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.

NAMING

   A number of different naming conventions are required to meet system
   and application programmer needs.

      The Display Pointer

         Nodes and branches in the system are named by assigning an
         integer or array location as an argument in the call used to
         create them.  The system places in these variables a number
         which points to the physical location of the branch or node
         position in the picture-subpicture data structure.  We call
         this name the DISPLAY POINTER.  As long as the user does not
         change the contents of these variables he can refer to
         particular nodes or branches in various subroutines by use of
         these integer variables as arguments.  In other words, to the
         user, the name of a picture or subpicture can be thought of as
         the variable used at the time of its creation.  Such a naming
         scheme is clearly required if pictures or subpictures are to be
         manipulated by the programmer.







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RFC 192          Some Factors which a Network Graphics      12 July 1971


      The Light Button Code

         Additional identification is useful to the application
         programmer in order to simplify his programming task.  A user
         has no control over the number assigned by the system to a
         Display Pointer.  There are situations in which the user would
         like to associate a particular known number with a branch.  One
         common example is in the use of "light buttons".  A light
         button is a displayed object that the user wants to be able to
         point at in order to command the controlling application
         program to do something.  A light button is commonly a string
         of characters forming an English word or words, but could be
         any picture.  When the user picks or hits the light button,
         information identifying the object must be transmitted to the
         timesharing application program.  The program must then branch
         to an appropriate statement or subroutine to perform the
         operations required to execute the command.  The Display
         Pointer uniquely identifies the object hit, but because its
         value is not under the programmers control, writing the code
         necessary to test it against the various Display Pointers
         considered legitimate to be hit at this point in the program is
         tedious.  If, however, the application programmer knew that at
         this point only objects with identification numbers 20-28 were
         legitimate to be hit, then testing to see that one was in this
         range and branching by use of a computed GOTO simplifies the
         programming of flow of control.  Often one does not need unique
         identification of an object, but wants to perform a certain
         action if any object in a class of objects is hit.

         The above need for identification is satisfied by allowing the
         application programmer the ability to assign a number, not
         necessarily unique, to a branch.  This number is called the
         Light Button Code.  This code can be used in any way the
         programmer desires, but is most commonly used, as its name
         implies, as a code identifying light buttons.  This number is
         sent to the application program along with the Display pointer
         of the object hit on the screen with a graphical input device.

      The Back Pointer

         We indicated earlier that it is required in interactive graphic
         programming to be able to associate application oriented data
         with picture and subpicture objects on the screen.  The data
         may be stored in many kinds of data structures depending on the
         nature of the application, examples being arrays, lists, trees,
         etc.  We meet the need by associating with each branch one word
         which could contain a pointer to the appropriate spot in the
         application data structure containing the data associated with



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         the branch.  We call this word the Back Pointer.  The
         application programmer can in fact store any code he desires in
         this word and use it in any way desired, but its common use as
         a pointer back into a data base in the application program
         dictated its name.

         For example, consider an application which would allow a
         chemical engineer to draw a chemical flow sheet on the screen
         and then input this flow sheet into a process calculation
         system.  There will be various symbol-pictures on the screen
         representing basic process units such as heat exchangers,
         mixers, columns, and so forth that can be copied and positioned
         on the screen.  These units will have to be connected together
         by streams.  The units and the streams will have names and data
         associated with them describing their contents and properties.
         Further, the node-branch structure. while visually indicating
         to the user what units are connected together and how, does not
         necessarily have the connecting information in a form easily
         handled by the application program.

         The continuity is best represented by a data structure using
         simple list processing in which each unit and stream has a
         block of cells associated with it containing data for it and
         pointers containing the connectivity information.  When a
         branch is created to position and display a unit, it will
         contain in the Back Pointer a pointer to the block of data
         associated with it.  The block of data will probably contain
         the Display Pointer for the associated branch so that one can
         go from the picture to the data block or from the data block to
         the picture.  For example, one may point at a unit for the
         purpose of deleting it.  Given the Back Pointer of the unit
         hit, one can find its associated block and return that block to
         free space.  One can then follow the appropriate chain of
         pointers to the blocks for the streams connected to the unit.
         In these blocks one has the Display Pointers for the branches
         displaying the stream and can then delete it from the node-
         branch structure, thus making it disappear from the screen.

         An additional form of name is to allow the programmer to store
         an alphanumeric string with each branch or node.  This form of
         name is not required for most applications, but can be useful
         with the picture editor.

         To review, each node and branch has associated with it a unique
         identifier named by the user and called the Display Pointer;
         its value is assigned by the system.  Each branch has two
         additional pieces of information which can be assigned to it by
         the programmer, called the Light Button Code and Back Pointer.



