6 Copyright (C) 2011-2012 by Werner Lemberg.
8 This file is part of the ttfautohint library, and may only be used,
9 modified, and distributed under the terms given in `COPYING'. By
10 continuing to use, modify, or distribute this file you indicate that you
11 have read `COPYING' and understand and accept it fully.
13 The file `COPYING' mentioned in the previous paragraph is distributed
14 with the ttfautohint library.
22 **ttfautohint** is a library written in\ C which takes a TrueType font as
23 the input, removes its bytecode instructions (if any), and returns a new
24 font where all glyphs are bytecode hinted using the information given by
25 FreeType's autohinting module. The idea is to provide the excellent quality
26 of the autohinter on platforms which don't use FreeType.
28 The library has a single API function, `TTF_autohint`, which is described
29 [below](#the-ttfautohint-api).
31 Bundled with the library there are two front-end programs, [`ttfautohint`
32 and `ttfautohintGUI`](#ttfautohint-and-ttfautohintgui), being a command line
33 and an application with a Graphics User Interface (GUI), respectively.
36 What exactly are hints?
37 -----------------------
39 To cite [Wikipedia](http://en.wikipedia.org/wiki/Font_hinting):
41 > **Font hinting** (also known as **instructing**) is the use of
42 > mathematical instructions to adjust the display of an outline font so that
43 > it lines up with a rasterized grid. At low screen resolutions, hinting is
44 > critical for producing a clear, legible text. It can be accompanied by
45 > antialiasing and (on liquid crystal displays) subpixel rendering for
48 and Apple's [TrueType Reference
49 Manual](https://developer.apple.com/fonts/TTRefMan/RM03/Chap3.html#features):
51 > For optimal results, a font instructor should follow these guidelines:
53 > - At small sizes, chance effects should not be allowed to magnify small
54 > differences in the original outline design of a glyph.
56 > - At large sizes, the subtlety of the original design should emerge.
59 In general, there are three possible ways to hint a glyph.
61 1. The font contains hints (in the original sense of this word) to guide
62 the rasterizer, telling it which shapes of the glyphs need special
63 consideration. The hinting logic is partly in the font and partly in
64 the rasterizer. More sophisticated rasterizers are able to produce
65 better rendering results.
67 This is how Type\ 1 and CFF font hints work.
69 2. The font contains exact instructions (also called *bytecode*) on how to
70 move the points of its outlines, depending on the resolution of the
71 output device, and which intentionally distort the (outline) shape to
72 produce a well-rasterized result. The hinting logic is in the font;
73 ideally, all rasterizers simply process these instructions to get the
74 same result on all platforms.
76 This is how TrueType hints work.
78 3. The font gets auto-hinted (at run-time). The hinting logic is
79 completely in the rasterizer. No hints in the font are used or needed;
80 instead, the rasterizer scans and analyzes the glyphs to apply
81 corrections by itself.
83 This is how FreeType's auto-hinter works; see
84 [below](#background-and-technical-details) for more.
87 What problems can arise with TrueType hinting?
88 ----------------------------------------------
90 While it is relatively easy to specify PostScript hints (either manually or
91 by an auto-hinter which works at font creation time), creating TrueType
92 hints is far more difficult. There are at least two reasons:
94 - TrueType instructions form a programming language, operating at a very
95 low level. They are comparable to assembler code, thus lacking all
96 high-level concepts to make programming more comfortable.
98 Here an example how such code looks like:
102 PUSHB[ ] /* 3 values pushed */
105 PUSHB[ ] /* 2 values pushed */
108 PUSHB[ ] /* 3 values pushed */
113 Another major obstacle is the fact that font designers usually aren't
116 - It is very time consuming to manually hint glyphs. Given that the
117 number of specialists for TrueType hinting is very limited, hinting a
118 large set of glyphs for a font or font family can become very expensive.
124 The ttfautohint library brings the excellent quality of FreeType rendering
125 to platforms which don't use FreeType, yet require hinting for text to look
126 good -- like Microsoft Windows. Roughly speaking, it converts the glyph
127 analysis done by FreeType's auto-hinting module to TrueType bytecode.
