interactive Programming Environments By Robert F Sproull Raster Graphics for Interactive Programming Environments by Robert F Sproull CSL 79 6 June 1979 Abstract Raster scan display terminals can significantly improve the quality of interaction with conventional computer systems The design of a graphics package to provide a window into the extensive programming environment of Interlisp is presented Two aspects of the package are described first the functional view of display output and interactive input facilities as seen by the programmer and second the methods used to link the display terminal to the main computer via a packet switched computer network Recommendations are presented for designing operating systems and programming languages so as to simplify attacning display terminals An appendix contains detailed documentation of the graphics package A shortened version of this paper appeared in Computer Graphics Summer 1979 CR Categories 8 2 6 35 4 35 Key words and phrases computer graphics raster scan display frame buffer computer networks network graphics XEROX PALO ALTO RESEARCH CENTER 3333 Coyote Hill Road Palo Alto California 94304 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS 1 Introduction Computer graphics has yet to have much impact on interactive computing as a whole Several arguments sugg
2. Any Tenex program may generate terminal output In particular if Interlisp starts an inferior executive the characters typed out by programs running in that executive and echoed characters are ordinary terminal output and will be processed like characters typed to the terminal by the Interlisp job Thus the terminal activity of the inferior fork can be confined to a given region of the screen which can scroll as new text arrives etc Tenex also performs formatting operations on characters destined for terminal output For example it may convert control characters into two character sequences tG for example so that they become visible Much of the formatting can be controlled by appropriate system calls ADIS requires one special character code currently 3 control C for communication with Chat over the teletype connection ADISInit informs Tenex that this character is to be passed on unmolested User programs should not alter this setting and should not attempt to type the character over the teletype connection ADISTTYRegion n Flush Sets the teletype region to n All characters transmitted from MAXC over the normal teletype connection will be displayed in the specified region ADISInit does ADISTTYRegion 0 Because this is a region like any other parameters such as margin font limits can all be set by ADISxxx functions ADIS functions that modify teletype region parameters are synchronized with the teletype connection so that
3. buttons it is DownMask 256 UpMask where cach mask uses the same bit RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS assignments as for the BUTTONS entry in the EVENT record For example to enable for down transitions of all mouse buttons EventEnables would be 7 256 0 In order to detect double clicks of keys there is a timer facility The idea is that TimerStartEnables describes what button transitions should start the timer TimerStopEnables describes what button transitions should stop the timer When the timer is started it will run for Timerlnterval 27 seconds and then expire if it is not stopped in the interim If the timer ever expires a timeout event is generated and the timer is stopped See ADISEvent for a discussion of timeout events Null arguments except EventEnables default to 0 30 ApisStartTimer timeOut Flush This function starts the timer under program control The argument is the amount of time in units of 1 27 second to wait When that time expires unless some other event resets the timer a timeout event is generated AbIsEvent wait activateontypein oldevent This function returns a button event or NIL if there is none waiting If wait is not NIL the function will hang until an event is generated The function returns an EVENT record describing the event or NIL if a timeout event occurs In retrospect it would probably have been better to return TIMEOUT ratherr than NIL If
4. 64 LOGOR LLSH WIDTH a LOGAND 3329220 13 3 Fi 64 NIL NIL 2 Loren WONK ADLA J 15 me about f 11 6 60 8 6 1 about searchi SEARCHING FILES loading from lt TETTELMAN gt PACKAGES 462 z r L i filepos xifite start end LAMBROS FL PRIN1 11 from FL FILEPQS Fram HAILF LE GETMATLPOS NIL NIL T SENDER sa MATLFILE OLSPLAYROTBL Ev ispes VAL PINE L NER L GASE SENDER T PRIN FL PRINA 97 0 34309144576 it FL Figure 1 Typical DLISP displays This image and the one on the next page illustrate two successive images during an interactive session The second image is created from the first by slight rearrangement of some windows and by adding new text to the Work Area window The DLISP extension of these facilities is intended not only to use a fast display output device The spirit of DLISP is illustrated in Figure 1 the display shows many windows each of which presents information but also to take advantage of other opportunities offered by the display about a programming context One window contains a function definition being edited one a selected portion of the user s manual one a message received from a colleague one a typescript of the user s activities and several contain menus The user may alter the size and location of RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS
5. although responsibility for screen management is borne by the application program the technique offers advantageous flexibility as well as a burden Experience with ADIS suggests that it is reasonable to require the application program to assume substantial responsibility for screen RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS 9 updates as it has sufficient contextual information to plan an update sequence using simple frame buffer editing functions that are very fast Expensive although general update schemes such as hidden surface elimination can be avoided This approach is possible only because the notion of a frame buffer representation of an image is a simple one that no application programmer finds bewildering Although a plethora of off by one bugs crop up do the limits of a region include the points at the edge a frame buffer has none of the complexities of display file formats for calligraphic displays 3 2 Graphical Input DLISP s needs for interactive input are divided into two classes character typein from a conventional keyboard destined to be used by the Interlisp program with exactly the same conventions as teletype input and graphical input obtained from the 5 button keyset or the 3 button mouse coordinate input device An important objective of the design is to require no modifications to the Interlisp functions already provided for processing teletype input The only direct form of input feedback provid
6. careful timing that is not provided by the event mechanism alone For example consider distinguishing a single click key goes down then up from a double click key goes down up down again within 1 3 second then up After observing the first two transition events down up the application program needs to wait for either a down transition or 1 3 second whichever comes first Because DLISP runs in a time sharing environment it cannot perform such accurate timing ADIS provides an internal timer that is started whenever one of a set of starting transitions occurs and is stopped whenever one in a set of stopping transitions occurs If the timer expires before a stopping transition Occurs a timeout event is generated Thus the program trying to distinguish single and double clicks will receive one of two input sequences down up timeout single click down up down up timeout double click Both events and typein characters are queued A knowledgeable user is therefore free to work occasionally faster than the application program can process inputs This technique works only if the application program refrains from changing frequently the parameters that govern the reporting of events e g which transitions cause events which transitions start and stop the timer the length of the timeout period etc If such changes are made frequently a difficult synchronization problem arises events already recorded in the queue may no
7. connection is opened If other ADISxxx routines find ADISOFILE null they return without transmitting protocol over the non existent connection If the program becomes detached if the Chat program crashes or if the message IO DATA ERROR appears on your Lisp job and you are forced to re enter the setting of ADISOFILE is likely to be invalid Before calling any other ADIS routines ADISCheck should be called to test carefully the state of the connection ADISCheck sets ADISOFILE accordingly ADISFlush wait This function causes any partially filled output buffers on the graphics connection to be transmitted to Chat Most of the ADISxxx functions given below do not flush output 24 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS buffers because further commands may be issued Those functions which flush output are marked Flush The wait argument is used to synchronize Chat and Interlisp if wait is non NIL the ADISFlush function will not return until Chat has successfully processed all the protocol and characters already shipped from MAXC Waiting requires an end to end exchange of protocol 2 2 Display output The ADIS output functions change the display image by changing the contents of a frame buffer used to refresh the display screen Display output for Chat is always directed at some region Each region has state associated with it and ADISxxx functions for changing the state ApisLimits The limits are coordinates of the
