Commit 4814030bf696bf552c0ee8cb7ea00c62f46ff50e
2005-08-31T07:13:27
* src/gxvalid/README: Revised.
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diff --git a/ChangeLog b/ChangeLog
index f3d4c91..54ef5eb 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2005-08-30 Werner Lemberg <wl@gnu.org>
+
+ * src/gxvalid/README: Revised.
+
2005-08-29 Werner Lemberg <wl@gnu.org>
* include/freetype/freetype.h, include/freetype/ftchapters.h: Add
diff --git a/src/gxvalid/README b/src/gxvalid/README
index ac5ad63..a370cfc 100644
--- a/src/gxvalid/README
+++ b/src/gxvalid/README
@@ -1,60 +1,91 @@
- gxvalid: TrueType GX validator
- ==============================
+gxvalid: TrueType GX validator
+==============================
- 1. What is this
- ---------------
- "gxvalid" is a module to validate TrueType GX tables: a collection of
- additional tables in TrueType font which is used by "QuickDraw GX
- Text", Apple Advanced Typography (AAT). In addition, gxvalid can
- validates "kern" table which had been extended for AAT. Like otvalid,
- gxvalid uses Freetype2's validator framework(ftvalid).
+
+1. What is this
+---------------
+
+ `gxvalid' is a module to validate TrueType GX tables: a collection of
+ additional tables in TrueType font which are used by `QuickDraw GX
+ Text', Apple Advanced Typography (AAT). In addition, gxvalid can
+ validates `kern' tables which have been extended for AAT. Like the
+ otvalid module, gxvalid uses Freetype 2's validator framework
+ (ftvalid).
You can link gxvalid with your program; before running your own layout
- engine, gxvalid validates a font file. As the result, you can reduce
- error-checking code from the layout engine. You can use gxvalid as a
- stand-alone font validator; ftvalid command included in ft2demo calls
- gxvalid internally. Stand-alone font validator may be useful for font
- developers.
+ engine, gxvalid validates a font file. As the result, you can remove
+ error-checking code from the layout engine. It is also possible to
+ use gxvalid as a stand-alone font validator; the `ftvalid' test
+ program included in the ft2demo bundle calls gxvalid internally.
+ A stand-alone font validator may be useful for font developers.
+
+ This documents documents the following issues.
- This documents contains following informations:
- supported TrueType GX tables
- - validation limitation in principle
+ - fundamental validation limitations
- permissive error handling of broken GX tables
- - "kern" table issue.
+ - `kern' table issue.
+
+
+2. Supported tables
+-------------------
+
+ The following GX tables are currently supported.
+ bsln
+ feat
+ just
+ kern(*)
+ lcar
+ mort
+ morx
+ opbd
+ prop
+ trak
- 2. Supported tables
- -------------------
- Following GX tables are currently supported.
- bsln feat just kern(*) lcar mort morx opbd prop trak
+ The following GX tables are currently unsupported.
- Following GX tables are currently unsupported.
- cvar fdsc fmtx fvar gvar Zapf
+ cvar
+ fdsc
+ fmtx
+ fvar
+ gvar
+ Zapf
- Following GX tables won't be supported.
- acnt(**) hsty(***)
+ The following GX tables won't be supported.
- Undocumented tables in TrueType fonts designed for Apple platform.
- CVTM TPNM addg umif
+ acnt(**)
+ hsty(***)
- *) "kern" validator includes both of classic kern (format supported
- by both of Microsoft and Apple platforms) and new kern (a format
- supported by Apple platform only).
+ The following undocumented tables in TrueType fonts designed for Apple
+ platform aren't handled either.
- **) "acnt" tables is not supported by currently available Apple font
+ addg
+ CVTM
+ TPNM
+ umif
+
+
+ *) The `kern' validator handles both the classic and the new kern
+ formats; the former is supported on both Microsoft and Apple
+ platforms, while the latter is supported on Apple platforms.
+
+ **) `acnt' tables are not supported by currently available Apple font
tools.
- ***) There is one more Apple extension "hsty" but it is for Newton-OS,
- not GX (Newton-OS is a platform by Apple, but it can use sfnt-
- housed bitmap fonts only. Therefore, it should be excluded from
- "Apple platform" in the context of TrueType. gxvalid ignores it
- as Apple font tools do so.
-
- We have checked 183 fonts bundled to MacOS 9.1, MacOS 9.2, MacOS 10.0,
- MacOS X 10.1, MSIE for MacOS and AppleWorks 6.0. In addition, we have
- checked 67 Dynalab fonts (designed for MacOS) and 189 Ricoh fonts
- (designed for Windows and MacOS dual platforms). The number of fonts
- including TrueType GX tables are listed in following:
+ ***) There is one more Apple extension, `hsty', but it is for
+ Newton-OS, not GX (Newton-OS is a platform by Apple, but it can
+ use sfnt- housed bitmap fonts only). Therefore, it should be
+ excluded from `Apple platform' in the context of TrueType.
