





		   Compound Text Encoding
			Version 1.1
		   X Consortium Standard
		 X Version 11, Release 6.8
		    Robert W. Scheifler



Copyright (C) 1989 by X Consortium

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1.  Overview

Compound Text is a format for multiple character set data,
such as multi-lingual text.  The format is based on ISO
standards for encoding and combining character sets.  Com-
pound Text is intended to be used in three main contexts:
inter-client communication using selections, as defined in
the Inter-Client Communication Conventions Manual (ICCCM);
window properties (e.g., window manager hints as defined in
the ICCCM); and resources (e.g., as defined in Xlib and the
Xt Intrinsics).

Compound Text is intended as an external representation, or
interchange format, not as an internal representation.	It
is expected (but not required) that clients will convert









			     -2-


Compound Text to some internal representation for processing
and rendering, and convert from that internal representation
to Compound Text when providing textual data to another
client.

2.  Values

The name of this encoding is ``COMPOUND_TEXT''.  When text
values are used in the ICCCM-compliant selection mechanism
or are stored as window properties in the server, the type
used should be the atom for ``COMPOUND_TEXT''.

Octet values are represented in this document as two decimal
numbers in the form col/row.  This means the value (col *
16) + row.  For example, 02/01 means the value 33.

For our purposes, the octet encoding space is divided into
four ranges:

     C0   octets from 00/00 to 01/15
     GL   octets from 02/00 to 07/15
     C1   octets from 08/00 to 09/15
     GR   octets from 10/00 to 15/15


C0 and C1 are ``control character'' sets, while GL and GR
are ``graphic character'' sets.  Only a subset of C0 and C1
octets are used in the encoding, and depending on the char-
acter set encoding defined as GL or GR, a subset of GL and
GR octets may be used; see below for details.  All octets
(00/00 to 15/15) may appear inside the text of extended seg-
ments (defined below).

[For those familiar with ISO 2022, we will use only an 8-bit
environment, and we will always use G0 for GL and G1 for
GR.]

3.  Control Characters

In C0, only the following values will be used:

     00/09   HT    HORIZONTAL TABULATION
     00/10   NL    NEW LINE
     01/11   ESC   (ESCAPE)


In C1, only the following value will be used:

     09/11   CSI   CONTROL SEQUENCE INTRODUCER


[The alternate 7-bit CSI encoding 01/11 05/11 is not used in
Compound Text.]










			     -3-


No control sequences are defined in Compound Text for chang-
ing the C0 and C1 sets.

A horizontal tab can be represented with the octet 00/09.
Specification of tabulation width settings is not part of
Compound Text and must be obtained from context (in an
unspecified manner).

[Inclusion of horizontal tab is for consistency with the
STRING type currently defined in the ICCCM.]

A newline (line separator/terminator) can be represented
with the octet 00/10.

[Note that 00/10 is normally LINEFEED, but is being inter-
preted as NEWLINE.  This can be thought of as using the
(deprecated) NEW LINE mode, E.1.3, in ISO 6429.  Use of this
value instead of 08/05 (NEL, NEXT LINE) is for consistency
with the STRING type currently defined in the ICCCM.]

The remaining C0 and C1 values (01/11 and 09/11) are only
used in the control sequences defined below.

4.  Standard Character Set Encodings

The default GL and GR sets in Compound Text correspond to
the left and right halves of ISO 8859-1 (Latin 1).  As such,
any legal instance of a STRING type (as defined in the
ICCCM) is also a legal instance of type COMPOUND_TEXT.