8. four edges of a rectangular box which surrounds the region If information directed to be displayed in the region lies outside these limits it is clipped off AbisSetX ADISSetY or ADISSetXY A current position CP within the region This position is used to determine where text or lines will be displayed The current position is changed both by the three set functions and by the display of a line or character ADIsBold AbisItalic ADISSetCR ADISSetLF ADISSetTab ADISFont ADISPrintMode These state variables govern the display of characters Chat provides a number of independent regions All functions for changing region state operate On one current region CR as set by ADISRegion Because Chat stores the basic region state internally it may not be necessary to change region variables often but simply to switch regions using ADISRegion ADISInit initializes all the regions with reasonable defaults sets the limits of region 0 to a teletype simulation area at the bottom of the screen sets the limits of other regions to the entire screen and makes region 1 the current region A fixed coordinate system is used to direct the display of objects on the screen regions do not establish new coordinate systems even though they clip output In the parlance of conventional graphics systems the region is a viewport but not a window No coordinate transformation takes place ADISInit will set four global variables which give the dimension
9. g DECSystem 10 are familiar with the result typein and typeout are thoroughly intermingled preventing careful presentation of either Terminal input output facilities in the programming environment also need careful attention to support formatted communication protocol with a terminal Although the ideal solution is to funnel all terminal input and output through the graphics package almost no programming language systems permit it Instead existing facilities for terminal input and output must remain intact and additional independent mechanisms can be provided by the graphics package Problems with output are modest provided the terminal interprets normal text characters generated by the system in the proper way Calls on the graphics package can generate terminal output protocol sequences to display graphical objects or to alter the interpretation of text characters This is the approach taken by the Tektronix terminals and is quite successful Input presents a more intricate problem descriptions of graphical input arriving in the midst of character typein will interfere with the character input expected by the programming system The use of READ macros to solve this problem in the ADIS package is an idiosyncratic solution applicable only in Interlisp Programming environments that allow careful control of terminal 1 0 e g SAIL present fewer difficulties Manufacturers of graphics terminals have chosen to solve this problem by tran
10. graphic displays The primitives resolve this problem in a reasonably effective way the unformatted teletype output may be directed to a specific region of the screen under complete control of the LISP program However we are forced to acknowledge the existence of two independent sources of information for Chat a teletype connection that carries the stream of teletype characters emerging from Tenex or Interlisp and a separate graphics connection that carries the characters and graphics protocol generated by Lisp As with the Network Graphics Protocol Sproull74 it is important that these connections be kept separate the graphics connection must transmit highly structured protocol messages that cannot suffer interference from system messages and other uncontrolled teletype transmissions INTERLISP DISPLAY PRIMITIVES 23 2 The ADIS functions This report does not cover the detailed implementation of the system but concentrates on a description of the collection of Lisp functions that are provided and their intended effects Each function is named ADISxxx the actual names of the functions use all upper case letters even though the description below does not ResetSave Several of the functions for setting state variables use argument conventions that permit use with ResetSave They have the following properties 1 called with no arguments they return a current state 2 when called with legitimate arguments they interpret the argume
11. oldevent is provided it is an EVENT record that is smashed when reporting the new event If activateontypein and wait are both non NIL the LISP job will hang waiting for either 1 an event to occur in which case the EVENT record described above is returned or 2 the user types some character on the keyboard in which case ADiSEvent returns T Note that there is a potential race because ADISEvent must inform the Chat program to indeed activate if a character is typed The character may be typed before the information is received by Chat in which case the wakeup will not occur for that character ApbIsTypeOnEvent cl c2 Flush ResetSave When an event is generated by Chat it is sometimes preferrable if it signals the LISP program by typing a character in addition to sending the event description via protocol This permits the LISP job to be blocked for terminal input the character typed when an event Occurs can activate a READ macro that calls ADISEvent to read the event details The arguments cl and c2 are character codes for up to two characters that will be typed If a character code is Q it will not be typed Null arguments except cl default to 0 Thus AbISsTypeOnEvent 0 resets the feature In retrospect a single character would have been sufficient ADISReadStatef Flush This function records the current state of the mouse and buttons and returns an EVENT structure that describes it It is absolutely independent of wh
12. on the Chat screen This function depends on internal conventions within the PARC environment 3 Conclusion The ADIS package has seen considerable use in its present form for the past two years It supports DLISP a display oriented extension of the Interlisp user interface DWIM MASTERSCOPE HELPSYS HISTORY etc worked out by Warren Teitelman Teitelman77a Teitelman77b In turn DLISP has been used as a basis for other display related projects Model79 The necessity of interposing a communications system between the interactive program Interlisp and the display 32 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS and input devices has generated some of the unpleasant aspects of the design when necessary we have chosen solutions that make a cleaner user interface and a happier uscr rather than a cleaner programming interface and a happier programmer e ii 8 _ ad n _ 7 z z gt s Api er r gt aaea H ev sm La gt y E 22 Fx D g R a gt a E a P Fe 5 s AT 1 9 z 5 2 4 ras A pa 0 et ta 1 a lt gt 7 z fi r s z _ 2 2 ai a gt 2 inn ie gt 5 gt
13. passes it on to G o simulating the effect of a Text call The text characters are displayed on the screen by G 0o using parameters supplied by a region current position font scrolling options etc In this way text generated in a conventional way by Interlisp modules or by Tenex can be presented in a carefully controlled way on the screen The ARPA network graphics protocol does not allow precise treatment of such unescorted text the DLISP application clearly requires it The Interlisp programming environment on which the DLISP application is based depends critically on the properties of the terminal 1 0 streams These streams are subjected to special processing within the Tenex operating system For example normally invisible control characters can optionally be converted to two character sequences e g control C is printed as tC More importantly Tenex implements an echo path on which Interlisp depends normally as each character of terminal input is read by Interlisp Tenex echos the character by sending it as output to the terminal Because it is impractical and inefficient to replicate these functions within DLISP the ADIS design had to continue to use the normal terminal I O streams for all character input and Output and use the graphical streams for all other traffic 14 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS Unfortunately the exisiting termin l input and output streams cannot be used reliably to
14. system The design builds on issues raised in the ARPA network graphics protocol Sproull74 and solves the problem of synchronizing teletype and graphics information The design shows that shortcomings of input output handling by programming language systems and by operating systems present thorny obstacles to a clean design The success of the network connection is demonstrated by routine use of the final system 2 The DLISP Application DLISP enriches the already copious interactive programming facilities of Interlisp by making effective use of raster scan displays The programming environment developed by Warren Teitelman and others is designed to reduce the tedious mechanical aspects of program development by making generous use of interactive exchanges with the programmer and by exploiting information in the programming context Examples of these facilities are interactive access to the programmer s manual a structurc oriented editor for modifying LISP function definitions and data a file package to keep track of function definitions in text files reading in functions as needed and writing out functions that have been modified a translator that converts into LISP any function definitions written in CLISP a Icgible Algol like form a facility for inserting break points in functions and for examining and modifying program state when in a break or when an error occurs and the DWIM package that attempts to fix crrors by using c
15. the diagram of Figure 4 were an accurate picture of the processes involved in the DLISP ADIS system the three process design could be easily and cleanly implemented Unfortunately various deficiencies of the Interlisp environment and of the Tenex operating system make such a simple implementation impossible The problems stem from the ways in which terminal 1 0 is handled by the two villains This section explains the difficulties in the hope that future operating system and language system designs will avoid the problems RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS 13 Frame buffer Figure 5 Actual implementation of ADIS D is the application program T are operating system processes and G are processes in the graphics terminal Communication is achieved with four streams S transported by a packet switched network uninvited text S to echo path Q HHE The basic problem is that systems make it impossible to funnel all input and output through the graphics package D g Instead two additional streams shown in Figure 5 provide communication to and from a terminal in the sense that a timesharing system views a terminal The two processes G ti and G to interact with G i and G o in very simple ways when a key is struck on the keyboard G i passes it to G ti which sends it over the terminal stream to Tenex When Tenex generates output characters the terminal text is sent to G to which
