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NETWORK WORKING GROUP                                       J. McConnellRequest for Comment: 177           Ames Research Center Moffet Field, CAObsoletes: none                                             15 June 1971Updates:125NIC: 7102           A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION   As more nodes are connected to the ARPA network, the types of   graphical display processors available to users is quite varied.  To   attempt to facilitate the transmission of graphical information over   the network, a device independent description of a display is   described.  The using host may make any conversions necessary to   realize the picture on a specific device.  It is also possible to   interpose a form machine as proposed by Heafner and Harslem in RFC   #94.  Some of the items and entities described herein were first   discussed by Steve Crocker in RFC #86.  It is intended that this   description is to be oriented to a refresh display with point,   vector, and character drawing capability, these types of devices   include the IBM 2250, IMLAC PDS-1, the DEC 338, and DEC 340, as well   as the Evans and Sutherland LDS-1.  However, direct video storage   tube devices represented by the ARDS, Tektronix and Computer devices   can also serve as clumsy interactive devices.  Hard copy devices,   such as microfilm or plotters, can also be used at the using host's   discretion.   There are several items and constructs which will be defined before   discussing the specifics of the description:      1.  The network standard graphics description stream (NGDS)      contains the description, data, and operators necessary to effect      a display.      2.  The network standard stream interpreter (NGSI) parses the NGDS      into its components.      3.   The network standard display list (NGDL) is the basic entity      which, when executed, causes graphical information to be      displayed.      4.  The network standard list interpreter (NGLI) is the entity      which executes the NGDL and controls the beam movement to effect      generation of graphical information.      5.  The network standard screen (NGS) is the entity on which the      information is to be displayed.  It may be divided into image      areas to be defined later.McConnell                                                       [Page 1]

RFC 177    A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION  June 1971      6.  The network standard graphics list pointer (NGLP) and the      network standard graphics execution stack (NGES) are entities      which describe the state of the NGLI at any given time and provide      the facilities to allow the NGLI to be a recursive interpreter.   Figure 1. summarizes the preceding discussion graphically, which is   appropriate.   The network standard graphics screen is rectangular and positions on   it are specified by an ordered pair of fractions representing the   horizontal distance from the left edge and the vertical distance from   the bottom edges respectively.  These shall be termed the (x,y)   coordinates as is standard.  There is no specification of resolution,   so that the quality of the picture will degrade with the decreasing   resolution of a poor display device, or be improved if it is   displayed on a higher resolution device.  Coordinate values are   specified by a 16 bit unsigned fractions with the binary point to the   left of the most significant bit - this provides a normalized screen   with coordinates between 0.0 and .999...9.   The NGS may be subdivided into rectangular image areas with   possibility unique displays in each space. An image area has three   attributes:  a 16 bit integer name, an x,y pair specifying the lower   left corner and an x,y pair to specify the upper right corner of the   area.  Image spaces may overlap, but may not be completely contained   within each other.  The main image space has the name 0 and is   coincident with the NGS.  The use of image spaces allows for   manipulation of part of the NGS's contents, as well as redefining the   coordinate space. Within an image area, coordinate values are   fractional displacements from the lower left hand corner of the image   space.  Thus an image area defined from (0.0, 0.0) to (0.5, 0.5)   would contain an image one-fourth of the size of what it would be on   the full NGS.   Character strings may be displayed at arbitrary points in an image   area.  Because many display devices have hardware character   generators capable of producing one, or a few sizes, character   scaling within an image area will not be expected.  Characters shall   be assumed to be .014 screen width wide, and .025 screen height high   including spacing.  This gives a screen capacity of 72 characters and   40 lines.  When the beam is moved to be a screen position prior to   drawing a character, it is assumed to be in the center of the   rectangle defining the character space.  The beam position after   drawing a character, or a string of characters, is undefined.   The format of the NGDS can now be specified.  The NGSI parses the   NGDS into commands.  The commands are in a prefix format with an   eight bit command followed by the necessary parameters.  TheirMcConnell                                                       [Page 2]

RFC 177    A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION  June 1971   descriptions follow the numerical order of the command codes.   ERASE (commands = 0) consists of a command byte and no parameters.   Its effect is to erase the full NGS.   DEFINE LIST (commands = 1) has three parameters.  A 16 bit integer   for the list name is first.  If the name has already been used, then   this command redefines the list.  The count of the items in the list   is also a 16 bit integer defining the length of the list in items.   The list items follow the count terminating with an end list item.   The items themselves are used to control the NGLI and the movement of   the beam when drawing pictures.  They are also in prefix format, with   an eight bit item code followed by the necessary parameters.  All   names are 16 bit integers, coordinate data are 16 bit unsigned   fractions as previously described.  Any other parameters will have   their type and size specified in individual descriptions.      a.  OPEN IMAGE AREA (name) (item type = 0) instructs the NGLI that      list commands which follow are to be adjusted to be contained      within the named area.  A nonexistent area name is treated as a      NOP.  If no image area is specified for the NGLI the default area      is area 0, the NGS.  This directive remains in effect until      another type 0 item is encountered by the NGLI.      b.  RESET IMAGE AREA (name) (item type = 1) causes the NGLI to      move the beam to the lower left corner of the image area.  A      nonexistent name is again treated as a NOP.      c.  CLEAR IMAGE AREA (name) (item type = 2) causes the NGLI to      erase all graphical information in the specified area, and then to      perform a reset command.   The following items are display items which actually cause beam   movement by the NGLI.  Coordinate data are relative to the origin of   the current image area and are taken as fractional displacements   within this area.      d.  MOVE BEAM (x,y) (item type = 3) causes the NGLI to move the      beam to (x,y) relative to the current origin.      e.  DISPLAY CURRENT POSITION (item type = 4) causes the NGLI to      display the current point on the NGS.McConnell                                                       [Page 3]