128 Internally, the auto-hinter's algorithm resembles PostScript hinting
129 methods; it thus combines all three hinting methods discussed
130 [previously](#what-exactly-are-hints).
132 The simple interface of the front-ends (both on the command line and with
133 the GUI) allows quick hinting of a whole font with a few mouse clicks or a
134 single command on the prompt. As a result, you get better rendering results
135 with web browsers, for example.
137 Across Windows rendering environments today, fonts processed with
138 ttfautohint look best with ClearType enabled. This is the default for
139 Windows\ 7. Good visual results are also seen in recent MacOS\ X versions
140 and GNU/Linux systems that use FreeType for rendering.
142 The goal of the project is to generate a 'first pass' of hinting that font
143 developers can refine further for ultimate quality.
147 `ttfautohint` and `ttfautohintGUI`
148 ==================================
150 On all supported platforms (GNU/Linux, Windows, and Mac OS\ X), the GUI
151 looks quite similar; the used toolkit is [Qt], which in turn uses the
152 platform's native widgets.
154 ![`ttfautohintGUI` on GNU/Linux running KDE](img/ttfautohintGUI.png)
156 Both the GUI and console version share the same features, to be discussed in
159 **Warning: ttfautohint cannot always process a font a second time.**
160 If the font contains composite glyphs, and option `-c` is not used,
161 reprocessing with ttfautohint will fail. For this reason it is strongly
162 recommended to *not* delete the original, unhinted font so that you can
163 always rerun ttfautohint.
166 Calling `ttfautohint`
167 ---------------------
170 ttfautohint [OPTION]... [IN-FILE [OUT-FILE]]
173 The TTY binary, `ttfautohint`, works like a Unix filter, this is, it reads
174 data from standard input if no input file name is given, and it sends its
175 output to standard output if no output file name is specified.
177 A typical call looks like the following.
180 ttfautohint -v -f foo.ttf foo-autohinted.ttf
183 For demonstration purposes, here the same using a pipe and redirection.
184 Note that Windows's default command line interpreter, `cmd.exe`, doesn't
185 support piping with binary files, unfortunately.
188 cat foo.ttf | ttfautohint -v -f > foo-autohinted.ttf
192 Calling `ttfautohintGUI`
193 ------------------------
196 ttfautohintGUI [OPTION]...
199 `ttfautohintGUI` doesn't send any output to a console; however, it accepts
200 the same command line options as `ttfautohint`, setting default values for
207 Long options can be given with one or two dashes, and with and without an
208 equal sign between option and argument. This means that the following forms
209 are acceptable: `-foo=`*bar*, `--foo=`*bar*, `-foo`\ *bar*, and
212 Below, the section title refers to the command's label in the GUI, then
213 comes the name of the corresponding long command line option and its short
214 equivalent, followed by a description.
216 Background and technical details on the meaning of the various options are
217 given [afterwards](#background-and-technical-details).
219 ### Hint Set Range Minimum, Hint Set Range Maximum
221 See ['Hint Sets'](#hint-sets) for a definition and explanation.
223 `--hinting-range-min=`*n*, `-l`\ *n*
224 : The minimum PPEM value (in pixels) at which hint sets are created. The
225 default value for *n* is\ 8.
227 `--hinting-range-max=`*n*, `-r`\ *n*
228 : The maximum PPEM value (in pixels) at which hint sets are created. The
229 default value for *n* is 50.
233 `--latin-fallback`, `-f`
234 : Set fallback script to 'latin', this is, use the 'latin' auto-hinting
235 module instead of 'none' for all glyphs which can't be assigned to a
236 script. See [below](#scripts) for more details.
240 `--hinting-limit=`*n*, `-G`\ *n*
241 : The *hinting limit* is the PPEM value (in pixels) where hinting gets
242 switched off (using the `INSTCTRL` bytecode instruction); it has zero
243 impact on the file size. The default value for *n* is 200 which means
244 that the font is not hinted for PPEM values larger than 200.