16. the implementation of terminal communications in computer networks and would also allow one user defined process to invoke another in a simple way Some operating systems implement pseudo teletypes that attempt to serve this function but most implementations are either too inefficient or too awkward to use uninvited text S to Figure 8 User defined processes control the graphics terminal Any processes wishing to use the terminal e g the application program and the operating system the source of uninvited text communicate with the terminal controller processes 4 3 Network Issues A packet switched network is used to implement four of the interprocess communication streams used by ADIS S8 go S gi S to S ti The network introduces no special problems it is the introduction of several processes and the confrontation with programming language and operating system limitations described in the previous section that complicate the implementation All four streams are implemented with a byte stream protocol Boggs80 similar to TCP Cerf74 The two streams that carry terminal traffic S to and S ti use a Telnet protocol implemented with streams Telnet is the name of the terminal protocol in the ARPAnet Davidson77 The Tenex operating system interfaces the character input output facilities available to the Interlisp program to the network protocol This interface ensures that characters output from RASTER GRAPHICS FO
17. to which it is destined Its baseline is aligned with the current y position and its left edge is aligned with the x position After the character is displayed the current x position is advanced to x width where width is the width of the character This process is repeated for all characters received There are two different methods for transmitting characters to Chat for display over the teletype connection and over the graphics connection Any characters transmitted over the graphics connection will be displayed in the current region as identified by ADISRegion Characters transmitted over the teletype connection will be displayed in a region specified by ADISTTYRegion see below Note that the two transmission methods are asynchronous if both teletype and graphics traffic is directed to the same region the characters may become intermixed on the screen Using the teletype connection The standard 1 0 facilities of Interlisp and Tenex are used to output characters on the teletype connection Whenever terminal input is received e g by READ characters may be echoed and therefore appear as output characters on the teletype connection as well The functions for controlling terminal I o in Interlisp are documented in the language manual The Interlisp system performs a number of formatting operations e g insertion of carriage return and line feed when lines exceed a certain length that will affect the characters output to the terminal
18. typists often make mistakes and will want to backspace over characters words or entire lines As this happens we want the display to show the current input characters without extraneous editing characters so often scen on teletype paper Control of typeout Characters typed with operating system assistance by Interlisp and other programs must be directed to an appropriate window This requirement arises because Interlisp allows a user to initiate inferior executives that invoke arbitrary Tenex programs and yet to return to the retained Interlisp context The display context must likewise be untouched Graphical detail The flexibility of the raster scan frame buffer display should be exploited to use graphical details to provide cues to the programmer Examples are italic text boldface text different character font styles special iconic symbols that have mnemonic value attention directing forms such as white text on black background different texture patterns etc Graphical input The user is provided with a mouse a coordinate input device with three pushbuttons The mouse rolls along a table top and is tracked by a cursor that moves on the screen The mouse can thus be used to point to objects already displayed on the screen or simply to identify a coordinate position The three keys are remarkably versatile provided the software for handling them is sufficient for example combinations of keys can be struck as chords a single k
19. 2 or 3 _ a all os e st 4 e a 1 a a K 3 4 Pk Sey lt gt E E A gt y gt full r 7 E A lt s A a 7 m go ae gt Af oe 5 gt j e p o s n P E e F v an e 7 gt m ir at gt 3 d m J 5 v z Z e 3 y 2 i 4 a z P
20. 3 windows and may bring partially obscured windows on top for a clear view Commands to DLISP are entered with a keyboard and a mouse coordinate input device The primary objective of the ADIS graphics package is to provide a high performance service to Interlisp programmers As a consequence highest priority is placed on providing pleasant interaction and avoiding errors of all sorts Although the system offers some general purpose graphics facilities its objectives are focused on the needs of DLISP which are outlined below INFORMAIL changed Input material TYPESCRIPT 4 0 1PM SAS 3 Tif bits 4 10 JSYS 143 64 MODE STPAR sets voy new mode word wry SETPAGE 124 setpage 72 100 NIL if 13 27 18 load lt MASINTER gt GETMAILPOS ii FILE CREATED 23 JUN 77 0 red GETMAILPOS td lt MASINTERSGETMAILPOS 1 ie LAA 4 ge ve Readmali u 18 informail t zef 59 Mail from Masinter 3 30PM Helpsys Ty A Masterscope anny 2 Prettyprint sird 494 Mait from Burton 3 55PM Refresh screen Want to see it now Yes ane TET di i informail t v2 4 Mail from Burton at bbn tenexd 3 55PM wea Read selections oy aor Move i EDE EEE HIS TONY COMMANDS SEERE Grow shank 9 a Redo Use Fix Undo 13 ve Put on top tL Point ae ot Gata fat Put on bottom Make invisible Mover gt 72 SNDMSG Done eee WORK AREA i From BURTON
21. INF ORMAIL VP 000 AD 52 LAMBDA FL id Tor PRIN1 Mail from FL 1 i FILEPOS From MAILFILE GETMAILPOS NIL NIL T py ee TEER SENDER RSTRING MAILFILE OLSPLAYROTBL table s PRINA SETQ SENDER L CASE SENDER T vid 16 6 me about searchi FL PRIN FL PRIND GOATE LASTKNOWNMAIL 34309144576 SEARCHING FILES 2 29 ne FL 3 Filepos x file start end if filepos Subject a MPRINA J tb FL filepos was written Figure 1 continued Speed and capacity The context surrounding a working programmer is enormous DLISP aims to display sizable chunks of this context as carefully formatted text in windows see Figure 1 Remaining portions of the display are devoted to menus cues pointing to more context and the like When the user asks that a window be moved or that an obscured window be placed on top of those hiding it a considerable portion of the display will need to be altered rapidly For further 4 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS reference we shall refer to this as the window update operation Control of typein Part of the window idiom requires that typein activity appear to take place in arbitrary windows Because Interlisp terminal facilities are full duplex this is quite simple the program will wait for a key to be struck and will then display an echo character in the proper window But
22. PRIMITIVES 1 Introduction This report describes briefly a set of display primitives that we have developed at PARC to extend the capabilities of Interlisp Teitelman78 These primitives are designed to operate a raster scanned display and concentrate on facilities for placing text carefully on the display and for moving chunks of an already created display The primitives are deliberately designed to provide a low level interface to the display A display output primitive will cause a specific change to appear on the display by changing the contents of a frame buffer that is used to refresh the raster scanned image The primitives make no assumptions about the sorts of data structures for describing the display that an application program may wish to build Our implementation of these primitives involves two computers Interlisp is executed on MAXC and communicates with a program called Chat which maintains the frame buffer that drives a 808 by 606 point raster display The existence of the communications link is not entirely invisible to the programmer Although it ably provides the bandwidth necessary to achieve rapid screen changes the mechanics of transmitting and receiving data are complicated both by the Interlisp programming environment and the Tenex operating system One source of complication comes from the need to accommodate on the display ordinary teletype output generated by Interlisp and Tenex as well as carefully constructed
23. R INTERACTIVE PROGRAMMING ENVIRONMENTS 19 the Interlisp program are forwarded through the stream in a timely fashion The two streams used for graphical information S go and S gi simply use the ability to send bytes from one process to another The Tenex network interface for these streams is rather different than that for terminal streams In particular bytes output by D g are not forwarded over S go in a timely way but are buffered until a sizeable number roughly 500 are available to fill a packet This buffering has the unfortunate effect of requiring the application program to issue periodically calls to an ADIS function that instructs the operating system to transmit a buffer even though it is not full Buffering of this sort may be acceptable for streams that access disk files but is quite unacceptable for streams that carry timely information This fact is all too often overlooked when a network interface is designed for an operating system it is slipped into the file machinery of the system without recognizing that a network connects computational processes operating in a timely way What is needed is the ability to declare that a stream contains timely information and that no bytes should be buffered for very long Such a facility is necessary for terminal and network streams it also has applications for file streams e g transaction recording The particular network implementation in ADIS seems to introduce no noticeable performanc
