                    How to further enhance XKB configuration

                     Abstract
                     This guide is aimed to relieve one's
                     labour to create a new
                     (internationalized) keyboard layout.
                     Unlike other documents this guide
                     accents the keymap developer's point
                     of view.

            -------------------------------------------------------

   Table of Contents

   [1]Overview

   [2]The Basics

   [3]Enhancing XKB Configuration

   [4]Defining New Layouts

Overview

   The developer of a new layout should read the xkb protocol specification
   ([5]The X Keyboard Extension: Protocol Specification) at least to clarify
   for himself some xkb-specific terms used in this document and elsewhere in
   xkb configuration. Also it shows wise to understand how the X server and a
   client digest their keyboard inputs (with and without xkb).

   A useful source is also [6]Ivan Pascal's text about xkb configuration
   often referenced throughout this document.

   Note that this document covers only enhancements which are to be made to
   XFree86 version 4.3 and X11R6.7.0 and above.

   --------------------------------------------------------------------------

The Basics

   At the startup (or at later at user's command) X server starts its xkb
   keyboard module extension and reads data from a compiled configuration
   file.

   This compiled configuration file is prepared by the program xkbcomp which
   behaves altogether as an ordinary compiler (see man xkbcomp). Its input
   are human readable xkb configuration files which are verified and then
   composed into a useful xkb configuration. Users don't need to mess with
   xkbcomp themselves, for them it is invisible. Usually, it is started upon
   X server startup.

   As you probably already know, the xkb configuration consists of five main
   modules:

   Keycodes

           Tables that defines translation from keyboard scan codes into
           reasonable symbolic names, maximum, minimum legal keycodes,
           symbolic aliases and description of physically present
           LED-indicators. The primary sence of this component is to allow
           definitions of maps of symbols (see below) to be independent of
           physical keyboard scancodes. There are two main naming conventions
           for symbolic names (always four bytes long):

              * names which express some traditional meaning like <SPCE>
                (stands for space bar) or

              * names which express some relative positioning on a keyboard,
                for example <AE01> (an exclamation mark on US keyboards), on
                the right there are keys <AE02>, <AE03> etc.



   Types

           Types describe how the produced key is changed by active modifiers
           (like Shift, Control, Alt, ...). There are several predefined
           types which cover most of used combinations.

   Compat

           Compatibility component defines internal behaviour of modifiers.
           Using compat component you can assign various actions (elaborately
           described in xkb specification) to key events. This is also the
           place where LED-indicators behaviour is defined.

   Symbols

           For i18n purposes, this is the most important table. It defines
           what values (=symbols) are assigned to what keycodes (represented
           by their symbolic name, see above). There may be defined more than
           one value for each key and then it depends on a key type and on
           modifiers state (respective compat component) which value will be
           the resulting one.

   Geometry

           Geometry files aren't used by xkb itself but they may be used by
           some external programs to depict a keyboard image.

   All these components have the files located in xkb configuration tree in
   subdirectories with the same names (usually in /usr/lib/X11/xkb).

   --------------------------------------------------------------------------

Enhancing XKB Configuration

   Most of xkb enhancements concerns a need to define new output symbols for
   the some input key events. In other words, a need to define a new symbol
   map (for a new language, standard or just to feel more comfortable when
   typing text).

   What do you need to do? Generally, you have to define following things:

     * the map of symbols itself

     * the rules to allow users to select the new mapping

     * the description of the new layout



   First of all, it is good to go through existing layouts and to examine
   them if there is something you could easily adjust to fit your needs. Even
   if there is nothing similar you may get some ideas about basic concepts
   and used tricks.

   --------------------------------------------------------------------------

  Levels And Groups

   Since XFree86 4.3.0 and X11R6.7.0 you can use multi-layout concept of xkb
   configuration. Though it is still in boundaries of xkb protocol and
   general ideas, the keymap designer must obey new rules when creating new
   maps. In exchange we get a more powerful and cleaner configuration system.