246 Note that hinting in the range 'hinting-range-max' up to 'hinting-limit'
247 uses the hinting configuration for 'hinting-range-max'.
249 To omit a hinting limit, use `--hinting-limit=0` (or check the 'No
250 Hinting Limit' box in the GUI). Since this will cause internal math
251 overflow in the rasterizer for large pixel values (>\ 1500px approx.) it
252 is strongly recommended to not use this except for testing purposes.
254 ### x Height Increase Limit
256 `--increase-x-height=`*n*, `-x`\ *n*
257 : Normally, ttfautohint rounds the x\ height to the pixel grid, with a
258 slight preference for rounding up. If this flag is set, values in the
259 range 6\ PPEM to *n*\ PPEM are much more often rounded up. The default
260 value for *n* is 14. Use this flag to increase the legibility of small
261 sizes if necessary; you might get weird rendering results otherwise for
262 glyphs like 'a' or 'e', depending on the font design.
264 To switch off this feature, use `--increase-x-height=0` (or check the
265 'No x\ Height Increase' box in the GUI).
267 The following images again use the font 'Mertz Bold'.
269 ![At 17px, without option `-x` and '`-w ""`', the hole in glyph 'e'
270 looks very grey in the FontForge snapshot, and the GDI ClearType
271 rendering (which is the default on older Windows versions) fills it
272 completely with black because it uses B/W rendering along the y\ axis.
273 FreeType's 'light' autohint mode (which corresponds to ttfautohint's
274 'smooth' stem width algorithm) intentionally aligns horizontal lines
275 to non-integer (but still discrete) values to avoid large glyph shape
276 distortions.](img/e-17px-x14.png)
278 ![The same, this time with option `-x 17` (and
279 '`-w ""`').](img/e-17px-x17.png)
281 ### Windows Compatibility
283 `--windows-compatibility`, `-W`
284 : This option makes ttfautohint add two artificial blue zones, positioned
285 at the `usWinAscent` and `usWinDescent` values (from the font's `OS/2`
286 table). The idea is to help ttfautohint so that the hinted glyphs stay
287 within this horizontal stripe since Windows clips everything falling
290 There is a general problem with tight values for `usWinAscent` and
291 `usWinDescent`; a good description is given in the [Vertical Metrics
292 How-To](http://typophile.com/node/13081). Additionally, there is a
293 special problem with tight values if used in combination with
294 ttfautohint because the auto-hinter tends to slightly increase the
295 vertical glyph dimensions at smaller sizes to improve legibility. This
296 enlargement can make the heights and depths of glyphs exceed the range
297 given by `usWinAscent` and `usWinDescent`.
299 If ttfautohint is part of the font creation tool chain, and the font
300 designer can adjust those two values, a better solution instead of using
301 option `-W` is to reserve some vertical space for 'padding': For the
302 auto-hinter, the difference between a top or bottom outline point before
303 and after hinting is less than 1px, thus a vertical padding of 2px is
304 sufficient. Assuming a minimum hinting size of 6ppem, adding two pixels
305 gives an increase factor of 8÷6 = 1.33. This is near to the default
306 baseline-to-baseline distance used by TeX and other sophisticated text
307 processing applications, namely 1.2×designsize, which gives satisfying
308 results in most cases. It is also near to the factor 1.25 recommended
309 in the abovementioned How-To. For example, if the vertical extension of
310 the largest glyph is 2000 units (assuming that it approximately
311 represents the designsize), the sum of `usWinAscent` and `usWinDescent`
312 could be 1.25×2000 = 2500.
314 In case ttfautohint is used as an auto-hinting tool for fonts which can
315 be no longer modified to change the metrics, option `-W` in combination
316 with '`-X ""`' to suppress any vertical enlargement should prevent almost
319 [Option `-X` is not implemented yet.]
323 `--pre-hinting`, `-p`
324 : *Pre-hinting* means that a font's original bytecode is applied to all
325 glyphs before it is replaced with bytecode created by ttfautohint.
