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.

         identification of picture parts when pointed at or "picked" or
         "hit" with graphical input devices such as light pen,
         electronic pen-tablet, Joy Stick, SRI mouse, or other supplying
         x, y information.

      3) The computer representation of the picture must allow linking
         of picture parts with data about these parts appropriate to the
         application using the terminal.  There should be an appropriate
         data management system for use with interactive application
         programming.




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RFC 192          Some Factors which a Network Graphics      12 July 1971


      4) There must be some way of communicating events taking place at
         the terminal in real-time, such as picking objects with the
         light pen, with the application program running in the
         timesharing system.

      5) The user should be able to save and restore pictures from one
         console session to the next.

      6) If possible, the user should be able to use the display as a
         stand-alone terminal or in conjunction with a teletype or other
         typewriter terminal.

      7) The user should be able to do some graphic programming by
         drawing directly at the console.

   The choice of an appropriate data structure for picture
   representation simplifies the handling of requirements one to five.
   It is this data structure that we consider now in more detail.

Picture-Related Structures

   If a picture displayed on the console had meaning only in the
   physical position of its lines and characters, the system would be
   little more effective than an easily erased piece of paper.  To
   significantly enhance the capabilities of the system, we must be able
   to express relations between displayed entities.  A line is much more
   than just a line when it represents a boundary or a part of some more
   complex unit.  Such units in turn may be related in a similar way to
   higher level units.  Furthermore, we may wish to create picture
   elements that may be used repeatedly so that a change in the one
   master copy will be reflected in every use of that copy.

   To illustrate the usefulness of this picture-subpicture relationship,
   we shall consider the three houses of Figure 1.  While the two types
   of houses differ in appearance, it is obvious that they have picture
   elements that could be drawn by a designer of prefabricated houses
   and that the designer wished to incorporate a new standard window
   unit into all houses.  The use of conventional pencil and paper
   techniques would require that he redraw or overlay each window on his
   diagram to reflect the changed component.  If the window were,
   instead, drawn by the graphics system within a common subroutine,
   only that one master copy would have to be modified in order to
   change the appearance of every reference to that kind of window on
   the diagram.







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RFC 192          Some Factors which a Network Graphics      12 July 1971


Nodes and Branches

   To facilitate the discussion we will introduce the terms "node" and
   "branch".  A node is a form of picture subroutine that may cause the
   display of lines and characters and may also call other nodes.  The
   subroutine call is called a "branch".  Nodes may also be thought of
   as representing PICTURES or SUBPICTURES and the branches to these
   nodes as uses or instances of these subpictures.

Directed Graph Structure

   The nodes and branches form a directed graph.  The branches contain
   positioning information indicating the beam location to be used by
   the called node.  This location is relative to the position of the
   node in which the branch is made.  This use of relative beam
   positions allows the user of the system to create subroutine
   structures that make multiple branches to common nodes.  Branches may
   also set other display parameters such as intensity and character
   size.  A subroutine calling structure appropriate to the requirements
   of our hypothetical designer is shown schematically in Figure 2.
   Nodes are shown as circles and branches are shown as connecting
   lines.  The picture of the house is composed of wall unit and roof
   SUBPICTURES.  The wall unit is in turn composed of subpictures.

Node and Branch Display Parameters

   Branches may contain the setting of parameters which will be in
   effect when the called node is executed.  The parameters which may be
   set are the beam position to be used (relative to the current beam
   position, i.e., a displacement value), intensity, character size,
   line type, visibility, (the display of vectors and characters may be
   suppressed), "hitablility" (whether or not vectors and text may be
   "viewed" by devices such as the light pen), and blinking.

   Coding within nodes may modify only the parameters controlling
   position, intensity, character size, and line type to be used by
   subsequent display coding or branches.  It is not necessary that a
   node or branch specify every parameter.  For those parameters other
   than position, the system allows a "don't care" option; the parameter
   setting in effect when the node or branch is executed will be
   retained and used in this case.










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RFC 192          Some Factors which a Network Graphics      12 July 1971


Identification of Graphic Entities with Graphic Input Devices

      Structural Hits

         A console operator or application program may modify, add, or
         delete branches to any of the nodes as well as add new nodes.
         To allow a console operator to manipulate any branch in such a
         structure, we have implemented a "structural hit
         identification" scheme.  To illustrate the following
         discussion, we refer the reader to Figures 1 and 2.

