1gxvalid: TrueType GX validator 2============================== 3 4 51. What is this 6--------------- 7 8 `gxvalid' is a module to validate TrueType GX tables: a collection of 9 additional tables in TrueType font which are used by `QuickDraw GX 10 Text', Apple Advanced Typography (AAT). In addition, gxvalid can 11 validates `kern' tables which have been extended for AAT. Like the 12 otvalid module, gxvalid uses FreeType 2's validator framework 13 (ftvalid). 14 15 You can link gxvalid with your program; before running your own layout 16 engine, gxvalid validates a font file. As the result, you can remove 17 error-checking code from the layout engine. It is also possible to 18 use gxvalid as a stand-alone font validator; the `ftvalid' test 19 program included in the ft2demo bundle calls gxvalid internally. 20 A stand-alone font validator may be useful for font developers. 21 22 This documents documents the following issues. 23 24 - supported TrueType GX tables 25 - fundamental validation limitations 26 - permissive error handling of broken GX tables 27 - `kern' table issue. 28 29 302. Supported tables 31------------------- 32 33 The following GX tables are currently supported. 34 35 bsln 36 feat 37 just 38 kern(*) 39 lcar 40 mort 41 morx 42 opbd 43 prop 44 trak 45 46 The following GX tables are currently unsupported. 47 48 cvar 49 fdsc 50 fmtx 51 fvar 52 gvar 53 Zapf 54 55 The following GX tables won't be supported. 56 57 acnt(**) 58 hsty(***) 59 60 The following undocumented tables in TrueType fonts designed for Apple 61 platform aren't handled either. 62 63 addg 64 CVTM 65 TPNM 66 umif 67 68 69 *) The `kern' validator handles both the classic and the new kern 70 formats; the former is supported on both Microsoft and Apple 71 platforms, while the latter is supported on Apple platforms. 72 73 **) `acnt' tables are not supported by currently available Apple font 74 tools. 75 76 ***) There is one more Apple extension, `hsty', but it is for 77 Newton-OS, not GX (Newton-OS is a platform by Apple, but it can 78 use sfnt- housed bitmap fonts only). Therefore, it should be 79 excluded from `Apple platform' in the context of TrueType. 80 gxvalid ignores it as Apple font tools do so. 81 82 83 We have checked 183 fonts bundled with MacOS 9.1, MacOS 9.2, MacOS 84 10.0, MacOS X 10.1, MSIE for MacOS, and AppleWorks 6.0. In addition, 85 we have checked 67 Dynalab fonts (designed for MacOS) and 189 Ricoh 86 fonts (designed for Windows and MacOS dual platforms). The number of 87 fonts including TrueType GX tables are as follows. 88 89 bsln: 76 90 feat: 191 91 just: 84 92 kern: 59 93 lcar: 4 94 mort: 326 95 morx: 19 96 opbd: 4 97 prop: 114 98 trak: 16 99 100 Dynalab and Ricoh fonts don't have GX tables except of `feat' and 101 `mort'. 102 103 1043. Fundamental validation limitations 105------------------------------------- 106 107 TrueType GX provides layout information to libraries for font 108 rasterizers and text layout. gxvalid can check whether the layout 109 data in a font is conformant to the TrueType GX format specified by 110 Apple. But gxvalid cannot check a how QuickDraw GX/AAT renderer uses 111 the stored information. 112 113 3-1. Validation of State Machine activity 114 ----------------------------------------- 115 116 QuickDraw GX/AAT uses a `State Machine' to provide `stateful' layout 117 features, and TrueType GX stores the state transition diagram of 118 this `State Machine' in a `StateTable' data structure. While the 119 State Machine receives a series of glyph IDs, the State Machine 120 starts with `start of text' state, walks around various states and 121 generates various layout information to the renderer, and finally 122 reaches the `end of text' state. 123 124 gxvalid can check essential errors like: 125 126 - possibility of state transitions to undefined states 127 - existence of glyph IDs that the State Machine doesn't know how 128 to handle 129 - the State Machine cannot compute the layout information from 130 given diagram 131 132 These errors can be checked within finite steps, and without the 133 State Machine itself, because these are `expression' errors of state 134 transition diagram. 135 136 There is no limitation about how long the State Machine walks 137 around, so validation of the algorithm in the state transition 138 diagram requires infinite steps, even if we had a State Machine in 139 gxvalid. Therefore, the following errors and problems cannot be 140 checked. 