326 This makes only sense if your font already has some hints in it which
327 modify the shape even at EM size (normally 2048px); for example, some
328 CJK fonts need this because the bytecode is used to scale and shift
329 subglyphs. For most fonts, however, this is not the case.
331 ### Hint With Components
334 : Hint glyph components separately instead of hinting composite glyphs as
335 a whole. Using this flag reduces the bytecode size enormously, however,
336 it might yield worse results. In the GUI it is similar: If you uncheck
337 the 'Process With Components' box, glyph components are hinted
340 If a font contains composite glyphs and those glyphs are hinted as a
341 whole, ttfautohint cannot reprocess its own output.
346 : Use default values for standard (horizontal) stem width and height
347 instead of deriving them from latin character 'o'. Use this option
348 (usually in combination with option `--latin-fallback`) to hint symbol
349 or dingbat fonts or math glyphs, for example, which lack character 'o',
350 at the expense of possibly poor hinting results at small sizes.
352 ### Add ttfautohint Info
355 : Don't add ttfautohint version and command line information to the
356 version string or strings (with name ID\ 5) in the font's `name` table.
357 In the GUI it is similar: If you uncheck the 'Add ttfautohint info' box,
358 information is not added to the `name` table. Except for testing and
359 development purposes it is strongly recommended to not use this option.
361 ### Strong Stem Width and Positioning
363 `--strong-stem-width=`*string*, `-w`\ *string*
364 : ttfautohint offers two different routines to handle (horizontal) stem
365 widths and stem positions: 'smooth' and 'strong'. The former uses
366 discrete values which slightly increase the stem contrast with almost no
367 distortion of the outlines, while the latter snaps both stem widths and
368 stem positions to integer pixel values as much as possible, yielding a
369 crisper appearance at the cost of much more distortion.
371 These two routines are mapped onto three possible rendering targets:
373 - grayscale rendering, with or without optimization for subpixel
374 positioning (e.g. Mac OS\ X)
376 - 'GDI ClearType' rendering: the rasterizer version, as returned by the
377 GETINFO bytecode instruction, is in the range 36\ <= version <\ 38 and
378 ClearType is enabled (e.g. Windows XP)
380 - 'DirectWrite ClearType' rendering: the rasterizer version, as returned
381 by the GETINFO bytecode instruction, is >=\ 38, ClearType is enabled,
382 and subpixel positioning is enabled also (e.g. Internet Explorer\ 9
383 running on Windows\ 7)
385 GDI ClearType uses a mode similar to B/W rendering along the vertical
386 axis, while DW ClearType applies grayscale rendering. Additionally,
387 only DW ClearType provides subpixel positioning along the x\ axis. For
388 what it's worth, the rasterizers version\ 36 and version\ 38 in
389 Microsoft Windows are two completely different rendering engines.
391 The command line option expects *string* to contain up to three letters
392 with possible values '`g`' for grayscale, '`G`' for GDI ClearType, and
393 '`D`' for DW ClearType. If a letter is found in *string*, the strong
394 stem width routine is used for the corresponding rendering target. The
395 default value is '`G`' which means that strong stem width handling is
396 activated for GDI ClearType only. To use smooth stem width handling for
397 all three rendering targets, use the empty string as an argument,
398 usually connoted with '`""`'.
400 In the GUI, simply set the corresponding check box to select the stem
401 width routine for a given rendering target.
403 The following FontForge snapshot images use the font ['Mertz
404 Bold'](http://code.newtypography.co.uk/?p=2449) (still under
405 development) from [Vernon Adams].
407 ![The left part shows the glyph 'g' unhinted at 26px, the right part
408 with hints, using the 'smooth' stem algorithm.](img/ff-g-26px.png)
410 ![The same, but this time using the 'strong'
411 algorithm. Note how the stems are aligned to the pixel
412 grid.](img/ff-g-26px-wD.png)
414 ### Font License Restrictions
416 `--ignore-restrictions`, `-i`
417 : By default, fonts which have bit\ 1 set in the 'fsType' field of the
418 `OS/2` table are rejected. If you have a permission of the font's legal
419 owner to modify the font, specify this command line option.
