Compound Text Encoding

X Consortium Standard

Robert W. Scheifler

   X Consortium

   X Version 11, Release 7.7

   Version 1.1

   Copyright © 1989 X Consortium

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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} 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.

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 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

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.