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         Given a Display Pointer for a branch, the programmer can obtain
         the Light Button Code or the Back Pointer for the branch.
         Given a Light Button Code or the Back Pointer, the programmer
         can obtain a Display Pointer for a branch with such a code.
         This display pointer may not be unique if several branches have
         the same Light Button Code or Back Pointer.  The above naming
         and identification inventions have proven to be easy to
         understand and yet completely general and easy to use.

COORDINATE SYSTEMS

   We now consider the question of a coordinate system within which to
   describe picture position.  The actual display generation hardware in
   a terminal has a fixed coordinate system (commonly 1024 by 1024 units
   on a fixed size screen with the origin 0,0 in the left hand corner or
   center on the screen).  Ultimately, the user wants to work on a
   virtual screen much larger than the hardware screen and wants to
   consider the hardware screen as a window that he can move around to
   view this virtual screen.  Further, pictures are to be capable of
   being constructed out of subpictures as in the example of Figures 1
   and 2.  To be able to accomplish the latter and allow future
   expansion to allow the former, the following coordinate system
   conventions are used.

   Each node has its own coordinate system.  When a node A is created,
   the picture-drawing CRT beam is assumed by the programmer to be at
   the origin of the node's coordinate system.  When a node is used
   within a node B by use of a branch, the positioning of node A is
   relative to the beam position in the coordinate system of node B.
   All nodes are positioned relative to each other by x, y positioners
   in the corresponding branches.  When a picture is actually to be
   displayed, one node is indicated to the system as the initial or
   Universe Node.  This initial node is positioned absolutely on the
   screen and all other nodes appear relative to this one as specified
   in the branches pointing to them.  This scheme is required to give
   the flexibility and generality required in the picture-subpicture
   tree.

   Logical Completeness of Operation Set

      Throughout the system design one should try to follow the
      philosophy of incorporating a logically complete and consistent
      set of operations.  In particular, for each call that sets a value
      there should be another call to fetch the value.  That is, for
      each operation there is an inverse operation whenever it is
      meaningful to have one.  We see a need for a basic system with the
      calls as primarily primitives.  One can incorporate calls that
      could be created by the programmer from other calls, when it is



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      felt that usage would warrant the expansion.  We would expect a
      library of higher level routines in the language.

      It is beyond the scope of these notes to go into all the calls
      required except to indicate a few basic ones.  For structure
      creation, one needs to be able to create a node or branch, delete
      a branch, add a new branch to a node at run time.

      One needs to be able to specify beam movements in nodes and place
      text in nodes with the normal write-format statements of the host
      programming language.  This latter point is very important for
      easy programming.

      One needs to be able to set and test parameters and convert one
      form of name into others.

      We discuss Attention handling in more detail because of its
      importance in making interactive programming easy.

   Attention Handling

      The user sitting at the console is operating in real time while
      the application program is operating in timesharing time.  At any
      point where the user may perform some operation at the console,
      the application program may not be running.  A mechanism must be
      created to communicate between the user and the application
      program.  The design of this mechanism is very important and must
      be carefully considered.  There are many different operations that
      one might want to provide the user at the console.  A basic
      mechanism is discussed which will allow others to be added in the
      future.  When the application program gets to a point where it is
      expecting input from the terminal, it issues a call and passes an
      array as an argument.  The Attention handling mechanism dismisses
      the program until an event is reported from the console.  The
      information passed back to the application is the type of event
      which occurred and other relevant information for that event.

      On refreshed displays a common input device is the light pen.  The
      light pen has a physical field of view of about a 1/8-1/4 inch
      circle.  The most common use of the light pen is to point at an
      object to be hit or picked.  The logical field of view seen by the
      user is a branch in the node-branch structure.  The picture drawn
      by the structure below the branch is blinked to give feedback to
      the user about what object he is going to hit or operate upon.
      The level in the structure at which the logical view is given can
      be set under program control or adjusted by the user from the
      keyboard.  When the user obtains feedback indicating the correct
      object is in view, he then presses a button on the light pen to



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      generate an Attention.  He is said to obtain a "structural bit" at
      a branch at the level in the node-branch structure set by the
      application program or by himself.  When the hit occurs,
      appropriate information is then entered into the Attention queue
      as described below.

      The other type of graphical input device commonly in use on both
      refreshed and non-refreshed displays, such as electronic pen-
      tablets, Joy Sticks, SRI Mouse, etc., produce x, y position
      information which is fedback to the screen as some sort of cursor,
      such as a dot or a cross.  It is difficult, if not impossible,