NETWORK WORKING GROUP J. McConnell
Request for Comment: 177 Ames Research Center Moffet Field, CA
Obsoletes: none 15 June 1971
Updates: 125
NIC: 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.
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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. Their
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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.
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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 section
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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:
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RFC 177 A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION June 1971
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.
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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 being
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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:
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 coordinates
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RFC 177 A DEVICE INDEPENDENT GRAPHICAL DISPLAY DESCRIPTION June 1971