[The implied initial state in ISO 2022 is defined with the sequence:
 01/11 02/00 04/03  GO and G1 in an 8-bit environment only.  Designation also invokes.
 01/11 02/00 04/07  In an 8-bit environment, C1 represented as 8-bits.
 01/11 02/00 04/09  Graphic character sets can be 94 or 96.
 01/11 02/00 04/11  8-bit code is used.
 01/11 02/08 04/02  Designate ASCII into G0.
 01/11 02/13 04/01  Designate right-hand part of ISO Latin-1 into G1.
]

To define one of the approved standard character set encod-
ings to be the GL set, one of the following control
sequences is used:

     01/11 02/08 {I} F	       94 character set
     01/11 02/04 02/08 {I} F   94N character set


To define one of the approved standard character set encod-
ings to be the GR set, one of the following control
sequences is used:

     01/11 02/09 {I} F	       94 character set
     01/11 02/13 {I} F	       96 character set










			     -4-


     01/11 02/04 02/09 {I} F   94N character set


The ``F''in the control sequences above stands for ``Final
character'', which is always in the range 04/00 to 07/14.
The ``{I}'' stands for zero or more ``intermediate charac-
ters'', which are always in the range 02/00 to 02/15, with
the first intermediate character always in the range 02/01
to 02/03.  The registration authority has defined an ``{I}
F'' sequence for each registered character set encoding.

[Final characters for private encodings (in the range 03/00
to 03/15) are not permitted here in Compound Text.]

For GL, octet 02/00 is always defined as SPACE, and octet
07/15 (normally DELETE) is never used.	For a 94-character
set defined as GR, octets 10/00 and 15/15 are never used.

[This is consistent with ISO 2022.]

A 94N character set uses N octets (N > 1) for each charac-
ter.  The value of N is derived from the column value for F:

     column 04 or 05   2 octets
     column 06	       3 octets
     column 07	       4 or more octets


In a 94N encoding, the octet values 02/00 and 07/15 (in GL)
and 10/00 and 15/15 (in GR) are never used.

[The column definitions come from ISO 2022.]

Once a GL or GR set has been defined, all further octets in
that range (except within control sequences and extended
segments) are interpreted with respect to that character set
encoding, until the GL or GR set is redefined.	GL and GR
sets can be defined independently, they do not have to be
defined in pairs.

Note that when actually using a character set encoding as
the GR set, you must force the most significant bit (08/00)
of each octet to be a one, so that it falls in the range
10/00 to 15/15.

[Control sequences to specify character set encoding revi-
sions (as in section 6.3.13 of ISO 2022) are not used in
Compound Text.	Revision indicators do not appear to provide
useful information in the context of Compound Text.  The
most recent revision can always be assumed, since revisions
are upward compatible.]












			     -5-


5.  Approved Standard Encodings

The following are the approved standard encodings to be used
with Compound Text.  Note that none have Intermediate char-
acters; however, a good parser will still deal with Interme-
diate characters in the event that additional encodings are
later added to this list.

     ------------------------------------------------------------------
     {I} F   94/96   Description
     ------------------------------------------------------------------
     4/02    94      7-bit ASCII graphics (ANSI X3.4-1968),
		     Left half of ISO 8859 sets
     04/09   94      Right half of JIS X0201-1976 (reaffirmed 1984),
		     8-Bit Alphanumeric-Katakana Code
     04/10   94      Left half of JIS X0201-1976 (reaffirmed 1984),
		     8-Bit Alphanumeric-Katakana Code
     04/01   96      Right half of ISO 8859-1, Latin alphabet No. 1
     04/02   96      Right half of ISO 8859-2, Latin alphabet No. 2
     04/03   96      Right half of ISO 8859-3, Latin alphabet No. 3
     04/04   96      Right half of ISO 8859-4, Latin alphabet No. 4
     04/06   96      Right half of ISO 8859-7, Latin/Greek alphabet
     04/07   96      Right half of ISO 8859-6, Latin/Arabic alphabet
     04/08   96      Right half of ISO 8859-8, Latin/Hebrew alphabet
     04/12   96      Right half of ISO 8859-5, Latin/Cyrillic alphabet
     04/13   96      Right half of ISO 8859-9, Latin alphabet No. 5
     04/01   942     GB2312-1980, China (PRC) Hanzi
     04/02   942     JIS X0208-1983, Japanese Graphic Character Set
     04/03   942     KS C5601-1987, Korean Graphic Character Set
     ------------------------------------------------------------------


The sets listed as ``Left half of ...'' should always be
defined as GL.	The sets listed as ``Right half of ...''
should always be defined as GR.  Other sets can be defined
either as GL or GR.