   Remember that it is the application which must decide which symbol matches
   which keycode according to effective modifier state. The X server itself
   sends only an input event message to. Of course, usually the general
   interpretation is processed by Xlib, Xaw, Motif, Qt, Gtk and similar
   libraries. The X server only supplies its mapping table (usually upon an
   application startup).

   You can think of the X server's symbol table as of a irregular table where
   each keycode has its row and where each combination of modifiers
   determines exactly one column. The resulting cell then gives the proper
   symbolic value. Not all keycodes need to bind different values for
   different combination of modifiers. <ENTER> key, for instance, usually
   doesn't depend on any modifiers so it its row has only one column defined.

   Note that in XKB there is no prior assumption that certain modifiers are
   bound to certain columns. By editing proper files (see [7]keytypes) this
   mapping can be changed as well.

   Unlike the original X protocol the XKB approach is far more flexible. It
   is comfortable to add one additional XKB term - group. You can think of a
   group as of a vector of columns per each keycode (naturally the dimension
   of this vector may differ for different keycodes). What is it good for?
   The group is not very useful unless you intend to use more than one
   logically different set of symbols (like more than one alphabet) defined
   in a single mapping table. But then, the group has a natural meaning -
   each symbol set has its own group and changing it means selecting a
   different one. XKB approach allows up to four different groups. The
   columns inside each group are called (shift) levels. The X server knows
   the current group and reports it together with modifier set and with a
   keycode in key events.

   To sum it up:



     * for each keycode XKB keyboard map contains up to four one-dimensional
       tables - groups (logically different symbol sets)

     * for each group of a keycode XKB keyboard map contains some columns -
       shift levels (values reached by combinations of Shift, Ctrl, Alt, ...
       modifiers)

     * different keycodes can have different number of groups

     * different groups of one keycode can have different number of shift
       levels

     * the current group number is tracked by X server



   It is clear that if you sanely define levels, groups and sanely bind
   modifiers and associated actions you can have simultaneously loaded up to
   four different symbol sets where each of them would reside in its own
   group.

   The multi-layout concept provides a facility to manipulate xkb groups and
   symbol definitions in a way that allows almost arbitrary composition of
   predefined symbol tables. To keep it fully functional you have to:

     * define all symbols only in the first group

     * (re)define any modifiers with extra care to avoid strange
       (anisometric) behaviour



   --------------------------------------------------------------------------

Defining New Layouts

   See [8]Some Words About XKB internals for explanation of used xkb terms
   and problems addressed by XKB extension.

   See [9]Common notes about XKB configuration files language for more
   precise explanation of syntax of xkb configuration files.

   --------------------------------------------------------------------------

  Predefined XKB Symbol Sets

   If you are about to define some European symbol map extension, you might
   want to use on of four predefined latin alphabet layouts.

   Okay, let's assume you want extend an existing keymap and you want to
   override a few keys. Let's take a simple U.K. keyboard as an example
   (defined in pc/gb):



   partial default alphanumeric_keys
   xkb_symbols "basic" {
     include "pc/latin"

     name[Group1]="Great Britain";

     key <AE02>  { [         2,   quotedbl,  twosuperior,    oneeighth ] };
     key <AE03>  { [         3,   sterling, threesuperior,    sterling ] };
     key <AC11>  { [apostrophe,         at, dead_circumflex, dead_caron] };
     key <TLDE>  { [     grave,    notsign,          bar,          bar ] };
     key <BKSL>  { [numbersign, asciitilde,   dead_grave,   dead_breve ] };
     key <RALT>  { type[Group1]="TWO_LEVEL",
                   [ ISO_Level3_Shift, Multi_key ]   };

     modifier_map Mod5   { <RALT> };
   };



   It defines a new layout in basic variant as an extension of common latin
   alphabet layout. The layout (symbol set) name is set to "Great Britain".
   Then there are redefinitions of a few keycodes and a modifiers binding. As
   you can see the number of shift levels is the same for <AE02>, <AE03>,
   <AC11>, <TLDE> and <BKSL> keys but it differs from number of shift levels
   of <RALT>.

   Note that the <RALT> key itself is a binding key for Mod5 and that it
   serves like a shift modifier for LevelThree, together with Shift as a
   multi-key. It is a good habit to respect this rule in a new similar
   layout.

