rfc192.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                          R. Watson
Request for Comments: 192                                        SRI-ARC
NIC: 7137                                                   12 July 1971


      Some Factors which a Network Graphics Protocol must Consider

   After reading some of the RFC's on a network graphics protocol it
   seems that many are not providing general enough mechanisms to handle
   attention handling, picture structure, and other higher level
   processes involved in interactive graphics.

   Therefore for what it is worth I am sending out these rough
   introductory notes which contain ideas that I think any network
   graphics protocol must come to grips with.

   The network graphics protocol should allow one to operate the most
   sophisticated system with more general data structures and concepts
   than those described in these notes and allow very simple systems to
   function also.

Introduction

   It is our contention that, if computer graphics is to be widely
   useful, the graphics terminals must be just another type of terminal
   on a timesharing system with minimal special privileges.  In these
   brief notes we outline the basic features which we feel must be
   available in a graphics support package to allow easy interactive
   graphics application programming.

   If one examines the types of tasks in industry, government and
   universities which can avail themselves of timesharing support from
   graphics consoles, one can estimate that the large majority can
   effectively utilize quite simple terminals such as those employing
   storage tubes.  I would estimate 75% of the required terminals to
   fall in this class.  Another 15-20% of terminals may require higher
   response and some simple realtime picture movement, thus requiring
   simple refresh displays.  The remainder of terminals are needed for
   high payout tasks requiring all the picture processing power one can
   make available.  In this talk we are not considering support for this
   latter class of applications.

MAIN ASSUMPTIONS AND REQUIREMENTS FOR SYSTEM DESIGN

   The main assumptions and requirements underlying the interactive
   graphics are the following:





Watson                                                          [Page 1]

RFC 192          Some Factors which a Network Graphics      12 July 1971


      1) The user of the graphics terminal should be just another
         timesharing system user.

      2) The graphics software support should interface to existing
         timesharing programs.

      3) The software support should allow technicians, engineers,
         scientist, and business analysts as well as professional
         programmers to easily create applications using a graphic
         terminal.

      4) The software support should easily allow use of new terminals
         and types of terminals as they come on the market.

      5) The software support should be expandable as experience
         indicates new facilities are required.

      6) The software support should be portable from one timesharing
         service to another.

      7) Some form of hardcopy should be available.

MULTILEVEL MODULAR APPROACH TO SYSTEM DESIGN

   If one wants to create as system which is easy to use by
   inexperienced programmers and ultimately non-programmers, one needs
   to provide powerful problem-oriented aids to program writing.  One
   has to start with the primitive machine language used to command the
   graphics system hardware and build upward.  The philosophy of design
   chosen is the one becoming more common in the computer industry,
   which is to design increasingly more powerful levels of programming
   support, each of which interfaces to its surrounding levels and
   builds on the lower levels.  It is important to try to design these
   levels more or less at the same time so that the experience with each
   will feed back on the designs of the others before they are frozen
   and difficult to change.

   One can recognize five basic levels:

      1) The basic system level:

         This level provides facilities for use of the terminal by the
         assembly language programmers.








Watson                                                          [Page 2]

RFC 192          Some Factors which a Network Graphics      12 July 1971


      2) The problem programming language level:

         This level of support provides powerful facilities for
         interactive graphics programming from the commonly used higher
         level programming languages.

      3) The picture editor or drawing system:

         This level of support allows pictures to be drawn and linkage
         to these pictures and application programs.

      Data management support for interactive programming:

         This level of support is to provide facilities to aid creation
         and manipulation of data structures relating data associated
         with the pictures and the application.

      5) The application program level:

A REVIEW OF TERMINAL HARDWARE CHARACTERISTICS OF CONCERN TO THE USERS

   There are two basic kinds of general purpose cathode ray tube display
   systems available on the present market.  Within each class there are
   alternate forms and techniques of implementation which we do not
   discuss here.  One type is called a "refresh display".  The other
   type is called a "storage tube display".  The refresh display must
   keep repainting the picture on the screen at rates of from 20-60
   times per second.  Commands which instruct the system how to draw the
   picture are stored in a memory.  The storage tube display on the
   other hand, through its internal method of construction can maintain
   on the face of the display a picture for practical purposes,
   indefinitely once drawn.