RFC 177    A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION  June 1971      f.  DISPLAY VECTOR (x,y) (item type = 5) causes a vector to be      drawn from the current point to (x,y).  After the operation, (x,y)      becomes the current point.      g.  DISPLAY CHARACTERS (n, 'C1, C2...Cn') (item type = 6) causes      the n characters to be displayed starting from the current point.      N is the character count and is an eight bit byte.   The character set recognized by the NGLI is composed of eight bit   bytes interpreted in the following ways.  Any character with the most   significant bit a zero is interpreted as an ASCII character.  Some   non-graphic ASCII characters may have special functions within a   string:      1.  Line feed (LF) move the beam one line spacing downward.      2.  Carriage return (CR) move the beam to the left most character      position in the current line.      3.  Backspace (BS) move the beam one character position to the      left in the current line.      4.  Tab move the beam a predetermined number of character      positions to the right in the current line.  The number will      follow the tab character in the string, but may not exceed the      capacity of the current line.      5.  Vertical tab (VT) move the beam down a predetermined number of      line spacings.  The number of spacings follow the VT character and      may not exceed the line capacity of the screen.  This is expanded      as multiple line feeds.      6.  Any other non-graphic character - could cause a space      character to appear in its place on the display, or it may be used      for any purpose two processes choose.   Characters with the most significant bit a one are used for an   extended character set where a device can support this.  These may   also be used as an implicit stroke table, calls being the names of   special graphic lists that may be defined in the NGDS.  If no eight   bit list name exists, then the character is a space.      h.  EXECUTE LIST (name, x, y) (item type =7) causes the NGLI to      suspend interpretation of the list and interpret the named list.      It is equivalent to a subroutine call.  The (x,y) pair specifies      the origin of the new list relative to the current origin.  The      actions taken by the NGLI are specified in the next sectionMcConnell                                                       [Page 4]

RFC 177    A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION  June 1971      i.  COPY LIST (name, x,y) (item type = 8) cause the items      contained in the named list to be copied into the place of the      item with the origin of the list at (x,y) relative to the current      origin.  This is analogous to item (h) as a subroutine is to a      macro.      j.  EXECUTE TABLE (name, mode, length) (item = 9) causes the NGLI      to treat the named list as a special entity.  It is constrained to      contain only coordinate pairs.  It may be executed in either point      or vector mode as specified by the mode parameter 1= vector, 0=      point.  The length of the list is specified by a 16 bit integer      parameter.      k.  END LIST (item type = 10) has no parameters and informs the      NGLI that the end of a list has been reached.      l.  TABLE (n) (item type = 11) specifies that there are (x,y)      pairs in this list.  N is a 16 bit integer.      m.  Set vector mode (mode) (item type = 12)      n.  Set character mode (mode) (item type 13)   In addition to the above display items, some items exist which   control the display mode of the above where this is applicable.   There are two mode items, the one governing vector modes, and one for   character modes.  The quantities specified include brightness,   blinking, color selection, and mode dependent descriptions.   For vector modes, the texture of vectors such as dashed, solid or,   dotted can be specified.  For characters, size, and orientation can   be additionally specified.  The scope of a mode item extends to the   next encountered mode item of the same type.  Mode specifications   could have been included in the display items at the expense of more   parameters and in possibly more data to be transmitted.  The mode   specification consists of two eight bit bytes.  The bytes have the   following organization:McConnell                                                       [Page 5]

RFC 177    A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION  June 1971   1    4         3            2                     2    3 +-----------------------+  +-----------------+   +-------------------+ |    |    |             |  |     |           |   |     |     |       | |  B | BL |     CLR     |  | TX  |           |   | OR  |  SZ |       | |    |    |             |  |     |           |   |     |     |       | +-----------------------+  +-----------------+   +-------------------+ BYTEO - both modes         BYTE 1- vector        BYTE 1- Character mode                            mode Where      B     Blink if set            BL    Brightness level - 0 = invisible, IIII brightness            CLR   Color          One bit for each primary            TX    Texture        OO = solid                                 01 = dashed                                 10 = dotted                                 11 = dot - dash           OR     Orientation binary value x 90deg           SZ     Size   If the device will ot support them, or provide subroutines to perform   these enhanced functions, they are treated as NOPs.   Thus the NGDL is a set of named lists.      DEFINE IMAGE AREA (command = 2) has three parameters a 16 bit area      name; an (x,y) pair defining the lower left corner of the area and      an (x,y) pair defining the upper right corner.  If the name has      already been used to define an area then this command serves a      re-definition of that area.  If the two (x,y) information      displayed in it.  If X1 is greater than Xr or Y1 is greater than      Yr then the definition is treated as NOP.      The following commands are included to avoid retransmission of a      list when some minor changes to it are necessary.  All instances      of a list, where an instance is a command to execute the list will      be effected.  Where a list has been copied into another list, no      change will take place in the copied list.McConnell                                                       [Page 6]