   Okay, you could now define more variants of your new layout besides basic
   simply by including (augmenting/overriding/...) the basic definition and
   altering what may be needed.

   --------------------------------------------------------------------------

  Key Types

   The differences in the number of columns (shift levels) are caused by a
   different types of keys (see the types definition in section basics). Most
   keycodes have implicitly set the keytype in the included "pc/latin" file
   to "FOUR_LEVEL_ALPHABETIC". The only exception is <RALT> keycode which is
   explicitly set "TWO_LEVEL" keytype.

   All those names refer to pre-defined shift level schemes. Usually you can
   choose a suitable shift level scheme from default types scheme list in
   proper xkb component's subdirectory.

   The most used schemes are:

   ONE_LEVEL

           The key does not depend on any modifiers. The symbol from first
           level is always chosen.

   TWO_LEVEL

           The key uses a modifier Shift and may have two possible values.
           The second level may be chosen by Shift modifier. If Lock modifier
           (usually Caps-lock) applies the symbol is further processed using
           system-specific capitalization rules. If both Shift+Lock modifier
           apply the symbol from the second level is taken and capitalization
           rules are applied (and usually have no effect).

   ALPHABETIC

           The key uses modifiers Shift and Lock. It may have two possible
           values. The second level may be chosen by Shift modifier. When
           Lock modifier applies, the symbol from the first level is taken
           and further processed using system-specific capitalization rules.
           If both Shift+Lock modifier apply the symbol from the first level
           is taken and no capitalization rules applied. This is often called
           shift-cancels-caps behaviour.

   THREE_LEVEL

           Is the same as TWO_LEVEL but it considers an extra modifier -
           LevelThree which can be used to gain the symbol value from the
           third level. If both Shift+LevelThree modifiers apply the value
           from the third level is also taken. As in TWO_LEVEL, the Lock
           modifier doesn't influence the resulting level. Only Shift and
           LevelThree are taken into that consideration. If the Lock modifier
           is active capitalization rules are applied on the resulting
           symbol.

   FOUR_LEVEL

           Is the same as THREE_LEVEL but unlike LEVEL_THREE if both
           Shift+LevelThree modifiers apply the symbol is taken from the
           fourth level.

   FOUR_LEVEL_ALPHABETIC

           Is similar to FOUR_LEVEL but also defines shift-cancels-caps
           behaviour as in ALPHABETIC. If Lock+LevelThree apply the symbol
           from the third level is taken and the capitalization rules are
           applied. If Lock+Shift+LevelThree apply the symbol from the third
           level is taken and no capitalization rules are applied.

   KEYPAD

           As the name suggest this scheme is primarily used for numeric
           keypads. The scheme considers two modifiers - Shift and NumLock.
           If none of modifiers applies the symbol from the first level is
           taken. If either Shift or NumLock modifiers apply the symbol from
           the second level is taken. If both Shift+NumLock modifiers apply
           the symbol from the first level is taken. Again,
           shift-cancels-caps variant.

   FOUR_LEVEL_KEYPAD

           Is similar to KEYPAD scheme but considers also LevelThree
           modifier. If LevelThree modifier applies the symbol from the third
           level is taken. If Shift+LevelThree or NumLock+LevelThree apply
           the symbol from the fourth level is taken. If all
           Shift+NumLock+LevelThree modifiers apply the symbol from the third
           level is taken. This also, shift-cancels-caps variant.

   Besides that, there are several schemes for special purposes:

   PC_BREAK

           It is similar to TWO_LEVEL scheme but it considers the Control
           modifier rather than Shift. That means, the symbol from the second
           level is chosen by Control rather than by Shift.

   PC_SYSRQ

           It is similar to TWO_LEVEL scheme but it considers the Alt
           modifier rather than Shift. That means, the symbol from the second
           level is chosen by Alt rather than by Shift.

   CTRL+ALT

           The key uses modifiers Alt and Control. It may have two possible
           values. If only one modifier (Alt or Control) applies the symbol
           from the first level is chosen. Only if both Alt+Control modifiers
           apply the symbol from the second level is chosen.