141 142 - existence of states which the State Machine never transits to 143 - the possibility that the State Machine never reaches `end of 144 text' 145 - the possibility of stack underflow/overflow in the State Machine 146 (in ligature and contextual glyph substitutions, the State 147 Machine can store 16 glyphs onto its stack) 148 149 In addition, gxvalid doesn't check `temporary glyph IDs' used in the 150 chained State Machines (in `mort' and `morx' tables). If a layout 151 feature is implemented by a single State Machine, a glyph ID 152 converted by the State Machine is passed to the glyph renderer, thus 153 it should not point to an undefined glyph ID. But if a layout 154 feature is implemented by chained State Machines, a component State 155 Machine (if it is not the final one) is permitted to generate 156 undefined glyph IDs for temporary use, because it is handled by next 157 component State Machine and not by the glyph renderer. To validate 158 such temporary glyph IDs, gxvalid must stack all undefined glyph IDs 159 which can occur in the output of the previous State Machine and 160 search them in the `ClassTable' structure of the current State 161 Machine. It is too complex to list all possible glyph IDs from the 162 StateTable, especially from a ligature substitution table. 163 164 3-2. Validation of relationship between multiple layout features 165 ---------------------------------------------------------------- 166 167 gxvalid does not validate the relationship between multiple layout 168 features at all. 169 170 If multiple layout features are defined in TrueType GX tables, 171 possible interactions, overrides, and conflicts between layout 172 features are implicitly given in the font too. For example, there 173 are several predefined spacing control features: 174 175 - Text Spacing (Proportional/Monospace/Half-width/Normal) 176 - Number Spacing (Monospaced-numbers/Proportional-numbers) 177 - Kana Spacing (Full-width/Proportional) 178 - Ideographic Spacing (Full-width/Proportional) 179 - CJK Roman Spacing (Half-width/Proportional/Default-roman 180 /Full-width-roman/Proportional) 181 182 If all layout features are independently managed, we can activate 183 inconsistent typographic rules like `Text Spacing=Monospace' and 184 `Ideographic Spacing=Proportional' at the same time. 185 186 The combinations of layout features is managed by a 32bit integer 187 (one bit each for selector setting), so we can define relationships 188 between up to 32 features, theoretically. But if one feature 189 setting affects another feature setting, we need typographic 190 priority rules to validate the relationship. Unfortunately, the 191 TrueType GX format specification does not give such information even 192 for predefined features. 193 194 1954. Permissive error handling of broken GX tables 196------------------------------------------------ 197 198 When Apple's font rendering system finds an inconsistency, like a 199 specification violation or an unspecified value in a TrueType GX 200 table, it does not always return error. In most cases, the rendering 201 engine silently ignores such wrong values or even whole tables. In 202 fact, MacOS is shipped with fonts including broken GX/AAT tables, but 203 no harmful effects due to `officially broken' fonts are observed by 204 end-users. 205 206 gxvalid is designed to continue the validation process as long as 207 possible. When gxvalid find wrong values, gxvalid warns it at least, 208 and takes a fallback procedure if possible. The fallback procedure 209 depends on the debug level. 210 211 We used the following three tools to investigate Apple's error handling. 212 213 - FontValidator (for MacOS 8.5 - 9.2) resource fork font 214 - ftxvalidator (for MacOS X 10.1 -) dfont or naked-sfnt 215 - ftxdumperfuser (for MacOS X 10.1 -) dfont or naked-sfnt 216 217 However, all tests were done on a PowerPC based Macintosh; at present, 218 we have not checked those tools on a m68k-based Macintosh. 219 220 In total, we checked 183 fonts bundled to MacOS 9.1, MacOS 9.2, MacOS 221 10.0, MacOS X 10.1, MSIE for MacOS, and AppleWorks 6.0. These fonts 222 are distributed officially, but many broken GX/AAT tables were found 223 by Apple's font tools. In the following, we list typical violation of 224 the GX specification, in fonts officially distributed with those Apple 225 systems. 