421 If this option is not set, `ttfautohintGUI` shows a dialogue to handle
422 such fonts if necessary.
427 : On the console, print a brief documentation on standard output and exit.
428 This doesn't work with `ttfautohintGUI` on MS Windows.
431 : On the console, print version information on standard output and exit.
432 This doesn't work with `ttfautohintGUI` on MS Windows.
435 : Print *a lot* of debugging information on standard error while
436 processing a font (you should redirect stderr to a file). This
437 doesn't work with `ttfautohintGUI` on MS Windows.
441 Background and Technical Details
442 ================================
444 [Real-Time Grid Fitting of Typographic
445 Outlines](http://www.tug.org/TUGboat/tb24-3/lemberg.pdf) is a scholarly
446 paper which describes FreeType's auto-hinter in some detail. Regarding the
447 described data structures it is slightly out of date, but the algorithm
448 itself hasn't changed.
450 The next few subsections are mainly based on this article, introducing some
451 important concepts. Note that ttfautohint only does hinting along the
452 vertical direction (this is, modifying y\ coordinates).
458 A glyph consists of one or more *contours* (this is, closed curves). For
459 example, glyph 'O' consists of two contours, while glyph 'I' has only one.
461 ![The letter 'O' has two contours, an inner and an outer one, while letter
462 'I' has only an outer contour.](img/o-and-i)
464 A *segment* is a series of consecutive points of a contour (including its
465 Bézier control points) that are approximately aligned along a coordinate
468 ![A serif. Contour and control points are represented by squares and
469 circles, respectively. The bottom 'line' DE is approximately aligned
470 along the horizontal axis, thus it forms a segment of 7\ points. Together
471 with the two other horizontal segments, BC and FG, they form two edges
472 (BC+FG, DE).](img/segment-edge)
474 An *edge* corresponds to a single coordinate value on the main dimension
475 that collects one or more segments (allowing for a small threshold). While
476 finding segments is done on the unscaled outline, finding edges is bound to
477 the device resolution. See [below](#hint-sets) for an example.
479 The analysis to find segments and edges is specific to a script.
485 The auto-hinter analyzes a font in two steps. Right now, everything
486 described below happens for the horizontal axis only, providing vertical
491 This affects the hinting of all glyphs, trying to give them a uniform
494 + Compute standard stem widths and heights of the font. The values
495 are normally taken from the glyph of letter 'o'.
497 + Compute blue zones, see [below](#blue-zones).
499 If stem widths and heights of single glyphs differ by a large value, or
500 if ttfautohint fails to find proper blue zones, hinting becomes quite
501 poor, leading even to severe shape distortions.
505 This is a per-glyph operation.
507 + Find segments and edges.
509 + Link edges together to find stems and serifs. The abovementioned
510 paper gives more details on what exactly constitutes a stem or a
511 serif and how the algorithm works.
517 ![Two blue zones relevant to the glyph 'a'. Vertical point coordinates of
518 *all* glyphs within these zones are aligned.](img/blue-zones)
520 Outlines of certain characters are used to determine *blue zones*. This
521 concept is the same as with Type\ 1 fonts: All glyph points which lie in
522 certain small horizontal zones get aligned vertically.
524 Here a table which shows the characters used by the latin module; the values
525 are hard-coded in the source code.
527 ID Blue zone Characters
528 ---- ----------- ------------
529 1 top of capital letters THEZOCQS
530 2 bottom of capital letters HEZLOCUS
531 3 top of 'small f' like letters fijkdbh
532 4 top of small letters xzroesc
533 5 bottom of small letters xzroesc
534 6 bottom of descenders of small letters pqgjy
536 The 'round' characters (e.g. 'OCQS') from Zones 1, 2, and 5 are also used to
537 control the overshoot handling; to improve rendering at small sizes, zone\ 4
538 gets adjusted to be on the pixel grid; cf. the [`--increase-x-height`
539 option](#x-height-increase-limit).