+ gxvalid ignores it as Apple font tools do so.
+
+
+ We have checked 183 fonts bundled with MacOS 9.1, MacOS 9.2, MacOS
+ 10.0, MacOS X 10.1, MSIE for MacOS, and AppleWorks 6.0. In addition,
+ we have checked 67 Dynalab fonts (designed for MacOS) and 189 Ricoh
+ fonts (designed for Windows and MacOS dual platforms). The number of
+ fonts including TrueType GX tables are as follows.
+
bsln: 76
feat: 191
just: 84
@@ -65,371 +96,425 @@
opbd: 4
prop: 114
trak: 16
- Dynalab and Ricoh fonts didn't have GX tables except of feat and mort.
-
- 3. Validation limitations in principle
- --------------------------------------
- TrueType GX provides layout information to font-rasterize/text-layout
- libraries. gxvalid can check whether layout information is stored as
- TrueType GX format specified by Apple. But gxvalid cannot check how
- QuickDraw GX/AAT renderer uses the stored information.
-
- 3-1. Validation of State Machine activity
- -----------------------------------------
- QuickDraw GX/AAT has "State Machine" to provide "stateful" layout
- features, and TrueType GX stores the state transition diagram of
- "State Machine" in "StateTable" data structure. While State Machine
- receives a series of glyph ID, State Machine starts from "start of
- text" state, walks around various states and generates various
- layout informations to renderer, and finally reaches to "end of
- text".
+
+ Dynalab and Ricoh fonts don't have GX tables except of `feat' and
+ `mort'.
+
+
+3. Fundamental validation limitations
+-------------------------------------
+
+ TrueType GX provides layout information to libraries for font
+ rasterizers and text layout. gxvalid can check whether the layout
+ data in a font is conformant to the TrueType GX format specified by
+ Apple. But gxvalid cannot check a how QuickDraw GX/AAT renderer uses
+ the stored information.
+
+ 3-1. Validation of State Machine activity
+ -----------------------------------------
+
+ QuickDraw GX/AAT uses a `State Machine' to provide `stateful' layout
+ features, and TrueType GX stores the state transition diagram of
+ this `State Machine' in a `StateTable' data structure. While the
+ State Machine receives a series of glyph IDs, the State Machine
+ starts with `start of text' state, walks around various states and
+ generates various layout informations to the renderer, and finally
+ reaches the `end of text' state.
gxvalid can check essential errors like:
- - possibility of state transition to undefined states
- - existence of glyph ID that State Machine doesn't know how to
- handle it
- - State Machine cannot compute the layout information from given
- diagram
- these errors can be checked within finite steps, and without State
- Machine itself, because these are errors of "expression" of state
+
+ - possibility of state transitions to undefined states
+ - existence of glyph IDs that the State Machine doesn't know how
+ to handle
+ - the State Machine cannot compute the layout information from
+ given diagram
+
+ These errors can be checked within finite steps, and without the
+ State Machine itself, because these are `expression' errors of state
transition diagram.
- There's no limitation about how long State Machine walks around, so
- validation of the algorithm in the state transition diagram requires
- infinite steps, even if we have State Machine in gxvalid. Therefore,
- following "errors" cannot be checked.
- - existence of states which State Machine never transits to.
- - possibility that State Machine never reaches to "end of text".
- - possibility of stack underflow/overflow in State Machine
- (in ligature and contextual glyph substitution, State Machine
- can store 16 glyphs onto its stack)
-
- In addition, gxvalid doesn't check "temporal glyph ID" used in the
- chained State Machines (in "mort" and "morx" tables). When a layout
- feature is implemented by single State Machine, glyph ID converted
- by State Machine is passed to the glyph renderer, thus it should not
- point to undefined glyph ID. But if a layout feature is implemented
- by chained State Machines, the component State Machine (if it is not
- final one) is permitted to generate undefined glyph ID for temporal
- use, because it is handled by next component State Machine, instead
- of the glyph renderer. To validate such temporal glyph ID, gxvalid
- must stack all undefined glyph IDs which is possible in the output
- of previous State Machine and search them in "ClassTable" of current
- State Machine. It is too complexed work to list all possible glyph
- IDs from StateTable, especially from ligature substitution table.
-
- 3-2. Validation of relationship among multiple layout features
- --------------------------------------------------------------
- gxvalid does not validate the relationship among multiple layout
+ There is no limitation about how long the State Machine walks
+ around, so validation of the algorithm in the state transition
+ diagram requires infinite steps, even if we had a State Machine in
+ gxvalid. Therefore, the following errors and problems cannot be
+ checked.