24. The global variable ADISNFONTS is set by AbiSInit ADISBold h ADISItalic h ResetSave These functions turn on and off bold and italic features that will affect each character displayed in the region If h is ON or T subsequent characters will be bold or italic otherwise the feature is turned off but h NIL will not turn it off because of ResetSave conventions ApiIsSetCR leftx ApisSetLF deltay AbisSetTab tabx ResetSave These functions can be used to control the interpretation of carriage return line feed and tab characters When a carriage return is displayed the only effect is to set the current x to the value last given in ADISSetCR When a line feed is displayed the value deltay is added to the current y coordinate If the value of tabx is not zero it is interpreted as a INTERLISP DISPLAY PRIMITIVES 27 tab spacing relative to the carriage return position eftx Receipt of a tab character will move the horizontal position to the next tab stop If tabx 0 tab characters will not be subject to special interpretation but will be displayed Defaults leftx 0 deltay 12 tabx 0 ApisPrintMode CharDisplayOp ClearColor Scroll ResetSave Sets additional details pertaining to character display CharDisplayOp is the directive see ADISRegionOp above to use for displaying characters It defaults to 1 which causes characters to be painted from the font description onto the screen If Scroll is T or ON or EXPAND r
25. a Complex Computational Environment Xerox Palo Alto Research Center CSL 79 1 Jan 1979 Newman74 W M Newman and R F Sproull An Approach to Graphics System Design Proc IEEE 62 4 471 483 April 1974 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS 21 Newman79 W M Newman and R F Sprouil Principles of Interactive Computer Graphics second edition McGraw Hill 1979 Sproull74 R F Sproull and E L Thomas A Network Graphics Protocol Computer Graphics 8 3 27 Fall 1974 Sproull75 R F Sproull and W M Newman The Design of Gray Scale Graphics Software Proc Conf on Computer Graphics Pattern Recognition and Data Structure May 1975 Sproull78 R F Sproull and D Cohen High Level Protocols Proc IEEE 66 11 1371 1386 Nov 1978 Teitelman77a W Teitelman A Display Oriented Programmer s Assistant Xerox Palo Alto Research Center CSL 77 3 1977 Teitelman77b W Teitelman A Display Oriented Programmer s Assistant Proc Sth Int Joint Conf Artificial Intelligence pp 905 915 1977 Teitelman78 W Teitelman ed Interlisp Reference Manual Xerox Palo Alto Research Center Oct 1978 Thacker79 C P Thacker E M McCreight B W Lampson R F Sproull and D R Boggs Alto A Personal Computer to appear in D Siewiorek C G Bell and A Newell Computer Structures Readings and Examples second edition McGraw Hill 1979 Appendix INTERLISP DISPLAY
26. at lies on the line For each of these points it will store into the frame buffer the value F oldValue 1 Different choices of the combination function will produce different effects on the screen F or F will cause the line to appear black F will cause it to appear white and could be used to erase a black line by changing the value of each pixel along the line F will cause a line to appear regardless of what information has been previously recorded in the frame buffer The output functions are summarized in the following paragraphs Line This function generates the portion of a line that falls within the region limits The endpoints of the line are specified by a current position recorded in the region and a final position provided as an argument After the line is drawn the current position is changed to become the final position Arguments final position x y width of the line current position x y combination function region limits left right bottom top RasterOp This function copies the image in a rectangular area of the screen to another place on the screen applying one of several modifications to the image as it is copied The two rectangular areas are called the source rectangle and the destination rectangle The function may reduce the dimensions of the destinaticn rectangle to fit within the region limits The RasterOp function computes a new value for each pixel in the destination rectangle using the exp
27. carry graphical traffic The output stream is subject to invasion by uninvited text generated by the operating system or the Tenex executive Such text may garble the graphical output protocol In addition the application program may type arbitrary characters to the terminal and inadvertently generate sequences that interfere with the graphical protocol Graphical information encoded on the input stream is subject to echoing which may further garble the output stream The problem faced in the ADIS design is to provide a conceptual view similar to Figure 4 with the actual configuration of Figure 5 The design requires solving several synchronization problems rather than a single output stream in Figure 4 there are two in Figure 5 Similarly for input Each of these problems is considered separately below Input Input may be generated as graphical events or as typed characters The DLISP application cannot anticipate the kind of input that will be generated and must therefore wait for activity on either of the two input streams Tenex does not provide facilities to wait for input from either of two sources In addition the Interlisp functions that gather typed input READ and the like do not offer much opportunity for intervention once READ is called tests for graphical input cannot be performed The ADIS design uses the S ti stream to notify the Interlisp program that input has occurred Thus the program can request input from this stream bloc
28. e and the gray rectangle 12 The gray rectangle is used Gray rectangle is a psuedo rectangle that covers the entire screen and is filled uniformly with a pattern of ls and 0 s specified by the gray parameter a 16 bit constant The pattern is governed by the constant Simple cases 0 White throughout Black throughout General case The 16 bit constant is viewed as 4 4 bit bytes which define a 4x4 bit square pattern that is repeated throughout the entire screen The first byte is for the top scan line of the 4x4 square the second for the second etc For source types 0 and 4 the source RECTANGLE WIDTH and source RECTANGLE HEIGHT are ignored and a simple transfer between equally sized rectangles is performed For source types 0 4 the gray argument may be NIL for type 12 the altrectangle argument may be NIL ApisScroll rectangle deltay gray This command is a simplified form of ADisRegionOp It causes rectangle to be scrolled up or down by the amount deltay The information scrolled off the rectangle is lost The gap at the bottom or top of the rectangle will be set to the color specified by gray see discussion in ADISRegionOp Default gray white ApisFont n ResetSave Sets the font of the current region to n 0 lt n gt ADISNFONTS All characters subsequently displayed in the region will appear in the specified font The available fonts are either declared to Chat when it is initialized or read in with ADISReadFont
29. e left right bottom top array of pixel values region limits left right bottom top Caret The current position in a region may be illuminated with a blinking caret The facility is normally used to indicate where typed characters will next be displayed The caret is disabled by setting the blinking rate to zero Arguments 16x16 pattern to use for the caret blinking rate current position x y which will vary region limits left right bottom top Region data ADIS includes functions for reading and writing all state variables associated with regions These variables are flagged with asterisks in the descriptions above The region objects of ADIS are thus state transparent all state can be read and written DLISP uses this capability frequently Suppose that a function is using a region to place text and graphics on the display but that its computation is interrupted by a break point Such a break might occur because the user types a special interrupt character or because an error is detected The BREAK package is invoked which interacts with the user to repair the damage This package uses the display however and might disturb the display generation of the interrupted function To prevent this disturbance BREAK saves and restores the state of any regions it uses 8 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS Each output function makes changes directly in the frame buffer The image editing nature of
30. e problems During heavy output use only about 400 bytes second are transmitted over S go and S to combined Extensive use of the RasterOp function helps to keep this figure low Input traffic is of course lower still An event report even though it records the state of all input devices requires Only 9 bytes This message fits easily in a single packet and therefore requires no more network traffic that does a single character being typed 5 Conclusion Two aspects of the ADIS graphics package have been described in this paper The functions of the graphics package offer a small set of primitive ways to modify rapidly the contents of a frame buffer These primitives place on the application program the burden of managing screen updates However this burden is lessened by the ease with which the programmer understands and uses the frame buffer and by the power of the primitives especially RasterOp One of the rewards of this approach is that the application program can optimize its screen updating strategy thereby providing good interactive response A second interesting aspect of ADIS is its use of a packet switched network to link a timesharing computer and the display terminal The important point revealed by the design is that any trickiness in network graphics comes not from the presence of a network but because the system is implemented by several asynchronous processes Acknowledgements ADIS depends on superb workmanship contributed b
31. e that these operations are processed in the proper order is provided by the ADIS package Whenever ADIS sends commands over S go that might alter the interpretation of terminal text output it first sends a synchronization mark over both streams Over S to and thence to G o it sends a special character control C followed by a single digit this sequence is called SyncSto d where d is the digit used Via S go it sends a begin synchronization command followed by the same digit referred to as SyncBeginSgo d Then it sends via S go the commands that alter region parameters Finally it sends an end synchronization command over the S go stream followed by the single digit called SyncEndSgo d S to stream T1 T1 T1 Sync T2 T2 T2 char char char Sto char char char 4 S go stream P1 P1 P1 Sync char char char End Sgo 4 Figure 6 Sync markers tell when to switch interpretation from one stream to the other to assure processing of text and protocol in the proper order The graphical output process G o uses the synchronization marks to process commands from D g and text from G to in the proper order using the following algorithm see Figure 6 Initially 16 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS G o processes commands received on S go and text received from G to in arbitrary order Whenever a SyncSto d sequence is received from G to further processing of bytes from G to is delayed until a SyncEndSgo d c