226 227 4-1. broken BinSrchHeader (19/183) 228 ---------------------------------- 229 230 `BinSrchHeader' is a header of a data array for m68k platforms to 231 access memory efficiently. Although there are only two independent 232 parameters for real (`unitSize' and `nUnits'), BinSrchHeader has 233 three additional parameters which can be calculated from `unitSize' 234 and `nUnits', for fast setup. Apple font tools ignore them 235 silently, so gxvalid warns if it finds and inconsistency, and always 236 continues validation. The additional parameters are ignored 237 regardless of the consistency. 238 239 19 fonts include such inconsistencies; all breaks are in the 240 BinSrchHeader structure of the `kern' table. 241 242 4-2. too-short LookupTable (5/183) 243 ---------------------------------- 244 245 LookupTable format 0 is a simple array to get a value from a given 246 GID (glyph ID); the index of this array is a GID too. Therefore, 247 the length of the array is expected to be same as the maximum GID 248 value defined in the `maxp' table, but there are some fonts whose 249 LookupTable format 0 is too short to cover all GIDs. FontValidator 250 ignores this error silently, ftxvalidator and ftxdumperfuser both 251 warn and continue. Similar problems are found in format 3 subtables 252 of `kern'. gxvalid warns always and abort if the validation level 253 is set to FT_VALIDATE_PARANOID. 254 255 5 fonts include too-short kern format 0 subtables. 256 1 font includes too-short kern format 3 subtable. 257 258 4-3. broken LookupTable format 2 (1/183) 259 ---------------------------------------- 260 261 LookupTable format 2, subformat 4 covers the GID space by a 262 collection of segments which are specified by `firstGlyph' and 263 `lastGlyph'. Some fonts store `firstGlyph' and `lastGlyph' in 264 reverse order, so the segment specification is broken. Apple font 265 tools ignore this error silently; a broken segment is ignored as if 266 it did not exist. gxvalid warns and normalize the segment at 267 FT_VALIDATE_DEFAULT, or ignore the segment at FT_VALIDATE_TIGHT, or 268 abort at FT_VALIDATE_PARANOID. 269 270 1 font includes broken LookupTable format 2, in the `just' table. 271 272 *) It seems that all fonts manufactured by ITC for AppleWorks have 273 this error. 274 275 4-4. bad bracketing in glyph property (14/183) 276 ---------------------------------------------- 277 278 GX/AAT defines a `bracketing' property of the glyphs in the `prop' 279 table, to control layout features of strings enclosed inside and 280 outside of brackets. Some fonts give inappropriate bracket 281 properties to glyphs. Apple font tools warn about this error; 282 gxvalid warns too and aborts at FT_VALIDATE_PARANOID. 283 284 14 fonts include wrong bracket properties. 285 286 287 4-5. invalid feature number (117/183) 288 ------------------------------------- 289 290 The GX/AAT extension can include 255 different layout features, 291 but popular layout features are predefined (see 292 https://developer.apple.com/fonts/TrueType-Reference-Manual/RM09/AppendixF.html). 293 Some fonts include feature numbers which are incompatible with the 294 predefined feature registry. 295 296 In our survey, there are 140 fonts including `feat' table. 297 298 a) 67 fonts use a feature number which should not be used. 299 b) 117 fonts set the wrong feature range (nSetting). This is mostly 300 found in the `mort' and `morx' tables. 301 302 Apple font tools give no warning, although they cannot recognize 303 what the feature is. At FT_VALIDATE_DEFAULT, gxvalid warns but 304 continues in both cases (a, b). At FT_VALIDATE_TIGHT, gxvalid warns 305 and aborts for (a), but continues for (b). At FT_VALIDATE_PARANOID, 306 gxvalid warns and aborts in both cases (a, b). 307 308 4-6. invalid prop version (10/183) 309 ---------------------------------- 310 311 As most TrueType GX tables, the `prop' table must start with a 32bit 312 version identifier: 0x00010000, 0x00020000 or 0x00030000. But some 313 fonts store nonsense binary data instead. When Apple font tools 314 find them, they abort the processing immediately, and the data which 315 follows is unhandled. gxvalid does the same. 316 317 10 fonts include broken `prop' version. 318 319 All of these fonts are classic TrueType fonts for the Japanese 320 script, manufactured by Apple. 321 322 4-7. unknown resource name (2/183) 323 ------------------------------------ 324 325 NOTE: THIS IS NOT A TRUETYPE GX ERROR. 