+
+ - existence of states which the State Machine never transits to
+ - the possibility that the State Machine never reaches `end of
+ text'
+ - the possibility of stack underflow/overflow in the State Machine
+ (in ligature and contextual glyph substitutions, the State
+ Machine can store 16 glyphs onto its stack)
+
+ In addition, gxvalid doesn't check `temporary glyph IDs' used in the
+ chained State Machines (in `mort' and `morx' tables). If a layout
+ feature is implemented by a single State Machine, a glyph ID
+ converted by the State Machine is passed to the glyph renderer, thus
+ it should not point to an undefined glyph ID. But if a layout
+ feature is implemented by chained State Machines, a component State
+ Machine (if it is not the final one) is permitted to generate
+ undefined glyph IDs for temporary use, because it is handled by next
+ component State Machine and not by the glyph renderer. To validate
+ such temporary glyph IDs, gxvalid must stack all undefined glyph IDs
+ which can occur in the output of the previous State Machine and
+ search them in the `ClassTable' structure of the current State
+ Machine. It is too complex to list all possible glyph IDs from the
+ StateTable, especially from a ligature substitution table.
+
+ 3-2. Validation of relationship between multiple layout features
+ ----------------------------------------------------------------
+
+ gxvalid does not validate the relationship between multiple layout
features at all.
- If multiple layout features are defined in TrueType GX tables, the
- interactivity, overriding, and conflict among layout features are
- defined in the font too. For example, there are several predefined
- spacing control features:
+ If multiple layout features are defined in TrueType GX tables,
+ possible interactions, overrides, and conflicts between layout
+ features are implicitly given in the font too. For example, there
+ are several predefined spacing control features:
+
- Text Spacing (Proportional/Monospace/Half-width/Normal)
- Number Spacing (Monospaced-numbers/Proportional-numbers)
- Kana Spacing (Full-width/Proportional)
- Ideographic Spacing (Full-width/Proportional)
- CJK Roman Spacing (Half-width/Proportional/Default-roman
/Full-width-roman/Proportional)
- If all layout features are independently managed, we can set an
- inconsistent typographic rule, as like "Text Spacing=Monospace" and
- "Ideographic Spacing=Proportional", at the same time.
-
- The combination of each layout feature is managed by 32bit integer
- (1 bit for 1 selector setting), so we can define relationship among
- features up to 32 settings, theoretically. But if setting of
- a feature affects setting of another features, typographic priority
- of each layout feature is required to validate the relationship.
+
+ If all layout features are independently managed, we can activate
+ inconsistent typographic rules like `Text Spacing=Monospace' and
+ `Ideographic Spacing=Proportional' at the same time.
+
+ The combinations of layout features is managed by a 32bit integer
+ (one bit each for selector setting), so we can define relationships
+ between up to 32 features, theoretically. But if one feature
+ setting affects another feature setting, we need typographic
+ priority rules to validate the relationship. Unfortunately, the
TrueType GX format specification does not give such information even
for predefined features.
- 4. Permissive error handling of broken GX tables
- ------------------------------------------------
- When Apple's font rendering system finds an inconsistency, violation
- of specification or unspecified value in TrueType GX tables, they do
- not always return error. In most case, they silently ignore such wrong
- values or whole of table. In fact, MacOS is shipped with fonts
- including broken GX/AAT tables, but no harmful effects due to
- officially broken fonts are observed by end-users.
-
- gxvalid is designed to continue its validation as long as possible.
- When gxvalid find wrong value, gxvalid warns it at least, and take a
- fallback procedure if possible. The fallback procedure depends on the
- debug level.
-
- We used following 3 tools to refer Apple's error handling.
+
+4. Permissive error handling of broken GX tables
+------------------------------------------------
+
+ When Apple's font rendering system finds an inconsistency, like a
+ specification violation or an unspecified value in a TrueType GX
+ table, it does not always return error. In most cases, the rendering
+ engine silently ignores such wrong values or even whole tables. In
+ fact, MacOS is shipped with fonts including broken GX/AAT tables, but
+ no harmful effects due to `officially broken' fonts are observed by
+ end-users.
+
+ gxvalid is designed to continue the validation process as long as
+ possible. When gxvalid find wrong values, gxvalid warns it at least,
+ and takes a fallback procedure if possible. The fallback procedure
+ depends on the debug level.
+
+ We used the following three tools to investigate Apple's error handling.
+
- FontValidator (for MacOS 8.5 - 9.2) resource fork font
- ftxvalidator (for MacOS X 10.1 -) dfont or naked-sfnt
- ftxdumperfuser (for MacOS X 10.1 -) dfont or naked-sfnt
- However, all tests are on PowerPC based Macintosh, we have not tested
- on m68k-based Macintosh at all, at present.