   SHIFT+ALT

           The key uses modifiers Shift and Alt. It may have two possible
           values. If only one modifier (Alt or Shift) applies the symbol
           from the first level is chosen. Only if both Alt+Shift modifiers
           apply the symbol from the second level is chosen.

   If needed, special caps schemes may be used. They redefine the standard
   behaviour of all *ALPHABETIC types. The layouts (maps of symbols) with
   keys defined in respective types then automatically change their behaviour
   accordingly. Possible redefinitions are:

     * internal

     * internal_nocancel

     * shift

     * shift_nocancel

   None of these schemes should be used directly. They are defined merely for
   'caps:' xkb options (used to globally change the layouts behaviour).

   Don't alter any of existing key types. If you need a different behaviour
   create a new one.

   --------------------------------------------------------------------------

    More On Definitions Of Types

   When the XKB software deals with a separate type description it gets a
   complete list of modifiers that should be taken into account from the
   'modifiers=<list of modifiers>' list and expects that a set of
   'map[<combination of modifiers>]=<list of modifiers>' instructions that
   contain the mapping for each combination of modifiers mentioned in that
   list. Modifiers that are not explicitly listed are NOT taken into account
   when the resulting shift level is computed. If some combination is omitted
   the program (subroutine) should choose the first level for this
   combination (a quite reasonable behavior).

   Lets consider an example with two modifiers ModOne and ModTwo:

   type "..." {
       modifiers = ModOne+ModTwo;
       map[None] = Level1;
       map[ModOne] = Level2;
   };

   In this case the map statements for ModTwo only and ModOne+ModTwo are
   omitted. It means that if the ModTwo is active the subroutine can't found
   explicit mapping for such combination an will use the default level i.e.
   Level1.

   But in the case the type described as:

   type "..." {
       modifiers = ModOne;
       map[None] = Level1;
       map[ModOne] = Level2;
   };

   the ModTwo will not be taken into account and the resulting level depends
   on the ModOne state only. That means, ModTwo alone produces the Level1 but
   the combination ModOne+ModTwo produces the Level2 as well as ModOne alone.

   What does it mean if the second modifier is the Lock? It means that in the
   first case (the Lock itself is included in the list of modifiers but
   combinations with this modifier aren't mentioned in the map statements)
   the internal capitalization rules will be applied to the symbol from the
   first level. But in the second case the capitalization will be applied to
   the symbol chosen accordingly to he first modifier - and this can be the
   symbol from the first as well as from the second level.

   Usually, all modifiers introduced in 'modifiers=<list of modifiers>' list
   are used for shift level calculation and then discarded. Sometimes this is
   not desirable. If you want to use a modifier for shift level calculation
   but you don't want to discard it, you may list in 'preserve[<combination
   of modifiers>]=<list of modifiers>'. That means, for a given combination
   all listed modifiers will be preserved. If the Lock modifier is preserved
   then the resulting symbol is passed to internal capitalization routine
   regardless whether it has been used for a shift level calculation or not.

   Any key type description can use both real and virtual modifiers. Since
   real modifiers always have standard names it is not necessary to
   explicitly declare them. Virtual modifiers can have arbitrary names and
   can be declared (prior using them) directly in key type definition:

   virtual_modifiers <comma-separated list of modifiers>  ;

   as seen in for example basic, pc or mousekeys key type definitions.

   --------------------------------------------------------------------------

  Rules

   Once you are finished with your symbol map you need to add it to rules
   file. The rules file describes how all the five basic keycodes, types,
   compat, symbols and geometry components should be composed to give a
   sensible resulting xkb configuration.

   The main advantage of rules over formerly used keymaps is a possibility to
   simply parameterize (once) fixed patterns of configurations and thus to
   elegantly allow substitutions of various local configurations into
   predefined templates.

   A pattern in a rules file (often located in /usr/lib/X11/xkb/rules) can be
   parameterized with four other arguments: Model, Layout, Variant and
   Options. For most cases parameters model and layout should be sufficient
   for choosing a functional keyboard mapping.

