Provided by: m17n-docs_1.6.2-1_all bug


       mdbFLT - Font Layout Table


       For simple scripts, the rendering engine converts character codes into
       glyph codes one by one by consulting the encoding of each selected
       font. But, to render text that requires complicated layout (e.g. Thai
       and Indic scripts), one to one conversion is not sufficient. A sequence
       of characters may have to be drawn as a single ligature. Some glyphs
       may have to be drawn at 2-dimensionally shifted positions.

       To handle those complicated scripts, the m17n library uses Font Layout
       Tables (FLTs for short). The FLT driver interprets an FLT and converts
       a character sequence into a glyph sequence that is ready to be passed
       to the rendering engine.

       An FLT can contain information to extract a grapheme cluster from a
       character sequence and to reorder the characters in the cluster, in
       addition to information found in OpenType Layout Tables (CMAP, GSUB,
       and GPOS).

       An FLT is a cascade of one or more conversion stages. In each stage, a
       sequence is converted into another sequence to be read in the next
       stage. The length of sequences may differ from stage to stage. Each
       element in a sequence has the following integer attributes.

       · code
       In the first conversion stage, this is the character code in the
       original character sequence. In the last stage, it is the glyph code
       passed to the rendering engine. In other cases, it is an intermediate
       glyph code.
       · category
       The category code defined in the CATEGORY-TABLE of the current stage,
       or defined in the one of the former stages and not overwritten by later
       · combining-spec
       If nonzero, it specifies how to combine this (intermediate) glyph with
       the previous one.
       · left-padding-flag
       If nonzero, it instructs the rendering function to insert a padding
       space before this (intermediate) glyph so that the glyph does not
       overlap with the previous one.
       · right-padding-flag
       If nonzero, it instructs the rendering function to insert a padding
       space after this (intermediate) glyph so that the glyph does not
       overlap with the next one.
       When the layout engine draws text, it at first determines a font and an
       FLT for each character in the text. For each subsequence of characters
       that use the same font and FLT, the layout engine generates a
       corresponding intermediate glyph sequence. The code attribute of each
       element in the intermediate glyph sequence is its character code, and
       all other attributes are zeros. This sequence is processed in the first
       stage of FLT as the current run (substring).
       Each stage works as follows.
       At first, if the stage has a CATEGORY-TABLE, the category of each glyph
       in the current run is updated. If there is a glyph that has no
       category, the current run ends before that glyph.
       Then, the default values of code-offset, combining-spec, and
       left-padding-flag of this stage are initialized to zero.
       Next, the initial conversion rule of the stage is applied to the
       current run.
       Lastly, the current run is replaced with the newly produced
       (intermediate) glyph sequence.


       The m17n library loads an FLT from the m17n database using the tag
       <font, layouter, FLT-NAME>. The date format of an FLT is as follows:

       FLT-DECLARATION ::= '(' 'font' 'layouter' FLT-NAME nil PROP * ')'
       FLT-NAME ::= SYMBOL
       PROP :: = VERSION | FONT
       VERSION ::= '(' 'version' MTEXT ')'
       FONT ::= '(' 'font' FONT-SPEC ')'
       FONT-SPEC ::=
            '(' [[ FOUNDRY FAMILY
                  [ WEIGHT [ STYLE [ STRETCH [ ADSTYLE ]]]]]
                REGISTRY ]
             [ OTF-SPEC ] [ LANG-SPEC ] ')'



       CATEGORY-TABLE ::= '(' 'category' CATEGORY-SPEC + ')'

                         | '(' CODE CODE CATEGORY ')'

       CODE ::= INTEGER

       In the definition of CATEGORY-SPEC, CODE is a glyph code, and CATEGORY
       is ASCII code of an upper or lower letter, i.e. one of 'A', ... 'Z',
       'a', .. 'z'.
       The first form of CATEGORY-SPEC assigns CATEGORY to a glyph whose code
       is CODE. The second form assigns CATEGORY to glyphs whose code falls
       between the two CODEs.
       GENERATOR ::= '(' 'generator' RULE MACRO-DEF * ')'