32. eceipt of a line feed will cause all information within the region limits to be scrolled if the line feed would make new information lie partly below the region the entire region is scrolled up by the amount set by ADISSetLF if the line feed would not cause the new line to lie off the bottom scrolling moves information below the current position down and thereby opens up a new line If scrolling is enabled a small region will need to be cleared after scrolling the value of ClearColor same conventions as gray in ADISRegionOp above governs how the cleared area should appear If Scroll is set to EXPAND a line of text will be expanded before each new character is displayed in it This operation involves first translating all information to the right of the current position within the region to the right by the width of the character and then clearing out the small area opened up the small area will be just wide enough for the character After this horizontal scroll the new character will be displayed This feature allows simple text inserts in a line The arguments are individually defaulted numbers for CharDisplayOp and ClearColor are taken as new values non nil Scroll is taken as a new value ApisBackup x This function is designed to allow backspacing by erasing a character just displayed The argument x specifies how much to back up and is usually derived from a font width table in order to erase a character just disp
33. ed by ADIS is a cursor displayed on the screen to show the current position of the mouse An ADIS function is provided to specify a 16x16 pixel pattern that will be used as a cursor In addition two offsets are specified that identify the exact point within the pattern whose coordinates will be reported when an event occurs For example if the pattern is an arrow Figure 3 the offsets might identify the point of the arrow y offset x offset Figure 3 A cursor pattern and its reference point Graphical input is reported to the application program in the form of input events gen rated by explicit user actions This allows the application program running in a time sharing system to suspend execution awaiting the arrival of input An event is defined to occur whenever one of the 8 buttons makes a transition from up to down or down to up that has previously been enabled by the application program For each event the entire state of the input devices is recorded and 10 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS placed in an event queue that is later read by the application program The state includes 1 the x y location of the mouse cursor 2 current states of all 8 buttons 3 time since the most recent event 4 a specification of the button transition that caused the event Reporting the entire state reduces the likelihood that the application program will misinterpret an event Some input techniques require
34. est that the frame buffer raster scan display may substantially enrich the average interactive programming environment First a frame buffer display offers virtually limitless Opportunities in the range of graphical imagery that can be presented Second controlling such a display even with low level operations is straightforward and intuitive for programmers and designers of interactive systems Finally the displays will be increasingly economical as prices of memory and computing drop This paper describes a system that extends the interactive facilities of Interlisp Teitelman78 to make effective use of a display for communicating with the Lisp programmer The user s vicw of Disp the display oriented programmer s assistant is summarized in the first section of this paper and described in detail in Teitelman77a or Tcitelman77b The remaining two sections of the paper describe the design of ADIS the graphics package that supports DLISP The first shows how good interactive response can be provided in a modest raster scan system The pitfall that must be avoided is exhaustive scan conversion of the picture whenever a portion is altered The key idea used to address this problem is to provide mechanisms for copying and manipulating entire regions of a frame buffer at high speed The last section shows how a graphics terminal can be connected to an application program through a packet switched network without compromising the response of the
35. et to x y The optional parameter width controls the width of the line If width lt 0 the line will be exclusive or ed with the information already on the display this is can be used to erase an existing line AbisCurveSetup mode brushShape brush Width This function sets up to draw curves by tracing a brush over a trajectory Brush shapes are round 0 rectangular 1 horizontal bar 2 vertical bar 3 and diagonal bar 4 The brush size may be 1 2 4 8 or 16 screen units The mode is an operation in the sense of AbDISRegionOp explained below Defaults mode 1 brushShape 0 brush Width 1 ADISCurveTofx y dx dy ddx ddy dddx dddy n A curve is drawn from the current position to x y which becomes the current position The arguments specify finite differences as floating point numbers absolute value of all differences must be less than 1 dx is first order difference for x coordinate ddx second order and ddy third order Similarly for the y coordinate Chat will display the brush specified in ADISCurveSetup at the current position then compute a new position by applying the differences display the brush again etc The parameter n specifies the number of points along the curve that should be generated ADISData rectangle array Flush This function is available for writing arbitrary patterns into a rectangle of the display The rectangle argument specifies where the data should go Restriction X and WIDTH must be multiples o
36. ey action can be distinguished from two key depressions in rapid succession double clicking 3 ADIS The Graphics Package The ADIS graphics package provides a collection of Interlisp functions for controlling graphical Output and input Consistent with modern graphics system design the facilities for output and those for input are independent Newman74 This section provides an overview of the package more detail is presented in the appendix 3 1 Graphical Output The display image is represented in a frame buffer memory that records for each pixel on a 606 x 808 display whether the point should be white 0 or black 1 The output functions simply alter values in the memory in order to alter the image The frame buffer could be edited with only two very simple functions one to read the value of the pixel at a particular x y location and one RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS 5 to write the value Such an approach would be intolerably slow and the programmer would immediately begin to identify common editing operations and thereby create a larger set of functions that perform more specialized editing operations more efficiently Thus ADIS provides functions for drawing lines for adding text characters to the display and for moving existing portions of the display all coded to update the frame buffer as rapidly as possible These are common operations in the DLISP application which depends on the caref
37. f 16 The array is intrepreted as 16 bit numbers the first WIDTH 16 elements of the array correspond the to the top scan line of data the first word corresponds to the left most word of the top scan line ADISRegionOpf rectangle directive altrectangle gray This function causes a rectangular portion of the screen defined by the RECTANGLE record rectangle to be altered in one of several ways governed by the values of directive altrectangle and gray Roughly speaking directive tells what to do and altrectangle and or gray are arguments that together specify another rectangle the source rectangle The directive is itself the sum of two parts a number that specifies the operation to perform and a number that specifies the source type Thus directive operation source type The operations are 0 Replace The source rectangle is stored in the rectangle 1 Paint The source rectangle is or ed into the rectangle 26 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS 2 Invert The source rectangle is xor ed with rectangle and stored in rectangle 3 Erase The complement of the source rectangle is and ed with the rectangle The source type specifies how the source rectangle mentioned in the above list of operations is to be computed from the arguments 0 The source is the altrectangle on the screen 4 The source is the complement of the altrectangle on the screen 8 The source is the logical and of the altrectangl
38. from D g to G o to cause changes to the frame buffer does not require synchronization the flow of information is unidirectional The Interlisp portion of the graphics package D g actually saves a duplicate copy of most of the region data font italic bold scroll flag etc so that the Interlisp program may read this data without requiring communication with processes G o and G 12 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS A need to synchronize may arise if information input to D g depends on information recently output from D g This need arises because the interprocess communication paths have substantial buffering if D g completes output to G o and then reads some input from G i was the input actually generated by G i before or after the output was processed by G o For example suppose D g outputs a command to alter the timcout interval This command is transmitted to G o then to G i and the new timeout value is recorded in a table inside G i But the path from G i to D g may contain several buffered event descriptions as D g reads them it may wish to know which events were recorded before the change and which afterward Clearly this need can be met by propagating a special mark command around the loop D g to G o to G i to D g As D g reads input it will first find events recorded before the change then it will find the mark and finally it will find events recorded after the change Although such a synchronization p
