mdbFLT(5) The m17n Library mdbFLT(5)
NAME
mdbFLT - Font Layout Table
DESCRIPTION
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
stages.
• 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.
SYNTAX and SEMANTICS
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:
FONT-LAYOUT-TABLE ::= FLT-DECLARATION ? STAGE0 STAGE *
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 ] ')'
STAGE0 ::= CATEGORY-TABLE GENERATOR
STAGE ::= CATEGORY-TABLE ? GENERATOR
CATEGORY-TABLE ::= '(' 'category' CATEGORY-SPEC + ')'
CATEGORY-SPEC ::= '(' CODE CATEGORY ')'
| '(' CODE CODE CATEGORY ')'
CODE ::= INTEGER
CATEGORY ::= 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 * ')'
RULE ::= REGEXP-BLOCK | MATCH-BLOCK | SUBST-BLOCK | COND-BLOCK
FONT-FACILITY-BLOCK | DIRECT-CODE | COMBINING-SPEC | OTF-SPEC
| 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.
DIRECT-CODE ::= INTEGER
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 ::= INTEGER
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.
SUBST-BLOCK ::= '(' SOURCE-PATTERN RULE * ')'
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
zero.
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-BLOCK ::= '(' FONT-FACILITY RULE * ')'
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.
OTF-SPEC-NAME ::= ':otf=' SCRIPT LANGSYS ? GSUB-FEATURES ? GPOS-FEATURES ?
SCRIPT ::= SYMBOL
LANGSYS ::= '/' SYMBOL
GSUB-FEATURES ::= '=' FEATURE-LIST ?
GPOS-FEATURES ::= '+' FEATURE-LIST ?
FEATURE-LIST ::= ( SYMBOL ',' ) * [ SYMBOL | '*' ].fi
Each SYMBOL specifies a tag name defined in the OpenType specification.
For SCRIPT, SYMBOL specifies a Script tag name (e.g. deva for Devanagari).
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 GSUB-FEATURES.
Please note that all the tags above must be 4 ASCII printable characters.
See the following page for the OpenType specification.
http://www.microsoft.com/typography/otspec/default.htm
COMBINING ::= SYMBOL
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.
COMBINING-NAME ::= VPOS HPOS OFFSET VPOS HPOS
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 ::= SYMBOL
MACRO-NAME is a symbol that appears in one of MACRO-DEF. It is exapanded
to the sequence of the corresponding RULEs.
CONTEXT DEPENDENT BEHAVIOR
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.
SEE ALSO
mdbGeneral(5), FLTs provided by the m17n database
COPYRIGHT
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
<http://www.gnu.org/licenses/fdl.html>.
Version 1.8.4 Mon Sep 25 2023 mdbFLT(5)
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