- We checked 183 fonts bundled to MacOS 9.1, MacOS 9.2, MacOS 10.0,
- MacOS X 10.1, MSIE for MacOS and AppleWorks 6.0. These fonts are
- distributed officially, but many broken GX/AAT tables are found by
- Apple's font tools. In following, we list typical violation against GX
- specification, in Apple official fonts. At least, gxvalid warns them,
- and fallback method to continue
+ However, all tests were done on a PowerPC based Macintosh; at present,
+ we have not checked those tools on a m68k-based Macintosh.
+
+ In total, we checked 183 fonts bundled to MacOS 9.1, MacOS 9.2, MacOS
+ 10.0, MacOS X 10.1, MSIE for MacOS, and AppleWorks 6.0. These fonts
+ are distributed officially, but many broken GX/AAT tables were found
+ by Apple's font tools. In the following, we list typical violation of
+ the GX specification, in fonts officially distributed with those Apple
+ systems.
+
+ 4-1. broken BinSrchHeader (19/183)
+ ----------------------------------
+
+ `BinSrchHeader' is a header of a data array for m68k platforms to
+ access memory efficiently. Although there are only two independent
+ parameters for real (`unitSize' and `nUnits'), BinSrchHeader has
+ three additional parameters which can be calculated from `unitSize'
+ and `nUnits', for fast setup. Apple font tools ignore them
+ silently, so gxvalid warns if it finds and inconsistency, and always
+ continues validation. The additional parameters are ignored
+ regardless of the consistency.
+
+ 19 fonts include such inconsistencies; all breaks are in the
+ BinSrchHeader structure of the `kern' table.
+
+ 4-2. too-short LookupTable (5/183)
+ ----------------------------------
+
+ LookupTable format 0 is a simple array to get a value from a given
+ GID (glyph ID); the index of this array is a GID too. Therefore,
+ the length of the array is expected to be same as the maximum GID
+ value defined in the `maxp' table, but there are some fonts whose
+ LookupTable format 0 is too short to cover all GIDs. FontValidator
+ ignores this error silently, ftxvalidator and ftxdumperfuser both
+ warn and continue. Similar problems are found in format 3 subtables
+ of `kern'. gxvalid warns always and abort if the validation level
+ is set to FT_VALIDATE_PARANOID.
+
+ 5 fonts include too-short kern format 0 subtables.
+ 1 font includes too-short kern format 3 subtable.
+
+ 4-3. broken LookupTable format 2 (1/183)
+ ----------------------------------------
+
+ LookupTable format 2, subformat 4 covers the GID space by a
+ collection of segments which are specified by `firstGlyph' and
+ `lastGlyph'. Some fonts store `firstGlyph' and `lastGlyph' in
+ reverse order, so the segment specification is broken. Apple font
+ tools ignore this error silently; a broken segment is ignored as if
+ it did not exist. gxvalid warns and normalize the segment at
+ FT_VALIDATE_DEFAULT, or ignore the segment at FT_VALIDATE_TIGHT, or
+ abort at FT_VALIDATE_PARANOID.
- 4-1. broken BinSrchHeader ( 19/183)
- -----------------------------------
- BinSrchHeader is a header of data array, for m68k platform to access
- memory effectively. Although independent parameters for real use are
- only 2 (unitSize and nUnits), BinSrchHeader has 3 additional
- parameters which can be calculated from unitSize and nUnits, for
- fast setup. Apple font tools ignore them silently, so gxvalid warns
- inconsistency and always continues validation. The additional
- parameters are ignored regardless of the consistency.
-
- 19 fonts include inconsistent with calculated values
- all breaks are in BinSrchHeader of "kern" table.
-
- 4-2. too-short LookupTable ( 5/183)
- -----------------------------------
- LookupTable format 0 is simple array to get a value from given GID,
- the index of array is GID. Therefore, the length of array is
- expected to be same with max GID defined in "maxp" table, but there
- is some fonts whose LookupTable format 0 is too short to cover all
- GID. FontValidator ignores this error silently, ftxvalidator and
- ftxdumperfuser warns and continues. Similar shortage is found in
- format 3 subtable of "kern".
- gxvalid warns always and abort at FT_VALIDATE_PARANOID.
-
- 5 fonts include too-short kern format 0 subtables.
- 1 font includes too-short kern format 3 subtable.
-
-
- 4-3. broken LookupTable format 2 ( 1/183)
- -----------------------------------------
- LookupTable format 2, 4 covers GID space by collection of segments
- which specified by firstGlyph and lastGlyph. Some fonts stores
- firstGlyph and lastGlyph in reverse order, so segment specification
- is broken. Apple font tools ignores this error silently, broken
- segment is ignored as if it did not exist. gxvalid warns and
- normalize the segment at FT_VALIDATE_DEFAULT, or ignore the segment
- at FT_VALIDATE_TIGHT, or abort at FT_VALIDATE_PARANOID.
-
- 1 font includes broken LookupTable format 2, in "just" table.