                | PREDEFINED-RULE | MACRO-NAME

       MACOR-DEF ::= '(' MACRO-NAME RULE + ')'
       Each RULE specifies glyphs to be consumed and glyphs to be produced.
       When some glyphs are consumed, they are taken away from the current
       run. A rule may fail in some condition. If not described explicitly to
       fail, it should be regarded that the rule succeeds.
       This rule consumes no glyph and produces a glyph which has the
       following attributes:
       · code : INTEGER plus the default code-offset
       · combining-spec : default value
       · left-padding-flag : default value
       · right-padding-flag : zero
       After having produced the glyph, the default code-offset,
       combining-spec, and left-padding-flag are all reset to zero.
       PREDEFINED-RULE ::= '=' | '*' | '<' | '>' | '|' | '[' | ']'
       They perform actions as follows.
       · =
       This rule consumes the first glyph in the current run and produces the
       same glyph. It fails if the current run is empty.
       · *
       This rule repeatedly executes the previous rule. If the previous rule
       fails, this rule does nothing and fails.
       · <
       This rule specifies the start of a grapheme cluster.
       · >
       This rule specifies the end of a grapheme cluster.
       · @[
       This rule sets the default left-padding-flag to 1. No glyph is
       consumed. No glyph is produced.
       · @]
       This rule changes the right-padding-flag of the lastly generated glyph
       to 1. No glyph is consumed. No glyph is produced.
       · |
       This rule consumes no glyph and produces a special glyph whose category
       is ' ' and other attributes are zero. This is the only rule that
       produces that special glyph.
       REGEXP-BLOCK ::= '(' REGEXP RULE * ')'

       REGEXP ::= MTEXT
       MTEXT is a regular expression that should match the sequence of
       categories of the current run. If a match is found, this rule executes
       RULEs temporarily limiting the current run to the matched part. The
       matched part is consumed by this rule.
       Parenthesized subexpressions, if any, are recorded to be used in
       MATCH-BLOCK that may appear in one of RULEs.
       If no match is found, this rule fails.
       MATCH-BLOCK ::= '(' MATCH-INDEX RULE * ')'

       MATCH-INDEX is an integer specifying a parenthesized subexpression
       recorded by the previous REGEXP-BLOCK. If such a subexpression was
       found by the previous regular expression matching, this rule executes
       RULEs temporarily limiting the current run to the matched part of the
       subexpression. The matched part is consumed by this rule.
       If no match was found, this rule fails.
       If this is the first rule of the stage, MATCH-INDEX must be 0, and it
       matches the whole current run.

       SOURCE-PATTERN ::= '(' CODE + ')'
                          | (' 'range' CODE CODE ')'
       If the sequence of codes of the current run matches SOURCE-PATTERN,
       this rule executes RULEs temporarily limiting the current run to the
       matched part. The matched part is consumed.
       The first form of SOURCE-PATTERN specifies a sequence of glyph codes to
       be matched. In this case, this rule resets the default code-offset to
       The second form specifies a range of codes that should match the first
       glyph code of the code sequence. In this case, this rule sets the
       default code-offset to the first glyph code minus the first CODE
       specifying the range.
       If no match is found, this rule fails.
       FONT-FACILITY = '(' 'font-facility' CODE * ')'
                      | '(' 'font-facility' FONT-SPEC ')'
       If the current font has glyphs for CODEs or matches with FONT-SPEC,
       this rule succeeds and RULEs are executed. Otherwise, this rule fails.
       COND-BLOCK ::= '(' 'cond' RULE + ')'
       This rule sequentially executes RULEs until one succeeds. If no rule
       succeeds, this rule fails. Otherwise, it succeeds.
       OTF-SPEC ::= SYMBOL

       OTF-SPEC is a symbol whose name specifies an instruction to the OTF
       driver. The name has the following syntax.

         SCRIPT ::= SYMBOL

         LANGSYS ::= '/' SYMBOL



         FEATURE-LIST ::= ( SYMBOL ',' ) * [ SYMBOL | '*' ]

       Each SYMBOL specifies a tag name defined in the OpenType specification.
       For SCRIPT, SYMBOL specifies a Script tag name (e.g. deva for
       For LANGSYS, SYMBOL specifies a Language System tag name. If LANGSYS is
       omitted, the Default Language System table is used.
       For GSUB-FEATURES, each SYMBOL in FEATURE-LIST specifies a GSUB Feature
       tag name to apply. '*' is allowed as the last item to specify all
       remaining features. If SYMBOL is preceded by '~' and the last item is
       '*', SYMBOL is excluded from the features to apply. If no SYMBOL is
       specified, no GSUB feature is applied. If GSUB-FEATURES itself is
       omitted, all GSUB features are applied.
       When OTF-SPEC appears in a FONT-SPEC, FEATURE-LIST specifies features
       that the font must have (or must not have if preceded by '~'), and the
       last'*', even if exists, has no meaning.
       The specification of GPOS-FEATURES is analogous to that of
       Please note that all the tags above must be 4 ASCII printable
       See the following page for the OpenType specification.
       COMBINING is a symbol whose name specifies how to combine the next
       glyph with the previous one. This rule sets the default combining-spec
       to an integer code that is unique to the symbol name. The name has the
       following syntax.