326 327 If a TrueType font is stored in the resource fork or in dfont 328 format, the data must be tagged as `sfnt' in the resource fork index 329 to invoke TrueType font handler for the data. But the TrueType font 330 data in `Keyboard.dfont' is tagged as `kbd', and that in 331 `LastResort.dfont' is tagged as `lst'. Apple font tools can detect 332 that the data is in TrueType format and successfully validate them. 333 Maybe this is possible because they are known to be dfont. The 334 current implementation of the resource fork driver of FreeType 335 cannot do that, thus gxvalid cannot validate them. 336 337 2 fonts use an unknown tag for the TrueType font resource. 338 3395. `kern' table issues 340---------------------- 341 342 In common terminology of TrueType, `kern' is classified as a basic and 343 platform-independent table. But there are Apple extensions of `kern', 344 and there is an extension which requires a GX state machine for 345 contextual kerning. Therefore, gxvalid includes a special validator 346 for `kern' tables. Unfortunately, there is no exact algorithm to 347 check Apple's extension, so gxvalid includes a heuristic algorithm to 348 find the proper validation routines for all possible data formats, 349 including the data format for Microsoft. By calling 350 classic_kern_validate() instead of gxv_validate(), you can specify the 351 `kern' format explicitly. However, current FreeType2 uses Microsoft 352 `kern' format only, others are ignored (and should be handled in a 353 library one level higher than FreeType). 354 355 5-1. History 356 ------------ 357 358 The original 16bit version of `kern' was designed by Apple in the 359 pre-GX era, and it was also approved by Microsoft. Afterwards, 360 Apple designed a new 32bit version of the `kern' table. According 361 to the documentation, the difference between the 16bit and 32bit 362 version is only the size of variables in the `kern' header. In the 363 following, we call the original 16bit version as `classic', and 364 32bit version as `new'. 365 366 5-2. Versions and dialects which should be differentiated 367 --------------------------------------------------------- 368 369 The `kern' table consists of a table header and several subtables. 370 The version number which identifies a `classic' or a `new' version 371 is explicitly written in the table header, but there are 372 undocumented differences between Microsoft's and Apple's formats. 373 It is called a `dialect' in the following. There are three cases 374 which should be handled: the new Apple-dialect, the classic 375 Apple-dialect, and the classic Microsoft-dialect. An analysis of 376 the formats and the auto detection algorithm of gxvalid is described 377 in the following. 378 379 5-2-1. Version detection: classic and new kern 380 ---------------------------------------------- 381 382 According to Apple TrueType specification, there are only two 383 differences between the classic and the new: 384 385 - The `kern' table header starts with the version number. 386 The classic version starts with 0x0000 (16bit), 387 the new version starts with 0x00010000 (32bit). 388 389 - In the `kern' table header, the number of subtables follows 390 the version number. 391 In the classic version, it is stored as a 16bit value. 392 In the new version, it is stored as a 32bit value. 393 394 From Apple font tool's output (DumpKERN is also tested in addition 395 to the three Apple font tools in above), there is another 396 undocumented difference. In the new version, the subtable header 397 includes a 16bit variable named `tupleIndex' which does not exist 398 in the classic version. 399 400 The new version can store all subtable formats (0, 1, 2, and 3), 401 but the Apple TrueType specification does not mention the subtable 402 formats available in the classic version. 403 404 5-2-2. Available subtable formats in classic version 405 ---------------------------------------------------- 406 407 Although the Apple TrueType specification recommends to use the 408 classic version in the case if the font is designed for both the 409 Apple and Microsoft platforms, it does not document the available 410 subtable formats in the classic version. 411 412 According to the Microsoft TrueType specification, the subtable 413 format assured for Windows and OS/2 support is only subtable 414 format 0. The Microsoft TrueType specification also describes 415 subtable format 2, but does not mention which platforms support 416 it. Subtable formats 1, 3, and higher are documented as reserved 417 for future use. Therefore, the classic version can store subtable 418 formats 0 and 2, at least. `ttfdump.exe', a font tool provided by 419 Microsoft, ignores the subtable format written in the subtable 420 header, and parses the table as if all subtables are in format 0. 