1 /* Copyright (C) 2003, 2004
2 National Institute of Advanced Industrial Science and Technology (AIST)
3 Registration Number H15PRO112
4 See the end for copying conditions. */
8 @page mdbFLT Font Layout Table
10 @section flt-description DESCRIPTION
12 For simple scripts, the rendering engine converts character codes into glyph
13 codes one by one by consulting the encoding of each selected font.
14 But, to render text that requires complicated layout (e.g. Thai and
15 Indic scripts), one to one conversion is not sufficient. A sequence
16 of characters may have to be drawn as a single ligature. Some
17 glyphs may have to be drawn at 2-dimensionally shifted positions.
19 To handle those complicated scripts, the m17n library uses Font Layout
20 Tables (FLTs for short). The FLT driver interprets an FLT and
21 converts a character sequence into a glyph sequence that is ready to
22 be passed to the rendering engine.
24 An FLT can contain information to extract a grapheme cluster from a
25 character sequence and to reorder the characters in the cluster, in
26 addition to information found in OpenType Layout Tables (CMAP, GSUB,
29 An FLT is a cascade of one or more conversion stages. In each stage, a
30 sequence is converted into another sequence to be read in the
31 next stage. The length of sequences may differ from stage to
32 stage. Each element in a sequence has the following integer attributes.
37 In the first conversion stage, this is the character code in the
38 original character sequence. In the last stage, it is the glyph code
39 passed to the rendering engine. In other cases, it is an intermediate
44 The category code defined in the @c CATEGORY-TABLE of the current
45 stage, or defined in the one of the former stages and not overwritten
50 If nonzero, it specifies how to combine this (intermediate) glyph
51 with the previous one.
53 <li> left-padding-flag
55 If nonzero, it instructs the rendering function to insert a padding
56 space before this (intermediate) glyph so that the glyph does not
57 overlap with the previous one.
59 <li> right-padding-flag
61 If nonzero, it instructs the rendering function to insert a padding
62 space after this (intermediate) glyph so that the glyph does not
63 overlap with the next one.
67 When the layout engine draws text, it at first determines a font and
68 an FLT for each character in the text. For each subsequence of
69 characters that use the same font and FLT, the layout engine generates
70 a corresponding intermediate glyph sequence. The code attribute of
71 each element in the intermediate glyph sequence is its character code,
72 and all other attributes are zeros. This sequence is processed in the
73 first stage of FLT as the current @e run (substring).
75 Each stage works as follows.
77 At first, if the stage has a @c CATEGORY-TABLE, the category of each
78 glyph in the current run is updated. If there is a glyph that has no
79 category, the current run ends before that glyph.
81 Then, the default values of code-offset, combining-spec, and
82 left-padding-flag of this stage are initialized to zero.
84 Next, the initial conversion rule of the stage is applied to the
87 Lastly, the current run is replaced with the newly produced
88 (intermediate) glyph sequence.
90 @section flt-syntax SYNTAX and SEMANTICS
92 The m17n library loads an FLT from the m17n database using the tag
93 \<font, layouter, FLT-NAME\>. The date format of an FLT is as follows:
96 FONT-LAYOUT-TABLE ::= FLT-DECLARATION ? STAGE0 STAGE *
98 FLT-DECLARATION ::= '(' 'font' 'layouter' NAME nil PROP * ')'
100 PROP :: = VERSION | FONT
101 VERSION ::= '(' 'version' MTEXT ')'
102 FONT ::= '(' 'font' FONT-SPEC ')'
104 '(' [[ FOUNDRY FAMILY
105 [ WEIGHT [ STYLE [ STRETCH [ ADSTYLE ]]]]]
107 [ OTF-SPEC ] [ LANG-SPEC ] ')'
109 STAGE0 ::= CATEGORY-TABLE GENERATOR
111 STAGE ::= CATEGORY-TABLE ? GENERATOR
113 CATEGORY-TABLE ::= '(' 'category' CATEGORY-SPEC + ')'
115 CATEGORY-SPEC ::= '(' CODE CATEGORY ')'
116 | '(' CODE CODE CATEGORY ')'
123 In the definition of @c CATEGORY-SPEC, @c CODE is a glyph code, and @c
124 CATEGORY is ASCII code of an upper or lower letter, i.e. one of 'A',
125 ... 'Z', 'a', .. 'z'.