39. he LISP program The devices used in this way are a mouse which has three keys and additionally steers the cursor a five finger keyset and 7 uninterpreted keys on the keyboard Normal typing on the keyboard is not noticed by these functions but is instead transmitted to MAXC to serve as teletype input to the LISP program The functions deal with a single notion of event described by a LISP record RECORD EVENT Y BUTTONCHANGES BUTTONS OTHERBUTTONS ELAPSED where X and Y give the cursor location at the time of the event BUTTONCHANGES is an 8 bit number that has bits on corresponding to the buttons that changed to cause the event 200Q is the left handset button 100Q the next etc down to 10Q the rightmost 4Q is the top or left mouse button 1Q the middle 2Q the bottom or right BUTTONS gives the state of the buttons after the change that caused the event a bit on implies that the button is depressed OTHERBUTTONS is the present state of the various spare buttons on the keyboard 200Q lock 100Q lieft shift 40Q ctrl 1OQ right shift 4Q B3 2Q Bl 1Q B2 ELAPSED gives the time that elapsed since the previous event units of 1 27 second maximum value is 255 AbisButtonEnable EventEnables TimerStartEnables TimerStopEnables TimerInterval Flush ResetSave This function governs subsequent interpretation of button pushes An enable is a 16 bit mask that defines whether action should be taken on down or up transitions of the
40. he cursor the second for the second etc The optional parameters x and y are the location of the point of the cursor within the 16x16 square lower left corner is 0 0 These numbers are used for two purposes 1 to arrange that the point of the new cursor will lie precisely where the point of the old cursor was at the instant the change was made and 2 to offset the x and Y coordinates reported in the EVENT record so that these coordinates refer the the location of the point of the cursor The optional parameter invertFlag if non NIL will cause the cursor bit pattern to be set from array but with the bits inverted ApbisCursorMove dx dy Flush Nudges the current cursor position by dx dy AbisSyne Flush This function completes a profocol exchange that ensures that Chat and the LISP program are synchronized In routine use this function is not needed However a user may interrupt the Interlisp program in the middle of transmitting a protocol sequence i e when it is inside one of the ADIS functions and some parts of a protocol message have already been transmitted over the graphics connection but others have not In this case subsequent protocol transmitted over the graphics connection might be misinterpreted ADISSync uses an out of band signal a MARK signal described in 5 to insure that the protocol parser in Chat is reset Hardcopy A function is provided for producing a hard copy of the image currently
41. ich buttons are enabled for events AbisInputA vailf This function returns T if some form of graphical input from Chat is waiting for the LISP job NIL otherwise Note that this function does not look for teletype input but rather for input events or timeouts ApisFlushInput This function discards all input waiting on the graphics connection INTERLISP DISPLAY PRIMITIVES 31 2 4 Miscellaneous functions AbisReadFont n name Flush This function tries to replace font n with a font of the given name read from the Chat disk Storage for the old font n if any is reclaimed ADISReadFont updates the data structure of available fonts ADISNFONTS appropriately The function returns T if the font appears to have been successfully read otherwise NIL AbIsFontWidths n array Flush This function interrogates Chat to discover the parameters of the font numbered n The function returns NIL if the Chat program has no such font Otherwise the array is filled with font widths in screen units and the function returns CONS height baseline where height is the spacing in scan lines between baselines of consecutive lines of text in this font and baseline is the height above the baseline of the tallest character AbisCursorfarray x y invertFlag Flush ResetSave This function sets the bit pattern in the cursor that follows the mouse position Array is an array of 16 16 bit numbers the first is the pattern for the top scan line of t
42. king if none is yet present and receive information about the next input Characters typed at the keyboard are sent over this stream just as for normal terminal input If a graphical event occurs two actions are taken a full event record is delivered over the S gi stream and a single control character is sent over the S ti stream Thus the Interlisp program waits for input on the S ti stream reads the control character and then extracts the event descriptor from the S gi stream This design is not arbitrarily chosen but is a rather delicate path through the turmoil of Tenex and Interlisp character handling facilities For example the event description is not transmitted on the S ti stream because the bytes in the description might be echoed It turns out that the processing of input characters in Tenex allows echoing of control characters to be selectively disabled so the single control character transmitted over the S ti stream to herald an event is not subject to echoing The design is also delicately fitted into Interlisp The DLISP program calls READ or one of its variants to gather input from the terminal this function begins reading bytes from the S ti stream When READ encounters the control character it invokes a READ macro which can contain a call to an arbitrary Interlisp function This function can read the event description from the S gi stream and then return to READ where more typed characters can be assembled This mechanism al
43. layed The height of the region erased is determined by parameters of the current font ApisCaret region rate array x y This function sets up a blinking caret at the current x y position of the region specified As the current x y position changes so will the location of the caret The blinking rate is determined by the rate parameter Rate is the amount of time in units of 1 27th seconds that elapses before the caret state is flipped i e 2 Rate is the time for a complete cycle Default rate 13 ADISCaret will disable the caret entirely The appearance of the caret is controlled by the three arguments array x and y Array is an array of 16 16 bit numbers that determine the caret image the first is the pattern for the top scan line of the cursor the second for the second and so on similar to AbtsData The optional parameters x and y determine where within the 16x16 square the current position is located lower left corner is 0 0 Defaults x 0 y 0 Only one caret is available putting it in a region will remove it from any previous region In retrospect it would have been better to allow each region to have a separate caret Displaying characters The appearance of characters displayed by Chat is controlled carefully by the 28 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS functions given above e g ADISFont ADiSPrintMode When a character arrives it is displayed at the current x y position of the region
44. longer be classed as events using the new parameters other input actions not queued and forgotten should have been recorded as events given the new parameters ADIS assumes that the application program will set these event parameters once at the beginning of a session so as to generate events of any sort that might be needed by the application Thereafter the application can discard unwanted events as they are extracted from the queue It is a necessary consequence of letting the user get ahead i e of having a queue that the criteria for causing an event be more general than the criteria imposed by the exact context of a specific event 4 Implementing ADIS A graphics package such as ADIS can be implemented in a number of ways On a stand alone computer with a frame buffer ADIS could simply be a subroutine package linked into the application program The DLISP application requires ADIS to be partitioned between two computers Interlisp is available only on a large time sharing system that supports dozens of users while display and input facilities are available only on several dozen personal Alto minicomputers Thacker79 The machines are linked by the Ethernet a packet switched network Metcalfe76 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS 11 4 1 Multiple Processes Before exploring the way in which ADIS uses the communications network let us examine the implications of executing portions of the DLISP ADIS system as separa
45. lows a user to interrupt typein to generate graphical events with the mouse This rather intricate design allows the Interlisp program to receive input from several input devices The control character is used to synchronize the two input streams it marks the S ti stream at each place that input should be extracted from the S gi stream Thus although character and graphical input are forced to flow over separate asynchronous streams the synchronization RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS 15 mechanism allows the Interlisp program to receive the inputs in exactly the same sequence they were generated Output Providing output requires synchronizing the S to and S go output streams Synchronization is needed because characters to be displayed flow on S to while S go carries information that affects the display of the characters font changes changes to the current position in the region etc Consider the following sequence of operations performed by D a l Text string Tl is sent to the terminal 2 Protocol Pl changes region parameter to style italic 3 Text string 12 is sent to the terminal Operations 1 and 3 transmit text characters over S to while operation 2 transmits protocol on S go a stream that is independent and asynchronous relative to S to However it is essential that the Operations be performed by G o in proper order 1 2 3 to achieve the desired effect The synchronization necessary to guarante
