Compound Text Encoding

X Consortium Standard

Robert W. Scheifler

X Consortium

X Version 11, Release 7.7

Version 1.1

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Table of Contents

Overview
Values
Control Characters
Standard Character Set Encodings
Approved Standard Encodings
Non-Standard Character Set Encodings
Directionality
Resources
Font Names
Extensions
Errors

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. Compound 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 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.

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 character 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 segments (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.]

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.]

No control sequences are defined in Compound Text for changing 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 interpreted 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.

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 encodings 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 94^N character set

To define one of the approved standard character set encodings 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
01/11 02/04 02/09 {I} F 94^N 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 characters", 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 94^N character set uses N octets (N > 1) for each character. 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 94^N 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 revisions (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.]

Approved Standard Encodings

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

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

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.

Non-Standard Character Set Encodings

Character set encodings that are not in the list of approved standard encodings
can be included using "extended segments". An extended segment begins with one
of the following sequences:

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

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

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 Description.

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.]

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.]

Directionality can be nested. Logically, a stack of directions 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 directionality of GL, GR, or extended segments
differs, then directionality 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 directionality 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. However, 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 permitted whenever the directionality stack is empty.

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.

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      │ISO8859-1                     │Latinalphabet No. 1           │
├───────────────┼──────────────────────────────┼──────────────────────────────┤
│ISO8859-2      │ISO8859-2                     │Latinalphabet No. 2           │
├───────────────┼──────────────────────────────┼──────────────────────────────┤
│ISO8859-3      │ISO8859-3                     │Latinalphabet No. 3           │
├───────────────┼──────────────────────────────┼──────────────────────────────┤
│ISO8859-4      │ISO8859-4                     │Latinalphabet No. 4           │
├───────────────┼──────────────────────────────┼──────────────────────────────┤
│ISO8859-5      │ISO 8859-5                    │Latin/Cyrillic alphabet       │
├───────────────┼──────────────────────────────┼──────────────────────────────┤
│ISO8859-6      │ISO 8859-6                    │Latin/Arabic alphabet         │
├───────────────┼──────────────────────────────┼──────────────────────────────┤
│ISO8859-7      │ISO8859-7                     │Latin/Greekalphabet           │
├───────────────┼──────────────────────────────┼──────────────────────────────┤
│ISO8859-8      │ISO8859-8                     │Latin/Hebrew alphabet         │
├───────────────┼──────────────────────────────┼──────────────────────────────┤
│ISO8859-9      │ISO8859-9                     │Latinalphabet No. 5           │
├───────────────┼──────────────────────────────┼──────────────────────────────┤
│JISX0201.1976-0│JIS X0201-1976 (reaffirmed    │8-bit Alphanumeric-Katakana   │
│               │1984)                         │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  │
└───────────────┴──────────────────────────────┴──────────────────────────────┘

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 desirable 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 segment 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 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 indicates 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
implemented a general parser. The first control sequence indicates 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 general, it will be up to the client
generating the Compound Text to decide which control sequence to use.

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 segments), it is best to treat the entire string as invalid,
except as indicated by a version control sequence.