         VPOS ::= 't' | 'c' | 'b' | 'B'

         HPOS ::= 'l' | 'c' | 'r'

         OFFSET :: = '.' | XOFF | YOFF XOFF ?

         XOFF ::= ('<' | '>') INTEGER ?

         YOFF ::= ('+' | '-') INTEGER ?
       VPOS and HPOS specify the vertical and horizontal positions as
       described below.
                                       POINT VPOS HPOS
                                       ----- ---- ----
           0----1----2 <---- top       0     t    l
           |         |                 1     t    c
           |         |                 2     t    r
           |         |                 3     B    l
           9   10   11 <---- center    4     B    c
           |         |                 5     B    r
         --3----4----5-- <-- baseline  6     b    l
           |         |                 7     b    c
           6----7----8 <---- bottom    8     b    r
                                       9     c    l
           |    |    |                10     c    c
         left center right            11     c    r
       The left figure shows 12 reference points of a glyph by numbers 0 to
       11. The rectangle 0-6-8-2 is the bounding box of the glyph, the
       positions 3, 4, and 5 are on the baseline, 9-11 are on the vertical
       center of the box, 0-2 and 6-8 are on the top and on the bottom
       respectively. 1, 10, 4, and 7 are on the horizontal center of the box.
       The right table shows how those reference points are specified by a
       pair of VPOS and HPOS.
       The first VPOS and HPOS in the definition of COMBINING-NAME specify the
       reference point of the previous glyph, and the second VPOS and HPOS
       specify that of the next glyph. The next glyph is drawn so that these
       two reference points align.
       OFFSET specifies the way of alignment in detail. If it is '.', the
       reference points are on the same position.
       XOFF specifies how much the X position of the reference point of the
       next glyph should be shifted to the left ('<') or right ('>') from the
       previous reference point.
       YOFF specifies how much the Y position of the reference point the next
       glyph should be shifted upward ('+') or downward ('-') from the
       previous reference point.
       In both cases, INTEGER is the amount of shift expressed as a percentage
       of the font size, i.e., if INTEGER is 10, it means 10% (1/10) of the
       font size. If INTEGER is omitted, it is assumed that 5 is specified.
       Once the next glyph is combined with the previous one, they are treated
       as a single combined glyph.
       MACRO-NAME is a symbol that appears in one of MACRO-DEF. It is
       exapanded to the sequence of the corresponding RULEs.


       So far, it has been assumed that each sequence, which is drawn with a
       specific font, is context free, i.e. not affected by the glyphs
       preceding or following that sequence. This is true when sequence S1 is
       drawn with font F1 while the preceding sequence S0 unconditionally
       requires font F0.
         sequence                              S0      S1
         currently used font                   F0      F1
         usable font(s)                        F0      F1
       Sometimes, however, a clear separation of sequences is not possible.
       Suppose that the preceding sequence S0 can be drawn not only with F0
       but also with F1.
         sequence                              S0      S1
         currently used font                   F0      F1
         usable font(s)                        F0,F1   F1
       In this case, glyphs used to draw the preceding S0 may affect glyph
       generation of S1. Therefore it is necessary to access information about
       S0, which has already been processed, when processing S1. Generation
       rules in the first stage (only in the first stage) accept a special
       regular expression to access already processed parts.
         "RE0 RE1"
       RE0 and RE1 are regular expressions that match the preceding sequence
       S0 and the following sequence S1, respectively.
       Pay attention to the space between the two regular expressions. It
       represents the special category ' ' (see above). Note that the regular
       expression above belongs to glyph generation rules using font F1,
       therefore not only RE1 but also RE0 must be expressed with the
       categories for F1. This means when the preceding sequence S0 cannot be
       expressed with the categories for F1 (as in the first example above)
       generation rules having these patterns never match.


       mdbGeneral(5), FLTs provided by the m17n database


       Copyright (C) 2001 Information-technology Promotion Agency (IPA)
       Copyright (C) 2001-2011 National Institute of Advanced Industrial
       Science and Technology (AIST)
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License