6.  Non-Standard Character Set Encodings

Character set encodings that are not in the list of approved
standard encodings can be included using ``extended seg-
ments''.  An extended segment begins with one of the follow-
ing sequences:

     01/11 02/05 02/15 03/00 M L   variable number of octets per character
     01/11 02/05 02/15 03/01 M L   1 octet per character
     01/11 02/05 02/15 03/02 M L   2 octets per character
     01/11 02/05 02/15 03/03 M L   3 octets per character
     01/11 02/05 02/15 03/04 M L   4 octets per character

[This uses the ``other coding system'' of ISO 2022, using
private Final characters.]











			     -6-


The ``M'' and ``L'' octets represent a 14-bit unsigned value
giving the number of octets that appear in the remainder of
the segment.  The number is computed as ((M - 128) * 128) +
(L - 128).  The most significant bit M and L are always set
to one.  The remainder of the segment consists of two parts,
the name of the character set encoding and the actual text.
The name of the encoding comes first and is separated from
the text by the octet 00/02 (STX, START OF TEXT).  Note that
the length defined by M and L includes the encoding name and
separator.

[The encoding of the length is chosen to avoid having zero
octets in Compound Text when possible, because embedded NUL
values are problematic in many C language routines.  The use
of zero octets cannot be ruled out entirely however, since
some octets in the actual text of the extended segment may
have to be zero.]

The name of the encoding should be registered with the X
Consortium to avoid conflicts and should when appropriate
match the CharSet Registry and Encoding registration used in
the X Logical Font Description.  The name itself should be
encoded using ISO 8859-1 (Latin 1), should not use question
mark (03/15) or asterisk (02/10), and should use hyphen
(02/13) only in accordance with the X Logical Font Descrip-
tion.

Extended segments are not to be used for any character set
encoding that can be constructed from a GL/GR pair of
approved standard encodings. For example, it is incorrect to
use an extended segment for any of the ISO 8859 family of
encodings.

It should be noted that the contents of an extended segment
are arbitrary; for example, they may contain octets in the
C0 and C1 ranges, including 00/00, and octets comprising a
given character may differ in their most significant bit.

[ISO-registered ``other coding systems'' are not used in
Compound Text; extended segments are the only mechanism for
non-2022 encodings.]

7.  Directionality

If desired, horizontal text direction can be indicated using
the following control sequences:

     09/11 03/01 05/13	 begin left-to-right text
     09/11 03/02 05/13	 begin right-to-left text
     09/11 05/13	 end of string


[This is a subset of the SDS (START DIRECTED STRING) control
in the Draft Bidirectional Addendum to ISO 6429.]









			     -7-


Directionality can be nested.  Logically, a stack of direc-
tions is maintained.  Each of the first two control
sequences pushes a new direction on the stack, and the third
sequence (revert) pops a direction from the stack.  The
stack starts out empty at the beginning of a Compound Text
string.  When the stack is empty, the directionality of the
text is unspecified.

Directionality applies to all subsequent text, whether in
GL, GR, or an extended segment.  If the desired directional-
ity of GL, GR, or extended segments differs, then direction-
ality control sequences must be inserted when switching
between them.

Note that definition of GL and GR sets is independent of
directionality; defining a new GL or GR set does not change
the current directionality, and pushing or popping a direc-
tionality does not change the current GL and GR definitions.

Specification of directionality is entirely optional; text
direction should be clear from context in most cases.  How-
ever, it must be the case that either all characters in a
Compound Text string have explicitly specified direction or
that all characters have unspecified direction.  That is, if
directionality control sequences are used, the first such
control sequence must precede the first graphic character in
a Compound Text string, and graphic characters are not per-
mitted whenever the directionality stack is empty.