RFC 177    A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION  June 1971      TRANSLATE LIST (command = 3) provides for translation of all      instances of a list within all of the image spaces it occurs in.      Parameters are the 16 bit list name, and two 16 fractions for the      coordinate values of the translation.      SCALE LIST (command = 4) can change the size of the picture      created by a list in all image spaces.  The parameters are the 16      bit list name followed by two 16 bit (X,Y) unsigned fractions with      eight bits of exponent followed by eight bits of mantissa.  These      alter the scale factors in the X and Y directions.      ROTATE LIST (command = 5) will rotate all instances of a list.      The parameters are for the 16 bit list name, and a 16 bit value      corresponding to rotational angle in degrees with the sense being      counter-clockwise for increasing values and the possible axis      being zero degrees.   The NGLI executes in a loop, taking items from the main list.  When   the end of the main list is reached, the NGLI returns to the top of   it.  At any point, the NGLP gives the NGLI the positional   displacement of the next display item it is processing.  When a   display item of the type h. is encountered, the current lists's   execution is suspended,  the name of the list, the current origin,   and the NGLP are saved in the NGES.  The NGLP is re-set to zero, the   next current origin is calculated, and execution of the new list   commences.  When the end of the list is encountered, the old values   are restored from the NGES and execution of the suspended list   continues.  The NGLI is then a recursive interpreter.  Whenever a new   image area is opened, the name of that image area is stored in the   current image area name.   The problem of interacting with the displayed picture has yet to be   addressed since this is a more complicated area.  Interaction may   occur in two fashions:  the first, and the easiest to handle, are   those kinds of events which are separate from the picture itself.   This encompasses key boards and function key types of devices.  These   can be handled as standard messages from the graphics device to the   serving host.  The second class of interactions is with the picture   itself.  This is more difficult because of the problem of associating   the point selected with some meaningful entity such as a list.  This   association can probably only be made by the serving host since the   using host, or a Form Machine, may have transformed the NGDS in   unknown ways, and the NGDS may no longer exist at the using host's   site.   There are essentially two classes of devices that can interact with   the picture.  The first class is synchronous devices, ones who only   cause some attention to occur while a given display item is beingMcConnell                                                       [Page 7]

RFC 177    A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION  June 1971   executed.  Light pens are synchronous devices.  Asynchronous devices   are those which can interact regardless of which, or even when no,   display item is being executed.  Asynchronous devices make   associations more difficult since no relationship can be easily   inferred as they can for synchronous devices.   The NGES is created for the interaction with the picture.  For   synchronous devices the current values for the list name, origin, and   NGLP, as well as the contents of the NGES provides a hierarchical   structure where associations can be made.  For asynchronous devices,   the structure is probably not as necessary as the name and   coordinates within the image space in which the interaction occurred.   It will be necessary for the using host to understand which type of   devices are available and to supply proper type of interaction   information to the serving host.   The form of the interaction information for synchronous devices   follows:INTERACTION (type, cia, cln, cor, clp, n, l1, or1, lp1, ...ln, orn, lpn)   Where type describes the interaction type 0=synchronous   cia     current image area name   cln     current list name   cor     current origin value   clp     current NGLP value   n       number of entries in NGES, higher value of n is older entry   li      list name of the i th entry   ori     origin of the i th entry   lpi     NGLP of the i th entry   The form for asynchronous i.. teraction is:   INTERACTION (type, cia, iax, iay, sx, sy)   Where type = 1 for asynchronous interactions   cia     current image name   iax     image area x coordinate   iay     image area y coordinate   sx,sy   screen x,y coordinatesMcConnell                                                       [Page 8]

RFC 177    A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION  June 1971NETWORK  |    Using Host         |                +-------+         |                |       |         |                | NGDL  |          +--------+       +-------+         +--------+       |       |--------->|        |       |       |         |        |------>|       |          |  NGLI  |------>|  NGS  |         |  NGDS  |       +-------+          |        |       |       |         |        |       +-------+          |        |       |       |         |        |------>|       |--------->|        |       +-------+         +--------+       |IMAGE  |          +--------+         |                |AREA   |              ^         |                |DEFINI-|              |         |                |TIONS  |              V         |                +-------+          *--------+         |                                   |  NGLES |         |                                   |    &   |         |                                   |  NGLP  |         |                                   +--------+        [This RFC was put into machine readable form for entry]    [into the online RFC archives by Kelly Tardif, Viag�nie 12/1999]McConnell                                                       [Page 9]