   The rules file itself is composed of pattern lines and lines with rules.
   The pattern line starts with an exclamation mark ('!') and describes how
   will the xkb interpret the following lines (rules). A sample rules file
   looks like this:

   ! model                         =       keycodes
     macintosh_old                 =       macintosh
     ...
     *                             =       xorg

   ! model                         =       symbols
     hp                            =       +inet(%m)
     microsoftpro                  =       +inet(%m)
     geniuscomfy                   =       +inet(%m)

   ! model     layout[1]           =       symbols
     macintosh     us              =       macintosh/us%(v[1])
     *             *               =       pc/pc(%m)+pc/%l[1]%(v[1])

   ! model     layout[2]           =       symbols
     macintosh     us              =       +macintosh/us[2]%(v[2]):2
     *             *               =       +pc/%l[2]%(v[2]):2

   ! option                        =       types
     caps:internal                 =       +caps(internal)
     caps:internal_nocancel        =       +caps(internal_nocancel)



   Each rule defines what certain combination of values on the left side of
   equal sign ('=') results in. For example a (keyboard) model macintosh_old
   instructs xkb to take definitions of keycodes from file keycodes/macintosh
   while the rest of models (represented by a wild card '*') instructs it to
   take them from file keycodes/xorg. The wild card represents all possible
   values on the left side which were not found in any of the previous rules.
   The more specialized (more complete) rules have higher precedence than
   general ones, i.e. the more general rules supply reasonable default
   values.

   As you can see some lines contain substitution parameters - the parameters
   preceded by the percent sign ('%'). The first alphabetical character after
   the percent sign expands to the value which has been found on the left
   side. For example +%l%(v) expands into +cz(bksl) if the respective values
   on the left side were cz layout in its bksl variant. More, if the layout
   resp. variant parameter is followed by a pair of brackets ('[', ']') it
   means that xkb should place the layout resp. variant into specified xkb
   group. If the brackets are omitted the first group is the default value.

   So the second block of rules enhances symbol definitions for some
   particular keyboard models with extra keys (for internet, multimedia, ...)
   . Other models are left intact. Similarly, the last block overrides some
   key type definitions, so the common global behaviour ''shift cancels
   caps'' or ''shift doesn't cancel caps'' can be selected. The rest of rules
   produces special symbols for each variant us layout of macintosh keyboard
   and standard pc symbols in appropriate variants as a default.

   --------------------------------------------------------------------------

  Descriptive Files of Rules

   Now you just need to add a detailed description to <rules>.xml description
   file so the other users (and external programs which often parse this
   file) know what is your work about.

   --------------------------------------------------------------------------

    Old Descriptive Files

   The formerly used descriptive files were named <rules>.lst Its structure
   is very simple and quite self descriptive but such simplicity had also
   some cavities, for example there was no way how to describe local variants
   of layouts and there were problems with the localization of descriptions.
   To preserve compatibility with some older programs, new XML descriptive
   files can be converted to old format '.lst'.

   For each parameter of rules file should be described its meaning. For the
   rules file described above the .lst file could look like:

! model
  pc104         Generic 104-key PC
  microsoft     Microsoft Natural
  pc98          PC-98xx Series
  macintosh     Original Macintosh
  ...

! layout
  us            U.S. English
  cz            Czech
  de            German
  ...

! option
  caps:internal          uses internal capitalization. Shift cancels Caps
  caps:internal_nocancel uses internal capitalization. Shift doesn't cancel Caps




   And that should be it. Enjoy creating your own xkb mapping.

References

   Visible links
   1. file:///tmp/html-oq3178#AEN9
   2. file:///tmp/html-oq3178#AEN16
   3. file:///tmp/html-oq3178#AEN55
   4. file:///tmp/html-oq3178#AEN97
   5. http://www.x-docs.org/XKB/XKBproto.pdf
   6. http://www.tsu.ru/~pascal/en/xkb
   7. file:///tmp/html-oq3178#KEYTYPES
   8. http://www.tsu.ru/~pascal/en/xkb/internals.html
   9. http://www.tsu.ru/~pascal/en/xkb/gram-common.html