REFRESHED DISPLAYS

   There are limits to how much information can be drawn on the face of
   refreshed display before the time required to paint it forces the
   refresh rate below a critical value and the picture appears to
   flicker.  This quantity of information is a function of the type of
   phosphor on the tube face, the speed of display system in drawing
   lines and characters, and the ambient light level in the room.
   Refresh display systems range in cost upwards from $10,000 to several
   hundred thousand dollars.  Refresh displays, because the picture can
   be changed every few milliseconds by simply altering its command list
   (often called a display file or display buffer), allow the picture
   parts to be moved on the face of the screen either under operator
   control or computer control.  Objects on the screen can be
   selectively erased without affecting other objects on the screen.



Watson                                                          [Page 3]

RFC 192          Some Factors which a Network Graphics      12 July 1971


   These characteristics make refreshed displays suitable for a wide
   range of applications.

STORAGE TUBE DISPLAYS

   Storage tube based displays can display a large amount of information
   without a flicker, and generally cost under $20,000.  Present systems
   suffer from some limitations, however.  They cannot be selectively
   erased.  If an object is to be moved or deleted from the screen, the
   entire screen must be erased and then the new picture can be redrawn.
   Because this type of display generally operates over a communication
   line, the speed of the line may seriously restrict the amount of
   interaction if much erasing and redrawing is required.  The graphics
   software concepts to be described can be used with both a storage
   tube and refreshed display, although some features are only
   appropriate to the refreshed type of display.  The important point is
   that new storage tube technologies insure that this class of terminal
   will be with us a long time.

INPUT DEVICES

   It is necessary to allow a console user to communicate with the
   graphics system.  This is done through a keyboard and through
   specialized graphic input devices, the Light Pen, the Tablet, the SRI
   "Mouse", and the "Joy Stick".  These latter devices enable a console
   user to point to vectors and characters displayed on the CRT and to
   input position information to the graphics system.

   Comparison of the Graphics Input Devices -- Analog Comparitors

      The Joy Stick, Mouse, and Tablet are similar in that they both
      generate a two dimensional position address without the aid of the
      display processor, but cannot be directly used to identify
      displayed objects.  The light pen-display processor hardware
      combination and its associated software, on the other hand, can
      easily sense and identify displayed vectors and characters but
      does not generate directly any position data.  A "tracking cross"
      program is used to obtain the position data for the light pen.  To
      obtain the pointing capability for the Joy Stick, Mouse, and
      Tablet, we can use a pair of analog comparitors which generate
      interrupts when the beam is drawn on the CRT lies within a
      rectangular "viewing window" in much the same way that the light









Watson                                                          [Page 4]

RFC 192          Some Factors which a Network Graphics      12 July 1971


      pen generates interrupts when a beam is drawn under its circular
      viewing area.  These comparitors sense the x and y axis drive
      voltages of the display analog bus.

      A comparator will generate an output signal when the drive voltage
      is between two limits which may be set using special display
      processor commands.  When both comparitors generate a signal
      simultaneously, the output voltages on the analog buss correspond
      to a beam position within the rectangular viewing window.  The
      position of viewing window is set based on the position of the
      pen, Mouse, or Joy Stick.

      We can also use software to simulate the effect of hardware
      comparators.  Hardware comparators cannot be use with storage tube
      displays and, therefore, a software simulation is required.  This
      simulation is discussed later in these notes.

      The light pen can be used only with a refreshed display.  The
      other types of devices can be used with present storage tube
      displays and refreshed displays.  They are used with storage tube
      displays which have hardware which produces on the screen a dot,
      cross or other cursor, indicating the x, y position of the device.
      The reason one can move this cursor around it that the cursor is
      created using special techniques to avoid its storing on the
      screen.

USER SOFTWARE REQUIREMENTS

   The user requirements on a timesharing system based interactive
   graphics system are the following:

      1) The user should have available a language for creating a
         computer representation of the picture to be displayed.  This
         language should allow more complex pictures to be built up from
         simpler structures.

      2) The computer representation of the picture must allow easy