8.  Resources

To use Compound Text in a resource, you can simply treat all
octets as if they were ASCII/Latin-1 and just replace all
``\'' octets (05/12) with the two octets ``\\'', all newline
octets (00/10) with the two octets ``\n'', and all zero
octets with the four octets ``\000''.  It is up to the
client making use of the resource to interpret the data as
Compound Text; the policy by which this is ascertained is
not constrained by the Compound Text specification.

9.  Font Names

The following CharSet names for the standard character set
encodings are registered for use in font names under the X
Logical Font Description:

     --------------------------------------------------------------------------------------
     Name	       Encoding Standard		  Description
     --------------------------------------------------------------------------------------
     ISO8859-1	       ISO 8859-1			  Latin alphabet No. 1
     ISO8859-2	       ISO 8859-2			  Latin alphabet No. 2
     ISO8859-3	       ISO 8859-3			  Latin alphabet No. 3
     ISO8859-4	       ISO 8859-4			  Latin alphabet No. 4










			     -8-


     ISO8859-5	       ISO 8859-5			  Latin/Cyrillic alphabet
     ISO8859-6	       ISO 8859-6			  Latin/Arabic alphabet
     ISO8859-7	       ISO 8859-7			  Latin/Greek alphabet
     ISO8859-8	       ISO 8859-8			  Latin/Hebrew alphabet
     ISO8859-9	       ISO 8859-9			  Latin alphabet No. 5
     JISX0201.1976-0   JIS X0201-1976 (reaffirmed 1984)   8-bit Alphanumeric-Katakana Code
     GB2312.1980-0     GB2312-1980, GL encoding 	  China (PRC) Hanzi
     JISX0208.1983-0   JIS X0208-1983, GL encoding	  Japanese Graphic Character Set
     KSC5601.1987-0    KS C5601-1987, GL encoding	  Korean Graphic Character Set
     --------------------------------------------------------------------------------------



10.  Extensions

There is no absolute requirement for a parser to deal with
anything but the particular encoding syntax defined in this
specification.	However, it is possible that Compound Text
may be extended in the future, and as such it may be desir-
able to construct the parser to handle 2022/6429 syntax more
generally.

There are two general formats covering all control sequences
that are expected to appear in extensions:

01/11 {I} F

     For this format, I is always in the range 02/00 to
     02/15, and F is always in the range 03/00 to 07/14.

09/11 {P} {I} F

     For this format, P is always in the range 03/00 to
     03/15, I is always in the range 02/00 to 02/15, and F
     is always in the range 04/00 to 07/14.

In addition, new (singleton) control characters (in the C0
and C1 ranges) might be defined in the future.

Finally, new kinds of ``segments'' might be defined in the
future using syntax similar to extended segments:

01/11 02/05 02/15 F M L

     For this format, F is in the range 03/05 to 3/15.	M
     and L are as defined in extended segments.  Such a seg-
     ment will always be followed by the number of octets
     defined by M and L.  These octets can have arbitrary
     values and need not follow the internal structure
     defined for current extended segments.

If extensions to this specification are defined in the
future, then any string incorporating instances of such
extensions must start with one of the following control









			     -9-


sequences:

     01/11 02/03 V 03/00   ignoring extensions is OK
     01/11 02/03 V 03/01   ignoring extensions is not OK


In either case, V is in the range 02/00 to 02/15 and indi-
cates the major version minus one of the specification being
used.  These version control sequences are for use by
clients that implement earlier versions, but have imple-
mented a general parser.  The first control sequence indi-
cates that it is acceptable to ignore all extension control
sequences; no mandatory information will be lost in the
process.  The second control sequence indicates that it is
unacceptable to ignore any extension control sequences;
mandatory information would be lost in the process.  In gen-
eral, it will be up to the client generating the Compound
Text to decide which control sequence to use.

11.  Errors

If a Compound Text string does not match the specification
here (e.g., uses undefined control characters, or undefined
control sequences, or incorrectly formatted extended seg-
ments), it is best to treat the entire string as invalid,
except as indicated by a version control sequence.


