127 The first form of @c CATEGORY-SPEC assigns @c CATEGORY to a glyph
128 whose code @c CODE. The second form assigns @c CATEGORY to glyphs
129 whose code falls between the two @c CODEs.
132 GENERATOR ::= '(' 'generator' RULE MACRO-DEF * ')'
134 RULE ::= REGEXP-BLOCK | MATCH-BLOCK | SUBST-BLOCK | COND-BLOCK
135 FONT-FACILITY-BLOCK | DIRECT-CODE | COMBINING-SPEC | OTF-SPEC
136 | PREDEFINED-RULE | MACRO-NAME
138 MACOR-DEF ::= '(' MACRO-NAME RULE + ')'
141 Each @c RULE specifies glyphs to be consumed and glyphs to be
142 produced. When some glyphs are consumed, they are taken away from the
143 current run. A rule may fail in some condition. If not described
144 explicitly to fail, it should be regarded that the rule succeeds.
147 DIRECT-CODE ::= INTEGER
150 This rule consumes no glyph and produces a glyph which has the
151 following attributes:
154 <li> code : @c INTEGER plus the default code-offset
155 <li> combining-spec : default value
156 <li> left-padding-flag : default value
157 <li> right-padding-flag : zero
160 After having produced the glyph, the default code-offset,
161 combining-spec, and left-padding-flag are all reset to zero.
164 PREDEFINED-RULE ::= '=' | '*' | '<' | '>' | '|' | '[' | ']'
167 They perform actions as follows.
172 This rule consumes the first glyph in the current run and produces the
173 same glyph. It fails if the current run is empty.
177 This rule repeatedly executes the previous rule.
178 If the previous rule fails, this rule does nothing and fails.
182 This rule specifies the start of a grapheme cluster.
186 This rule specifies the end of a grapheme cluster.
190 This rule sets the default left-padding-flag to 1.
191 No glyph is consumed. No glyph is produced.
195 This rule changes the right-padding-flag of the lastly generated glyph
196 to 1. No glyph is consumed. No glyph is produced.
200 This rule consumes no glyph and produces a special glyph whose
201 category is ' ' and other attributes are zero. This is the only rule
202 that produces that special glyph.
207 REGEXP-BLOCK ::= '(' REGEXP RULE * ')'
212 @c MTEXT is a regular expression that should match the sequence of
213 categories of the current run. If a match is found, this rule
214 executes @c RULEs temporarily limiting the current run to the matched
215 part. The matched part is consumed by this rule.
217 Parenthesized subexpressions, if any, are recorded to be used in @c
218 MATCH-BLOCK that may appear in one of @c RULEs.
220 If no match is found, this rule fails.
223 MATCH-BLOCK ::= '(' MATCH-INDEX RULE * ')'
225 MATCH-INDEX ::= INTEGER
228 @c MATCH-INDEX is an integer specifying a parenthesized subexpression
229 recorded by the previous @c REGEXP-BLOCK. If such a subexpression was
230 found by the previous regular expression matching, this rule executes @c
231 RULEs temporarily limiting the current run to the matched part
232 of the subexpression. The matched part is consumed by this rule.
234 If no match was found, this rule fails.
236 If this is the first rule of the stage, @c MATCH-INDEX must be 0, and
237 it matches the whole current run.