-
- *) It seems that all fonts manufactured by ITC for AppleWorks have
+ 1 font includes broken LookupTable format 2, in the `just' table.
+
+ *) It seems that all fonts manufactured by ITC for AppleWorks have
this error.
+ 4-4. bad bracketing in glyph property (14/183)
+ ----------------------------------------------
- 4-4. bad bracketing in glyph property ( 14/183)
- -----------------------------------------------
- GX/AAT defines bracketing property of the glyphs by "prop" table, to
- control layout functionalities for string closed in brackets and out
- of brackets. Some fonts give inappropriate bracket properties to
- glyphs. Apple font tools warn this error. gxvalid warns always and
- abort at FT_VALIDATE_PARANOID.
+ GX/AAT defines a `bracketing' property of the glyphs in the `prop'
+ table, to control layout features of strings enclosed inside and
+ outside of brackets. Some fonts give inappropriate bracket
+ properties to glyphs. Apple font tools warn about this error;
+ gxvalid warns too and aborts at FT_VALIDATE_PARANOID.
+
+ 14 fonts include wrong bracket properties.
- 14 fonts include wrong bracket properties.
+ 4-5. invalid feature number (117/183)
+ -------------------------------------
- 4-5. invalid feature number (117/183)
- -------------------------------------
- GX/AAT extension can include 255 different features for layout, but
- popular layout features are predefined
- (see http://developer.apple.com/fonts/Registry/index.html).
- Some fonts include feature number which is incompatible with
- predefined feature registry.
+ The GX/AAT extension can include 255 different layout features, but
+ popular layout features are predefined (see
+ http://developer.apple.com/fonts/Registry/index.html). Some fonts
+ include feature numbers which are incompatible with the predefined
+ feature registry.
- In our survey, there are 140 fonts including "feat" table.
- a) 67 fonts uses feature number which should not be used.
- b) 117 fonts set wrong feature range (nSetting).
- this infraction is found in mort/morx.
+ In our survey, there are 140 fonts including `feat' table.
- Apple font tools gives no warning, although they cannot recognize
- what the feature is. At FT_VALIDATE_DEFAULT, gxvalid warns but
+ a) 67 fonts use a feature number which should not be used.
+ b) 117 fonts set the wrong feature range (nSetting). This is mostly
+ found in the `mort' and `morx' tables.
+
+ Apple font tools give no warning, although they cannot recognize
+ what the feature is. At FT_VALIDATE_DEFAULT, gxvalid warns but
continues in both cases (a, b). At FT_VALIDATE_TIGHT, gxvalid warns
and aborts for (a), but continues for (b). At FT_VALIDATE_PARANOID,
gxvalid warns and aborts in both cases (a, b).
- 4-6. invalid prop version ( 10/183)
- -----------------------------------
- As most TrueType GX tables, prop table must start with 32bit
- version: 0x00010000, 0x00020000 or 0x00030000. But some fonts store
- nonsense binary data in it. When Apple font tools find them, they
- abort the processing at once, and following data are unhandled.
- gxvalid does same always.
-
- 10 fonts include broken prop version.
-
- All of these fonts are classic TrueType for Japanese script,
- manufactured by Apple.
-
- 4-7. unknown resource name ( 2/183)
- ------------------------------------
- NOTE: THIS IS NOT TRUETYPE GX ERROR
- When TrueType font is stored in resource fork or dfont format,
- the data must be tagged as "sfnt" in resource fork index, to invoke
- TrueType font handler for the data. But the TrueType font data in
- "Keyboard.dfont" is tagged as "kbd", and that in "LastResort.dfont"
- is tagged as "lst". Apple font tools can detect the data is of
- TrueType and successfully validate them. Possibly this because they
- are known to be dfont. Current implementation of resource fork
- driver of FreeType cannot do that, thus gxvalid cannot validate them.
-
- 2 fonts use unknown tag for TrueType font resource.
-
- 5. "kern" table issue
- ---------------------
- In common terminology of TrueType, "kern" is classified to basic and
- platform-independent table. But there are Apple extensions of kern,
- and there is an extension which requires GX state machine for
- contextual kerning. Therefore, gxvalid includes validator for kern.
- Unfortunately, there is no exact algorithm to check Apple's extension,
- so gxvalid includes pragmatic detector of data format and validator
- for all possible data formats, including data format for Microsoft.
- By calling classic_kern_validate() instead of gxv_validate(), you can
- specify available "kern" format explicitly. However, current FreeType2
- uses Microsoft "kern" format only, others are ignored.
-
- 5-1. History
- ------------
- Original 16bit version of "kern" had been designed by Apple in pre-
- GX era, and it was also approved by Microsoft. Afterwards, Apple has
- designed new 32bit version "kern". Apple has noted as the difference
- between 16bit and 32bit version is only the size of variables in
- "kern" header. In following, we call the original 16bit version as
- "classic", and 32bit version as "new".