46. n of the image Unlike graphics packages for calligraphic displays these functions build no intermediate display file and retain no data structure that contains information about characters or lines ADIS provides only two structures the frame buffer which describes the image and the region objects which retain information that helps the output functions edit the frame buffer contents e g font descriptions Another concept used frequently in output operations is the combination function which describes how previously displayed information is to be treated A typical change to the frame buffer will compute the function F oldValue newValue where oldValue is the binary value of the pixel currently stored in the frame buffer and newValue is the binary value computed by the output function The value of the function F either 0 or 1 is stored into the frame buffer Only a few of the 16 possible binary functions are used heavily in graphics applications The functions provided are Copy new to old F old new Paint by adding black points 6 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS F old new old OR Erase by complement of paint F old new old AND NOT new Invert old image wherever new is present F old new old XOR new F oldnew old F old new NOT old F old new old AND new Fy old new new A function to draw a line for example will compute the coordinates of each point th
47. nts and then return the current state before the alteration induced by the arguments 3 called with a current state either a list or a number as argument they reset their state Functions with these properties are flagged ResetSave Note that the value returned by such a function is usually a pointer to an internal ADIS data structure If the calling program wishes to make any use of the value it should be copied 2 1 Initializing the Chat display ApisInit n Initializes a graphics connection to Chat for all graphics output If a connection already exists it is first closed cleanly and then re opened If the connection can be established the result of this function will be a LISP file that allows a program to perform I O to the connection otherwise the result will be NIL The parameter n specifies how many text lines at the bottom of the Chat display should be reserved for the initial teletype simulation area in which characters typed by MAXC over the teletype connection will be displayed Note that the size and location of this area can be altered with ADIS functions after the connection is initialized ApisCheck Checks the state of the display connection and closes it if something is found to be out of order ApisClosef Closes the connection cleanly and returns the entire Chat display to teletype simulation ADISOFILE The global variable ADISOFILE is set to the file returned by ADISInit or NIL if no
48. of RastecrOp can be found in Text The text output function is by far the most complicated as it must efficiently achieve a variety of cffects to support windows Each character of the output string is looked up in a font which records a small rectangular bit pattern for each character This pattern is used to modify the display in much the same way RasterOp does the combination function governs the display of the character Clipping to the region limits may discard entire characters or portions of characters After each character is displayed the current position is changed due either to the width of the character just displayed or to its formatting effect e g carriage returns tabs line feed A scrolling feature can be enabled to open up space between lines or space within a line when text is to be inserted The scrolling is achieved with RasterOp as described above Arguments for text output are text string current position x y region limits left right bottom top combination function style font bold italic formatting left margin for carriage return line spacing for line feed tab spacing scroll flag for vertical or horizontal scrolling pattern to clear scrolled area Pixels This call specifies values of pixels to store in a rectangular area of the screen This function is not used heavily it is used occasionally to produce complex patterns that the other functions cannot Arguments destination rectangl
49. ommand is received over S go Similarly whenever a SyncBeginSgo d command is received over S go further processing of bytes from S go is delayed until a SyncSto d sequence is received from G to This synchronization scheme is easy to implement and quite robust If processing of bytes from either stream S go or from G to is delayed for more than 10 seconds while waiting for a synchronization mark the wait is terminated and processing proceeds The only damage in this case is that some terminal text output will not be properly displayed The digit argument to the synchronization marks which is incremented modulo 10 for each new synchronization operation avoids global loss of synchronization due to program bugs or restarts that omit synchronization marks or operating system inserted text that garbles the SyncSto marks Discussion This section has described the modification of the simple view of Figure 4 in which Streams S go and S gi carry all traffic between the terminal and the application program to the more complicated view of Figure 5 in which four main streams are used S go S gi S to and S ti This arrangement was necessary because deficiencies in the Tenex operating system and Interlisp programming environment were too costly to remedy by invading these systems Instead ADIS was contorted to achieve the desired effect The contortions were necessary because the existing environment contained a view of terminals too thoroughl
50. ontext to Do What I Mean For 2 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS example DWIM will try to correct misspelled words in a function definition These facilities were originally designed for use on a teletype or character oriented terminal connected to the Tenex time sharing system Consequently they make use of typed input only and strive to limit the volume of typeout because of the slow speed of terminals INFONMAIL changed trace Documant a Look History r 3 Z Braak OHA LIONS pa Ean TYPESGRIEPT COM 08 MENUS et Window bits 4 10 SYS 143 64 MODE STPAR sats mode weord AGE wd tae E 1 72 100 re ISeload lt MASINTER gt GETMAILPOS FILE CREATED 23 JUN 77 2 11 GETMAILPO 8 11 MABINTER gt GE TMAILPOS 1 uh SET 0 1 wed POTEN Sndmsg Helpsys pyan from Mainiin 3 30PM MASSE oe J f Prattyprint 19 Mall from Burton 3 66PM Aefrash soruan if Want to see It how Yes Infarmait t oe Mali from Burton at bbn tenexd 3 56PM SEEN WINDOW oon oe Read selections oth 20 ris ORY COMMANDS 8 insert i s i ee F 7 7 p Redo Use Fix Undo 7 r Append E Replace Move e Move b NNN 9 3 JUN 77 15 SNOMSG Mall fram BBN REE Date 23 JUN 5 ewe From BURTON eS SETPAQEWIDTH To LAMBOA WIDTH ISYYS 143
51. ression destination F destination F source pattern where the destination combination function F ranges over F to F and the source function F over Fg to Fg The pattern is a pixel value computed by repeating a 4x4 pixel pattern over the entire screen It is very useful for filling rectangles with constant patterns such as black white or a gray half tone Arguments destination rectangle Ieft right bottom top source rectangle left right bottom top 4x4 pattern 16 bits combination functions Fy F region limits left right bottom top RasterOp is a very versatile function It allows the programmer to take advantage of imagery already displayed copying it to new places altering its appearance etc Choosing F F and F F the function will copy areas without alteration A scrolling operation uses the copy function to move a window s contents upward slightly the versatility of the RasterOp function RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS 7 makes scrolling down left or right equally easy The function can also be used to invert part of the screen changing images from black on white to white on black Fy F F Fg pattern all l s source and destination rectangles identical Inversion of this sort is useful for providing feedback to the user Figure 1 shows the word RSTRING highlighted in this way The reader is invited to ponder other combinations as well More information on the use
52. rimitive is technically easy to provide synchronization is almost never necessary We observed above that no information is returned to D g from G o so no synchronization of output is necessary The input process G i does return information to D g that depends on parameters sent from D g However as we remarked in discussing input facilities these parameters are best set only during initialization thus avoiding synchronization Although an implementation of a graphics system using several cooperating processes is not complex care must be taken to avoid needless synchronization difficulties Consider two implementations of the cursor function described above 1 D g transmits the 16x16 cursor pattern to G i via G o but saves values of the x and y offsets locally Whenever it extracts an input event from the queue being filled by G i it adds these offsets to the x y cursor position reported by G i 2 D g transmits the pattern and offsets to G i via G o Whenever G i detects an input event it adds the offsets to the mouse coordinates as it constructs an event description just prior to entering it in the queue Only the second method works properly because it intrinsically synchronizes the offsets and the cursor patterns Method 1 will not work because it may use offsets for a cursor shape that was not actually being displayed when the user struck the key that caused the event 4 2 Programming environments and operating systems If
53. s of the available Chat screen The coordinate system is SCREENXMIN SCREENYMIN at the lower left corner and SCREENXMAX SCREENYMAX at the upper right Since the Chat screen is fixed in size these values are actually 0 0 and 605 807 In the functions that follow a RECTANGLE is defined by RECORD RECTANGLE X Y WIDTH HEIGHT and defines a rectangular area of the screen X lt x lt X WIDTH 1l and Y lt y lt Y HEIGHT 1 ApbisRegion n ResetSave Sets the current region to n 0 lt n lt ADISNREGIONS It generates an error if no such region exists The global variable ADISNREGIONS is set by ADISInit ApisLimits rectangle or AbDISLimits I r b t ResetSave INTERLISP DISPLAY PRIMITIVES 25 Sets the limits of the current region to be the rectangle defined by the argument a RECTANGLE record The four argument version is an alternative for setting left right bottom or top limits individually if an argument is a number it is assumed to be a new value AbIssetX x AbisSetY y ADISSetX Y x y These functions set the current x y position of the current region ADISSetX and ApisSetY return their arguments AbisGetX Y This function returns CONS currentX currentY for the current region The function is quite slow as it must interrogate Chat for the current state of the region AbISsLineTo x y width Causes a line to be drawn in the current region from the current position to x y the current position is then s