240 SUBST-BLOCK ::= '(' SOURCE-PATTERN RULE * ')'
242 SOURCE-PATTERN ::= '(' CODE + ')'
243 | (' 'range' CODE CODE ')'
246 If the sequence of codes of the current run matches @c SOURCE-PATTERN,
247 this rule executes @c RULEs temporarily limiting the current run to
248 the matched part. The matched part is consumed.
250 The first form of @c SOURCE-PATTERN specifies a sequence of glyph codes to be
251 matched. In this case, this rule resets the default code-offset to
254 The second form specifies a range of codes that should match the first
255 glyph code of the code sequence. In this case, this rule sets the
256 default code-offset to the first glyph code minus the first @c CODE
257 specifying the range.
259 If no match is found, this rule fails.
262 FONT-FACILITY-BLOCK ::= '(' FONT-FACILITY RULE * ')'
263 FONT-FACILITY = '(' 'font-facility' CODE * ')'
264 | '(' 'font-facility' FONT-SPEC ')'
267 If the current font has glyphs for @c CODEs or matches with @c
268 FONT-SPEC, this rule succeeds and @c RULEs are executed. Otherwise,
272 COND-BLOCK ::= '(' 'cond' RULE + ')'
275 This rule sequentially executes @c RULEs until one succeeds. If no
276 rule succeeds, this rule fails. Otherwise, it succeeds.
282 @c OTF-SPEC is a symbol whose name specifies an instruction to the OTF
283 driver. The name has the following syntax.
286 OTF-SPEC-NAME ::= ':otf' SCRIPT LANGSYS ? GSUB-FEATURES ? GPOS-FEATURES ?
290 LANGSYS ::= '/' SYMBOL
292 GSUB-FEATURES ::= '=' FEATURE-LIST ?
294 GPOS-FEATURES ::= '+' FEATURE-LIST ?
296 FEATURE-LIST ::= ( SYMBOL ',' ) * [ SYMBOL | '*' ]
300 Each @c SYMBOL specifies a tag name defined in the OpenType
303 For @c SCRIPT, @c SYMBOL specifies a Script tag name (e.g. deva for
306 For @c LANGSYS, @c SYMBOL specifies a Language System tag name. If @c
307 LANGSYS is omitted, the Default Language System
310 For @c GSUB-FEATURES, each @c SYMBOL in @c FEATURE LIST specifies a
311 GSUB Feature tag name to apply. '*' is allowed as the last item to
312 specify all remaining features. If @c SYMBOL is preceded by '~' and
313 the last item is '*', @c SYMBOL is excluded from the features to
314 apply. If no @c SYMBOL is specified, no GSUB feature is applied. If
315 @c GSUB-FEATURES itself is omitted, all GSUB features are applied.
317 The specification of @c GPOS-FEATURES is analogous to that of @c
320 Please note that all the tags above must be 4 ASCII printable characters.
322 See the following page for the OpenType specification.\n
323 <http://www.microsoft.com/typography/otspec/default.htm>
329 @c COMBINING is a symbol whose name specifies how to combine the next
330 glyph with the previous one. This rule sets the default
331 combining-spec to an integer code that is unique to the symbol name.
332 The name has the following syntax.
335 COMBINING-NAME ::= VPOS HPOS OFFSET VPOS HPOS
337 VPOS ::= 't' | 'c' | 'b' | 'B'
339 HPOS ::= 'l' | 'c' | 'r'
341 OFFSET :: = '.' | XOFF | YOFF XOFF ?
343 XOFF ::= ('<' | '>') INTEGER ?
345 YOFF ::= ('+' | '-') INTEGER ?
348 @c VPOS and @c HPOS specify the vertical and horizontal positions
354 0----1----2 <---- top 0 t l
358 9 10 11 <---- center 4 B c
360 --3----4----5-- <-- baseline 6 b l
362 6----7----8 <---- bottom 8 b r
365 left center right 11 c r
368 The left figure shows 12 reference points of a glyph by numbers 0 to
369 11. The rectangle 0-6-8-2 is the bounding box of the glyph, the
370 positions 3, 4, and 5 are on the baseline, 9 and 11 are on the center
371 of the lines 0-6 and 2-8 respectively, 1, 10, 4, and 7 are on the
372 center of the lines 1-2, 3-5, 9-11, and 6-8 respectively.