541 ![This image shows the relevant glyph terms for vertical blue zone
542 positions.](img/glyph-terms)
548 Aligning outlines along the grid lines is called *grid fitting*. It doesn't
549 necessarily mean that the outlines are positioned *exactly* on the grid,
550 however, especially if you want a smooth appearance at different sizes.
551 This is the central routine of the auto-hinter; its actions are highly
552 dependent on the used script. Currently, only support for scripts which
553 work similarly to Latin (i.e. Greek and Cyrillic) is available.
555 * Align edges linked to blue zones.
557 * Fit edges to the pixel grid.
561 * Handle remaining 'strong' points. Such points are not part of an edge
562 but are still important for defining the shape. This roughly
563 corresponds to the `IP` TrueType instruction.
565 * Everything else (the 'weak' points) is handled with an `IUP`
568 The following images illustrate the hinting process, using glyph 'a' from
569 the freely available font ['Ubuntu Book'](http://font.ubuntu.com). The
570 manual hints were added by [Dalton Maag Ltd], the used application to create
571 the hinting debug snapshots was [FontForge].
573 ![Before hinting.](img/a-before-hinting.png)
575 ![After hinting, using manual hints.](img/a-after-hinting.png)
577 ![After hinting, using ttfautohint. Note that the hinting process
578 doesn't change horizontal positions.](img/a-after-autohinting.png)
584 In ttfautohint terminology, a *hint set* is the *optimal* configuration for
585 a given PPEM (pixel per EM) value.
587 In the range given by the `--hinting-range-min` and `--hinting-range-max`
588 options, ttfautohint creates hint sets for every PPEM value. For each
589 glyph, ttfautohint automatically determines if a new set should be emitted
590 for a PPEM value if it finds that it differs from a previous one. For some
591 glyphs it is possible that one set covers, say, the range 8px-1000px, while
592 other glyphs need 10 or more such sets.
594 In the PPEM range below `--hinting-range-min`, ttfautohint always uses just
595 one set, in the PPEM range between `--hinting-range-max` and
596 `--hinting-limit`, it also uses just one set.
598 One of the hinting configuration parameters is the decision which segments
599 form an edge. For example, let us assume that two segments get aligned on a
600 single horizontal edge at 11px, while two edges are used at 12px.
601 This change makes ttfautohint emit a new hint set to accomodate this
604 The next images illustrate this, using a Cyrillic letter (glyph 'afii10108')
605 from the 'Ubuntu book' font, processed with ttfautohint.
607 ![Before hinting, size 11px.](img/afii10108-11px-before-hinting.png)
609 ![After hinting, size 11px. Segments 43-27-28 and 14-15 are aligned on a
610 single edge, as are segments 26-0-1 and
611 20-21.](img/afii10108-11px-after-hinting.png)
613 ![Before hinting, size 12px.](img/afii10108-12px-before-hinting.png)
615 ![After hinting, size 12px. The segments are not aligned. While
616 segments 43-27-28 and 20-21 now have almost the same horizontal position,
617 they don't form an edge because the outlines passing through the segments
618 point into different directions.](img/afii10108-12px-after-hinting.png)
620 Obviously, the more hint sets get emitted, the larger the bytecode
621 ttfautohint adds to the output font. To find a good value\ *n* for
622 `--hinting-range-max`, some experimentation is necessary since *n* depends
623 on the glyph shapes in the input font. If the value is too low, the hint
624 set created for the PPEM value\ *n* (this hint set gets used for all larger
625 PPEM values) might distort the outlines too much in the PPEM range given
626 by\ *n* and the value set by `--hinting-limit` (at which hinting gets
627 switched off). If the value is too high, the font size increases due to
628 more hint sets without any noticeable hinting effects.
630 Similar arguments hold for `--hinting-range-min` except that there is no
631 lower limit at which hinting is switched off.