421 422 `kern' subtable format 1 uses a StateTable, so it cannot be 423 utilized without a GX State Machine. Therefore, it is reasonable 424 to assume that format 1 (and 3) were introduced after Apple had 425 introduced GX and moved to the new 32bit version. 426 427 5-2-3. Apple and Microsoft dialects 428 ----------------------------------- 429 430 The `kern' subtable has a 16bit `coverage' field to describe 431 kerning attributes, but bit interpretations by Apple and Microsoft 432 are different: For example, Apple uses bits 0-7 to identify the 433 subtable, while Microsoft uses bits 8-15. 434 435 In addition, due to the output of DumpKERN and FontValidator, 436 Apple's bit interpretations of coverage in classic and new version 437 are incompatible also. In summary, there are three dialects: 438 classic Apple dialect, classic Microsoft dialect, and new Apple 439 dialect. The classic Microsoft dialect and the new Apple dialect 440 are documented by each vendors' TrueType font specification, but 441 the documentation for classic Apple dialect is not available. 442 443 For example, in the new Apple dialect, bit 15 is documented as 444 `set to 1 if the kerning is vertical'. On the other hand, in 445 classic Microsoft dialect, bit 1 is documented as `set to 1 if the 446 kerning is horizontal'. From the outputs of DumpKERN and 447 FontValidator, classic Apple dialect recognizes 15 as `set to 1 448 when the kerning is horizontal'. From the results of similar 449 experiments, classic Apple dialect seems to be the Endian reverse 450 of the classic Microsoft dialect. 451 452 As a conclusion it must be noted that no font tool can identify 453 classic Apple dialect or classic Microsoft dialect automatically. 454 455 5-2-4. gxvalid auto dialect detection algorithm 456 ----------------------------------------------- 457 458 The first 16 bits of the `kern' table are enough to identify the 459 version: 460 461 - if the first 16 bits are 0x0000, the `kern' table is in 462 classic Apple dialect or classic Microsoft dialect 463 - if the first 16 bits are 0x0001, and next 16 bits are 0x0000, 464 the kern table is in new Apple dialect. 465 466 If the `kern' table is a classic one, the 16bit `coverage' field 467 is checked next. Firstly, the coverage bits are decoded for the 468 classic Apple dialect using the following bit masks (this is based 469 on DumpKERN output): 470 471 0x8000: 1=horizontal, 0=vertical 472 0x4000: not used 473 0x2000: 1=cross-stream, 0=normal 474 0x1FF0: reserved 475 0x000F: subtable format 476 477 If any of reserved bits are set or the subtable bits is 478 interpreted as format 1 or 3, we take it as `impossible in classic 479 Apple dialect' and retry, using the classic Microsoft dialect. 480 481 The most popular coverage in new Apple-dialect: 0x8000, 482 The most popular coverage in classic Apple-dialect: 0x0000, 483 The most popular coverage in classic Microsoft dialect: 0x0001. 484 485 5-3. Tested fonts 486 ----------------- 487 488 We checked 59 fonts bundled with MacOS and 38 fonts bundled with 489 Windows, where all font include a `kern' table. 490 491 - fonts bundled with MacOS 492 * new Apple dialect 493 format 0: 18 494 format 2: 1 495 format 3: 1 496 * classic Apple dialect 497 format 0: 14 498 * classic Microsoft dialect 499 format 0: 15 500 501 - fonts bundled with Windows 502 * classic Microsoft dialect 503 format 0: 38 504 505 It looks strange that classic Microsoft-dialect fonts are bundled to 506 MacOS: they come from MSIE for MacOS, except of MarkerFelt.dfont. 507 508 509 ACKNOWLEDGEMENT 510 --------------- 511 512 Some parts of gxvalid are derived from both the `gxlayout' module and 513 the `otvalid' module. Development of gxlayout was supported by the 514 Information-technology Promotion Agency(IPA), Japan. 515 516 The detailed analysis of undefined glyph ID utilization in `mort' and 517 `morx' tables is provided by George Williams. 518 519------------------------------------------------------------------------ 520 521Copyright (C) 2004-2020 by 522suzuki toshiya, Masatake YAMATO, Red hat K.K., 523David Turner, Robert Wilhelm, and Werner Lemberg. 524 525This file is part of the FreeType project, and may only be used, 526modified, and distributed under the terms of the FreeType project 527license, LICENSE.TXT. By continuing to use, modify, or distribute this 528file you indicate that you have read the license and understand and 529accept it fully. 530 531 532--- end of README --- 533