54. smitting graphical input Only when demanded by the application program this guarantees that the program is in a suitable state for accepting graphical input Unfortunately many have taken an additional step as well the graphical input device is active only when the program is demanding graphical input e g Tektronix terminals and the application program must activate only one input device at a time Perhaps the cleanest way to solve all of these problems is to solicit assistance from the Operating system to insert processes between application programs and the terminal Figure 8 These processes provide to the application program a simple view of the terminal as streams of characters if that is the view expected by the programming environment but can communicate with the terminal using carefully formatted protocols These processes may connect with other processes that desire communication with the user e g uninvited text These processes become in 18 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS effect a definition of how the user s single terminal is to be used to communicate with many application processes Most operating systems contain such processes they are shown in Figure 5 as T o and T i but prevent their alteration or substitution by the user The ability to connect the terminal streams of an application program to user defined processes would greatly ease the introduction of new terminal types would simplify
55. te processes The system divides naturally into three processes Figure 4 The DLISP application program D a which calls the ADIS functions D g G o a process responsible for graphical output and G i a graphical input process Timesharing system Minicomputer Application ADIS graphics package Figure 4 A _ three process implementation D a application and D g graphics package run on the timesharing system G o graphical output and G i graphical input run on a personal minicomputer Communication from one process to another is achieved with streams These are FIFO queues of 8 bit bytes the sending process inserts bytes into the stream and may block execution if the queue is full the receiving process extracts bytes in the same order and may block if the queue becomes empty The processes control each other by means of a high level protocol Sproull78 encoded as a sequence of 8 bit bytes and transported via the streams ADIS uses a protocol format in which the first byte is a coded form of a command followed by argument bytes Thus D g might use 4 bytes to instruct G o to set the left edge of a region the first byte is a number that identifies the set left edge command the second identifies a region instance and the last two form a 16 bit integer to be used as the new left edge The majority of the communication among these processes is of a very simple sort that requires no synchronization For example communication
56. the appearance of the teletype region can be carefully controlled INTERLISP DISPLAY PRIMITIVES 29 Using the graphics connection Any normal ASCII characters transmitted over the graphics connection will be displayed in the current region In gencrally it is good practice to use the graphics connection for text output whenever feasible as it is less susceptible to interference from Interlisp or Tenex formatting operations ADISOFILE ADIsInit establishes a LISP file that can be used to direct output over the graphics connection ADISOFILE can be used as a file argument in any I nterlisp output function e g PRIN OUTFILE as the 05 argument to COPYBYTES etc Bear in mind that characters will not be displayed until appropriate buffers in MAXC are flushed see ADISFlush above ADISBOUT n This function transmits the 8 bit byte n over the ADISOFILE connection to Chat Programmers should be warned that if n gt 127 Chat will interpret the byte as a protocol command with disastrous results ADISPRINI Obj and ADISPRIN2 obj These functions are exactly like PRIN1 and PRIN2 except the characters will be transmitted over the graphics connection and appear on the Chat display The ADISPRIN functions are provided so that 1 if the LISP program is not running under Chat no error is generated and 2 for documentation in your program 2 3 Input functions Chat is capable of reporting interactions with various graphical input devices to t
57. these functions forces onto the application program the burden of managing screen updates Figure 2 shows an cxample of a window being moved in DLISP The motion is achieved in four steps first RI is copied to RI with the RasterOp function then R2 is similarly copied to R2 then the region limits are set to R3 and R3 is cleared to white using the RasterOp function then whatever graphical material thought to lie within R3 is re displayed i e calls to the Line Text and Pixel functions are performed to alter the frame buffer within R3 The fact that all graphical output is clipped to the region limits greatly aids the last operation Figure 2 A partially hidden window is moved There are other solutions to the update problem By adding considerable hardware to the frame buffer dynamic 2 1 2 D effects can be provided by the hardware This technique requires several translatable image planes and a color map to resolve priorities On a simple frame buffer with a single bit per pixel the update sequence can be managed by software e g see PICO Sproull75 but to do so requires maintaining a large data structure describing all potentially visible graphical objects In general updating the screen requires an expensive computation much like hidden surface elimination The sequence described above for moving a window is much faster but it works well only for a limited number of cases e g rectangular regions of the screen Thus
58. ul presentation of formatted text Other applications might stress other graphical objects Each output operation is directed at a particular region of the screen A region is identified primarily by a rectangular boundary defined by left right bottom and top edges All output requests are clipped so that modifications are made only within the region in this respect a region is very similar to a viewport For example region limits may be set to the boundaries of a DLISP window thus ensuring that text will not appear outside the window either because too many lines are displayed or because too many characters are placed on a given line In fact DLISP uses regions to operate within windows as well to construct the label banner to provide scroll bars at a window s left edge etc A region is not just a rectangular boundary but also contains substantial information about how graphical objects are to be displayed within the window A region is thus like an abstract object in an object oriented programming language e g Simula Birtwistle73 or Smalltalk Goldberg76 it contains data and routines for producing output on the display In the description of output functions given below information marked with an asterisk is data associated with an instance of a region This data can be examined and modified by ADIS functions Each output function is designed to make specific changes to the frame buffer which is the ultimate representatio
59. y embedded to change uninvited text Figure 7 Terminal and graphical traffic are carefully multiplexed on two streams RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS 17 The operating system and language environment can be designed to allow the two stream implementation of Figure 4 What is required is a view of terminals that allows multiplexing of normal character input output and protocol messages that are not subject to modification by the operating system Figure 7 The program needs several output facilities 1 the ability to output text characters 2 the ability to output byte sequences of arbitrary length that will not be altered by the operating system and 3 some mechanism for dealing with other traffic that is sent to the terminal by the operating system These uninvited text messages such as System going down in 30 minutes are lethal invasions of any formatted communication between a computer and a terminal This is a severe problem with terminals such as storage tubes the system message may be interpreted as graphical commands Facilities needed for input are 1 a mechanism to allow protocol messages to bypass the echoing machinery and 2 a design for character echoing that echoes characters as they are read by the application program This last facility is available in Tenex without it echoed characters cannot be placed carefully on the screen Users of systems that echo characters as they are typed e
60. y many researchers at the Xerox Palo Alto Research Center Especially helpful for this effort was the network software developed by Ed Taft and Dave Boggs and the work on the RasterOp function by Dan Ingalls and 20 RASTER GRAPHICS FOR INTERACTIVE PROGRAMMING ENVIRONMENTS the Learning Research Group Of absolutely critical importance were Warren Teitelman and his DLISP application it was Warren s ambition for a usable system rather than a mere demonstration that spurred me to analyze and solve the more thorny problems References Birtwistle73 G M Birtwistle et al Simula Begin Petrocelli Charter 1973 Boggs80 D R Boggs J F Shoch E A Taft and R M Metcalfe Pup An Internetwork Architecture to appear in IEEE Trans Comm Jan 1980 Cerf74 V G Cerf and R E Kahn A Protocol for Packet Network Interconnection IEEE Trans Comm COM 22 637 May 1974 Davidson77 J Davidson W Hathaway J Postel N Mimno R Thomas and D Walden The ARPAnet Telnet Protocol Its Purpose Principles Implementation and Impact on Host Operating System Design Proc Sth Data Comm Symp Sept 1977 Goldberg76 A Goldberg and A C Kay Smalltalk 72 Instruction Manual Xerox Palo Alto Research Center SSL 76 6 1976 Metcalfe76 R M Metcalfe and D R Boggs Ethernet Distributed Packet Switching for Local Computer Networks CACM 19 7 395 404 July 1976 Model79 M L Model Monitoring System Behavior in

Raster Graphics for Interactive Programming

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