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 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 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.
- supported TrueType GX tables
- fundamental validation limitations
- permissive error handling of broken GX tables
- `kern' table issue.
2. Supported tables
-------------------
The following GX tables are currently supported.
bsln
feat
just
kern(*)
lcar
mort
morx
opbd
prop
trak
The following GX tables are currently unsupported.
cvar
fdsc
fmtx
fvar
gvar
Zapf
The following GX tables won't be supported.
acnt(**)
hsty(***)
The following undocumented tables in TrueType fonts designed for Apple
platform aren't handled either.
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 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
kern: 59
lcar: 4
mort: 326
morx: 19
opbd: 4
prop: 114
trak: 16
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 information to the renderer, and finally
reaches the `end of text' state.
gxvalid can check essential errors like:
- 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 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,
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 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, 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 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.
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)
----------------------------------------------
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.
4-5. invalid feature number (117/183)
-------------------------------------
The GX/AAT extension can include 255 different layout features,
but popular layout features are predefined (see
https://developer.apple.com/fonts/TrueType-Reference-Manual/RM09/AppendixF.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 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, 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
the version number.
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 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. Available 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. Subtable 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 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 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 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
format 0: 14
* classic Microsoft dialect
format 0: 15
- 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 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.
------------------------------------------------------------------------
Copyright (C) 2004-2021 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
accept it fully.
--- end of README ---