-
- 5-2. Versions and dialects which should be discriminated
- --------------------------------------------------------
- The "kern" table consists of the table header and several subtables.
- The version "classic" or "new" is explicitly written in the table
- header, but there are undocumented difference of font parser between
- Microsoft and Apple. It is called as "dialect" in following.
- There are 3 cases which should be discriminated: new Apple-dialect,
- classic Apple-dialect, and classic Microsoft-dialect. Analysis and
- auto detection algorithm of gxvalid is described in following.
-
- 5-2-1. Version detection: classic and new kern
- ----------------------------------------------
- According to Apple TrueType specification, the clarified
- difference between classic and new version are only 2:
- - "kern" table header starts with the version number.
+ 4-6. invalid prop version (10/183)
+ ----------------------------------
+
+ As most TrueType GX tables, the `prop' table must start with a 32bit
+ version identifier: 0x00010000, 0x00020000 or 0x00030000. But some
+ fonts store nonsense binary data instead. When Apple font tools
+ find them, they abort the processing immediately, and the data which
+ follows is unhandled. gxvalid does the same.
+
+ 10 fonts include broken `prop' version.
+
+ All of these fonts are classic TrueType fonts for the Japanese
+ script, manufactured by Apple.
+
+ 4-7. unknown resource name (2/183)
+ ------------------------------------
+
+ NOTE: THIS IS NOT A TRUETYPE GX ERROR.
+
+ If a TrueType font is stored in the resource fork or in dfont
+ format, the data must be tagged as `sfnt' in the resource fork index
+ to invoke TrueType font handler for the data. But the TrueType font
+ data in `Keyboard.dfont' is tagged as `kbd', and that in
+ `LastResort.dfont' is tagged as `lst'. Apple font tools can detect
+ that the data is in TrueType format and successfully validate them.
+ Maybe this is possible because they are known to be dfont. The
+ current implementation of the resource fork driver of FreeType
+ cannot do that, thus gxvalid cannot validate them.
+
+ 2 fonts use an unknown tag for the TrueType font resource.
+
+5. `kern' table issues
+----------------------
+
+ In common terminology of TrueType, `kern' is classified as a basic and
+ platform-independent table. But there are Apple extensions of `kern',
+ and there is an extension which requires a GX state machine for
+ contextual kerning. Therefore, gxvalid includes a special validator
+ for `kern' tables. Unfortunately, there is no exact algorithm to
+ check Apple's extension, so gxvalid includes a heuristic algorithm to
+ find the proper validation routines for all possible data formats,
+ including the data format for Microsoft. By calling
+ classic_kern_validate() instead of gxv_validate(), you can specify the
+ `kern' format explicitly. However, current FreeType2 uses Microsoft
+ `kern' format only, others are ignored (and should be handled in a
+ library one level higher than FreeType).
+
+ 5-1. History
+ ------------
+
+ The original 16bit version of `kern' was designed by Apple in the
+ pre-GX era, and it was also approved by Microsoft. Afterwards,
+ Apple designed a new 32bit version of the `kern' table. According
+ to the documentation, the difference between the 16bit and 32bit
+ version is only the size of variables in the `kern' header. In the
+ following, we call the original 16bit version as `classic', and
+ 32bit version as `new'.
+
+ 5-2. Versions and dialects which should be differentiated
+ ---------------------------------------------------------
+
+ The `kern' table consists of a table header and several subtables.
+ The version number which identifies a `classic' or a `new' version
+ is explicitly written in the table header, but there are
+ undocumented differences between Microsoft's and Apple's formats.
+ It is called a `dialect' in the following. There are three cases
+ which should be handled: the new Apple-dialect, the classic
+ Apple-dialect, and the classic Microsoft-dialect. An analysis of
+ the formats and the auto detection algorithm of gxvalid is described
+ in the following.
+
+ 5-2-1. Version detection: classic and new kern
+ ----------------------------------------------
+
+ According to Apple TrueType specification, there are only two
+ differences between the classic and the new:
+
+ - The `kern' table header starts with the version number.
The classic version starts with 0x0000 (16bit),
the new version starts with 0x00010000 (32bit).
- - In the "kern" table header, the number of subtables follows to
+
+ - In the `kern' table header, the number of subtables follows
the version number.
- In the classic version, it is stored in 16bit variable.
- In the new version, it is stored in 32bit variable.
+ In the classic version, it is stored as a 16bit value.
+ In the new version, it is stored as a 32bit value.
From Apple font tool's output (DumpKERN is also tested in addition
- to 3 Apple font tools in above), there is another undocumented
- difference. In new version, the subtable header includes a 16bit
- variable named "tupleIndex" which does not exist in the classic
- version.