374 The right table shows how those reference points are specified by a
375 pair of @c VPOS and @c HPOS.
377 The first @c VPOS and @c HPOS in the definition of @c COMBINING-NAME
379 reference point of the previous glyph, and the second @c VPOS and @c
380 HPOS specify that of the next glyph.
381 The next glyph is drawn so that these two reference points align.
383 @c OFFSET specifies the way of alignment in detail. If it is '.', the
384 reference points are on the same position.
386 @c XOFF specifies how much the X position of the reference point of
387 the next glyph should be shifted to the right ('<') or left ('>') from
388 the previous reference point.
390 @c YOFF specifies how much the Y position of the reference point the
391 next glyph should be shifted upward ('+') or downward ('-') from the
392 previous reference point.
394 In both cases, @c INTEGER is the amount of shift expressed as a
395 percentage of the font size, i.e., if @c INTEGER is 10, it means
396 10% (1/10) of the font size. If @c INTEGER is omitted, it is assumed that
399 Once the next glyph is combined with the previous one, they
400 are treated as a single combined glyph.
403 MACRO-NAME ::= SYMBOL
406 @c MACRO-NAME is a symbol that appears in one of @c MACRO-DEF. It is
407 exapanded to the sequence of the correponding @c RULEs.
409 @section flt-context-dependent CONTEXT DEPENDENT BEHAVIOR
411 So far, it has been assumed that each sequence, which is drawn with a
412 specific font, is context free, i.e. not affected by the glyphs
413 preceding or following that sequence. This is true when sequence S1
414 is drawn with font F1 while the preceding sequence S0 unconditionally
419 currently used font F0 F1
423 Sometimes, however, a clear separation of sequences is not possible.
424 Suppose that the preceding sequence S0 can be drawn not only with F0
429 currently used font F0 F1
430 usable font(s) F0,F1 F1
433 In this case, glyphs used to draw the preceding S0 may affect glyph
434 generation of S1. Therefore it is necessary to access information
435 about S0, which has already been processed, when processing S1.
436 Generation rules in the first stage (only in the first stage) accept a
437 special regular expression to access already processed parts.
443 @c RE0 and @c RE1 are regular expressions that match the preceding
444 sequence S0 and the following sequence S1, respectively.
446 Pay attention to the space between the two regular expressions. It
447 represents the special category ' ' (see above). Note that the
448 regular expression above belongs to glyph generation rules using font
449 F1, therefore not only RE1 but also RE0 must be expressed with the
450 categories for F1. This means when the preceding sequence S0 cannot
451 be expressed with the categories for F1 (as in the first example
452 above) generation rules having these patterns never match.
454 @section flt-seealso SEE ALSO
456 @ref mdbGeneral "mdbGeneral(5)",
457 @ref flt-list "FLTs provided by the m17n database"
461 Copyright (C) 2003, 2004
462 National Institute of Advanced Industrial Science and Technology (AIST)
463 Registration Number H15PRO112
465 This file is part of the m17n database; a sub-part of the m17n
468 The m17n library is free software; you can redistribute it and/or
469 modify it under the terms of the GNU Lesser General Public License
470 as published by the Free Software Foundation; either version 2.1 of
471 the License, or (at your option) any later version.
473 The m17n library is distributed in the hope that it will be useful,
474 but WITHOUT ANY WARRANTY; without even the implied warranty of
475 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
476 Lesser General Public License for more details.
478 You should have received a copy of the GNU Lesser General Public
479 License along with the m17n library; if not, write to the Free
480 Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
481 Boston, MA 02110-1301, USA.