         A viewing device, such as a light pen, can respond only to the
         individual vectors or characters displayed on the screen.  At
         the time a vector is drawn under the viewing area of the light
         pen, an interrupt is generated and, if enabled, will be sent to
         the central computer.  Even though the same node is used to
         display the eight windows in the diagram of Figure 1, we can
         tell which window and house is being pointed to by examining
         the sequence of branches taken to arrive at the window
         displayed at the time of interrupt.  If the console user points
         to the right hand window of the middle house of Figure 1
         (marked with an asterisk *) an examination of the subroutine
         return addresses in the push down stack would show that the
         current "window" node had been arrived at via the dotted line
         path shown on the network of Figure 2.

         There remains the question "Are we pointing at a window, at a
         wall, at the house, or at all three houses?"  The location of
         this structural hit depends on how many branches are counted in
         examination of the return addresses before one stops to
         consider to which branch that return jump points.  This is
         analogous to counting a fixed number of levels from the ends of
         the graph structure.  This number of jumps is set using
         reserved keys on the keyboard, one incrementing and the other
         decrementing the limit.  By manipulating these keys and
         pointing to various displayed objects with the light pen, it is
         possible to point to any branch in the network of subroutine
         calls.

         All information concerning the path in the node-branch network
         taken to arrive at any displayable coding is contained in a
         push down stack.  Return jumps are stored in the stack by the
         subroutine calls to nodes.  These jumps when executed will
         return the processor to the next instruction after the call.

         A greatly simplified version of the display coding used to
         generate the picture and tree of Figures 1 and 2 is shown in
         Figure 3.  The labels a through d on the diagram represent the



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RFC 192          Some Factors which a Network Graphics      12 July 1971


         address of the subroutine calls which cause the display of the
         subpicture hit by the viewing device -- in this case the right
         hand window of the second house.  The returns from the called
         subroutines are stored in the push down stack as jumps to the
         location following the calls.  The routine RETURN would merely
         execute POP instructions which ultimately will cause the
         execution of a jump instruction previously placed in the stack
         by the calling branch, thus returning control to the calling
         routine.  The stack is shown in the condition at the time of
         the hit on the right hand window of the middle house.  Note
         that by counting 3 jumps upward (downward in the diagram) in
         the memory containing the stack, we will arrive at the jump
         pointing to a structural hit at (b) in Figure 3, the call to
         model 120.

      Console Operator Feedback

         The console operator must be informed of where he is pointing
         in the network of nodes and branches.  This is accomplished by
         flashing all displayable coding below the structurally hit
         branch when a vector or character is viewed.  This flashing is
         a doubling of the intensity at 2 to 8 cycles per second.  In
         addition, a list of the names of all nodes and branches taken
         to arrive at the vector or character viewed is displayed in a
         corner of the screen.  The name of the branch selected is
         intensified somewhat brighter than the other names.

      Generating an Attention

         After the operator has confirmed the correctness of his choice,
         he need only terminate the view in order to generate an
         attention on the desired branch.  This is done by releasing the
         button on the light pen or lifting the pen from the Tablet.  A
         button on the mouse will perform the same function.  If the
         structural hit is not correct then the operator could move the
         viewing device to a new area.

         A termination of the view on a blank area of the screen will
         result in the generation of a "null" attention.  This attention
         returns only position data; no structural data is generated.
         The significance of this attention is determined by the
         application program.

         The above discussion assumed a refreshed display and use of a
         light pen, but it greatly simplifies interactive graphics
         programming if the above concepts can be implemented no matter
         what type of display or graphical input device is being used.
         This in fact can be accomplished as discussed later.



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RFC 192          Some Factors which a Network Graphics      12 July 1971


THE GRAPHICS LANGUAGE

   For the purpose of discussion we assume that the graphics language
   statements are a set of subroutine calls, although a more
   sophisticated syntax could be imbedded in the host programming
   language.  The statements required are:

      1) Subroutine calls for creation and manipulation of the picture-
         subpicture data structure.

      2) Subroutine calls to generate displayed pictures and picture
         parts such as lines and characters.

      3) Subroutine calls to input information about events or
         "attentions" occurring in real time at the console.

      4) Subroutine calls to manipulate picture parameters such as line
         type, (solid, dashed, dotted, etc.), brightness, character
         size, and so forth.

      5) Subroutine calls to perform utility functions such as saving
         and restoring pictures from disk files, initiating the display
         and so forth.