-
- New version can store all subtable formats (0, 1, 2 and 3), but
- Apple TrueType specification does not mention about subtable
- formats available in classic version.
-
-
- 5-2-2. Avaibale subtable format in classic version
- --------------------------------------------------
- Although Apple TrueType specification recommends to use classic
- version in the case if the font is designed for both of Apple and
- Microsoft platforms, it does not note about the available subtable
- formats in classic version.
-
- According to Microsoft TrueType specification, the subtable format
- assured for Windows & OS/2 support is only subtable format 0. Also
- Microsoft TrueType specification describes the subtable format 2,
- but does not mention about which platforms support it. About
- subtable format 1, 3 and later are noted as reserved for future
- use. Therefore, the classic version can store subtable formats 0
- and 2, at least. ttfdump.exe, a font tool provided by Microsoft
- ignores the subtable format written in the subtable header, and
- parse as if all subtables are in format 0.
-
- kern subtable format 1 uses StateTable, so it cannot be utilized
- without GX State Machine. Therefore, it is reasonable to assume
- format 1 (and 3) is introduced after Apple have introduced GX and
- moved to new 32bit version.
-
- 5-2-3. Apple and Microsoft dialects
- -----------------------------------
- The kern subtable has 16bit "coverage" to describe kerning
- attributions, but bit-interpretations by Apple and Microsoft are
- reverse ordered:
- e.g. Apple-dialect writes subtable format from 0x000F bit range,
- Microsoft-dialect writes subtable format from 0x0F00 bit range).
-
- In addition, from the outputs of DumpKERN and FontValidator,
- Apple's bit-interpretations of coverage in classic and new version
- are incompatible. In summary, there are 3 dialects: classic Apple-
- dialect, classic Microsoft-dialect, and new Apple-dialect.
- The classic Microsoft-dialect and new Apple-dialect are documented
- by each vendors' TrueType font specification, but the document for
- classic Apple-dialect had been lost.
+ to the three Apple font tools in above), there is another
+ undocumented difference. In the new version, the subtable header
+ includes a 16bit variable named `tupleIndex' which does not exist
+ in the classic version.
+
+ The new version can store all subtable formats (0, 1, 2, and 3),
+ but the Apple TrueType specification does not mention the subtable
+ formats available in the classic version.
+
+ 5-2-2. Avaibale subtable formats in classic version
+ ---------------------------------------------------
+
+ Although the Apple TrueType specification recommends to use the
+ classic version in the case if the font is designed for both the
+ Apple and Microsoft platforms, it does not document the available
+ subtable formats in the classic version.
+
+ According to the Microsoft TrueType specification, the subtable
+ format assured for Windows and OS/2 support is only subtable
+ format 0. The Microsoft TrueType specification also describes
+ subtable format 2, but does not mention which platforms support
+ it. Aubtable formats 1, 3, and higher are documented as reserved
+ for future use. Therefore, the classic version can store subtable
+ formats 0 and 2, at least. `ttfdump.exe', a font tool provided by
+ Microsoft, ignores the subtable format written in the subtable
+ header, and parses the table as if all subtables are in format 0.
+
+ `kern' subtable format 1 uses a StateTable, so it cannot be
+ utilized without a GX State Machine. Therefore, it is reasonable
+ to assume that format 1 (and 3) were introduced after Apple had
+ introduced GX and moved to the new 32bit version.
+
+ 5-2-3. Apple and Microsoft dialects
+ -----------------------------------
+
+ The `kern' subtable has a 16bit `coverage' field to describe
+ kerning attributes, but bit interpretations by Apple and Microsoft
+ are different: For example, Apple uses bits 0-7 to identify the
+ subtable, while Microsoft uses bits 8-15.
+
+ In addition, due to the output of DumpKERN and FontValidator,
+ Apple's bit interpretations of coverage in classic and new version
+ are incompatible also. In summary, there are three dialects:
+ classic Apple dialect, classic Microsoft dialect, and new Apple
+ dialect. The classic Microsoft dialect and the new Apple dialect
+ are documented by each vendors' TrueType font specification, but
+ the documentation for classic Apple dialect is not available.
- For example, in new Apple-dialect, the bit 0x8000 is documented as
- "set to 1 when the kerning is vertical". On the other hand, in
- classic Microsoft-dialect, the bit 0x0001 is documented as "set to
- 1 when the kerning is horizontal". From the outputs of DumpKERN
- and FontValidator, classic Apple-dialect recognizes the bit 0x8000
- as "set to 1 when the kerning is horizontal". From the results of
- similar experiments, classic Apple-dialect is ein ndian-reverse of
- classic Microsoft-dialect.
-
- It must be noted: no font tool can sense classic Apple-dialect or
- classic-Microsoft dialect automatically.
-
- 5-2-4. gxvalid auto dialect detection algorithm
- -----------------------------------------------
- The first 16bit of kern table is enough to sense the version:
- - if first 16bit is 0x0000,
- kern table is in classic Apple-dialect
- or classic Microsoft-dialect,
- - if first 16bit is 0x0001, and next 16bit is 0x0000,
- kern table is in new Apple-dialect.
- If kern table is classic version, 16bit coverage is checked for in
- next. For first, the coverage is decoded by classic Apple-dialect
- as following (it is based on DumpKERN output):
+ For example, in the new Apple dialect, bit 15 is documented as
+ `set to 1 if the kerning is vertical'. On the other hand, in
+ classic Microsoft dialect, bit 1 is documented as `set to 1 if the
+ kerning is horizontal'. From the outputs of DumpKERN and
+ FontValidator, classic Apple dialect recognizes 15 as `set to 1
+ when the kerning is horizontal'. From the results of similar
+ experiments, classic Apple dialect seems to be the Endian reverse
+ of the classic Microsoft dialect.
+
+ As a conclusion it must be noted that no font tool can identify
+ classic Apple dialect or classic Microsoft dialect automatically.
+
+ 5-2-4. gxvalid auto dialect detection algorithm
+ -----------------------------------------------
+
+ The first 16 bits of the `kern' table are enough to identify the
+ version:
+
+ - if the first 16 bits are 0x0000, the `kern' table is in
+ classic Apple dialect or classic Microsoft dialect
+ - if the first 16 bits are 0x0001, and next 16 bits are 0x0000,
+ the kern table is in new Apple dialect.
+
+ If the `kern' table is a classic one, the 16bit `coverage' field
+ is checked next. Firstly, the coverage bits are decoded for the
+ classic Apple dialect using the following bit masks (this is based
+ on DumpKERN output):
+
0x8000: 1=horizontal, 0=vertical
0x4000: not used
0x2000: 1=cross-stream, 0=normal
0x1FF0: reserved
0x000F: subtable format
- If any of reserved bits are set or subtable format is
- interpreted as 1 or 3, we take it as "impossible in classic
- Apple-dialect", and retry by classic Microsoft-dialect.
+
+ If any of reserved bits are set or the subtable bits is
+ interpreted as format 1 or 3, we take it as `impossible in classic
+ Apple dialect' and retry, using the classic Microsoft dialect.
+
The most popular coverage in new Apple-dialect: 0x8000,
The most popular coverage in classic Apple-dialect: 0x0000,
The most popular coverage in classic Microsoft dialect: 0x0001.
- 5-3. Tested fonts
- -----------------
- We checked 59 fonts bundled to MacOS which includes kern, and
- 38 fonts bundled to Windows which includes kern.
- - fonts bundled to MacOS
- * new Apple-dialect
+ 5-3. Tested fonts
+ -----------------
+
+ We checked 59 fonts bundled with MacOS and 38 fonts bundled with
+ Windows, where all font include a `kern' table.
+
+ - fonts bundled with MacOS
+ * new Apple dialect
format 0: 18
format 2: 1
format 3: 1
- * classic Apple-dialect
+ * classic Apple dialect
format 0: 14
- * classic Microsoft-dialect
+ * classic Microsoft dialect
format 0: 15
- - fonts bundled to Windows
- * classic Microsoft-dialect
+
+ - fonts bundled with Windows
+ * classic Microsoft dialect
format 0: 38
+
It looks strange that classic Microsoft-dialect fonts are bundled to
MacOS: they come from MSIE for MacOS, except of MarkerFelt.dfont.
ACKNOWLEDGEMENT
---------------
- Some part of gxvalid is derived from both gxlayout module and otvalid
- module. Development of gxlayout was support of Information-technology
- Promotion Agency(IPA), Japan.
- The detailed analysis of undefined glyph ID utilization in mort, morx
- is provided by George Williams.
+ Some parts of gxvalid are derived from both the `gxlayout' module and
+ the `otvalid' module. Development of gxlayout was supported by the
+ Information-technology Promotion Agency(IPA), Japan.
+
+ The detailed analysis of undefined glyph ID utilization in `mort' and
+ `morx' tables is provided by George Williams.
------------------------------------------------------------------------
@@ -437,10 +522,10 @@ Copyright 2004, 2005 by
suzuki toshiya, Masatake YAMATO, Red hat K.K.,
David Turner, Robert Wilhelm, and Werner Lemberg.
-This file is part of the FreeType project, and may only be used,
-modified, and distributed under the terms of the FreeType project
-license, LICENSE.TXT. By continuing to use, modify, or distribute this
-file you indicate that you have read the license and understand and
+This file is part of the FreeType project, and may only be used,
+modified, and distributed under the terms of the FreeType project
+license, LICENSE.TXT. By continuing to use, modify, or distribute this
+file you indicate that you have read the license and understand and
accept it fully.