633 An example. Let's assume that we have a hinting range 10\ <= ppem <=\ 100,
634 and the hinting limit is set to 250. For a given glyph, ttfautohint finds
635 out that four hint sets must be computed to exactly cover thes hinting
636 range: 10-15, 16-40, 41-80, and 81-100. For ppem values below 10ppem, the
637 hint set covering 10-15ppem is used, for ppem values larger than 100 the
638 hint set covering 81-100ppem is used. For ppem values larger than 250, no
639 hinting gets applied.
642 The '\.ttfautohint' Glyph
643 -------------------------
645 [The behaviour described in this section does not apply if [option
646 `--components`](#hint-with-components) is used.]
648 By default, ttfautohint doesn't hint subglyphs of composite glyphs
649 separately. Instead, it hints the whole glyph, this is, composites get
650 recursively expanded internally so that they form simple glyphs, then hints
651 are applied -- this is the normal working mode of FreeType's auto-hinter.
653 One problem, however, must be solved: Hinting for subglyphs (which usually
654 are used as normal glyphs also) must be deactivated so that nothing but the
655 final bytecode of the composite gets executed.
657 The trick used by ttfautohint is to prepend a composite element called
658 '\.ttfautohint', a dummy glyph with a single point, and which has a single
659 job: Its bytecode increases a variable (to be more precise, it is a CVT
660 register called `cvtl_is_subglyph` in the source code), indicating that we
661 are within a composite glyph. The final bytecode of the composite glyph
662 eventually decrements this variable again.
664 As an example, let's consider composite glyph 'Agrave' ('À') which has the
665 subglyph 'A' as the base and 'grave' as its accent. After processing with
666 ttfautohint it consists of three subglyphs: '\.ttfautohint', 'A', and
667 'grave' (in this order).
670 ------------- --------
671 .ttfautohint increase `cvtl_is_subglyph` (now: 1)
672 A do nothing because `cvtl_is_subglyph` > 0
673 grave do nothing because `cvtl_is_subglyph` > 0
674 Agrave decrease `cvtl_is_subglyph` (now: 0)\
675 apply hints because `cvtl_is_subglyph` == 0
677 Some technical details (which you might skip): All glyph point indices get
678 adjusted since each '\.ttfautohint' subglyph shifts all following indices by
679 one. This must be done for both the bytecode and one subformat of
680 OpenType's `GPOS` anchor tables.
682 While this approach works fine on all tested platforms, there is one single
683 drawback: Direct rendering of the '\.ttfautohint' subglyph (this is,
684 rendering as a stand-alone glyph) disables proper hinting of all glyphs in
685 the font! Under normal circumstances this never happens because
686 '\.ttfautohint' doesn't have an entry in the font's `cmap` table. (However,
687 some test and demo programs like FreeType's `ftview` application or other
688 glyph viewers which are able to bypass the `cmap` table might be affected.)
693 ttfautohint checks which auto-hinting module should be used to hint a
694 specific glyph. To do so, it checks a glyph's Unicode character code
695 whether it belongs to a given script. Currently, only FreeType's 'latin'
696 autohinting module is implemented, but more are expected to come. Here is
697 the hardcoded list of character ranges which are hinted by this 'latin'
698 module. As you can see, this also covers some non-latin scripts (in the
699 Unicode sense) which have similar typographical properties.
701 Character range Description
702 --------------------- -------------
703 `0x0020` - `0x007F` Basic Latin (no control characters)
704 `0x00A0` - `0x00FF` Latin-1 Supplement (no control characters)
705 `0x0100` - `0x017F` Latin Extended-A
706 `0x0180` - `0x024F` Latin Extended-B
707 `0x0250` - `0x02AF` IPA Extensions
708 `0x02B0` - `0x02FF` Spacing Modifier Letters
709 `0x0300` - `0x036F` Combining Diacritical Marks
710 `0x0370` - `0x03FF` Greek and Coptic
711 `0x0400` - `0x04FF` Cyrillic
712 `0x0500` - `0x052F` Cyrillic Supplement
713 `0x1D00` - `0x1D7F` Phonetic Extensions
714 `0x1D80` - `0x1DBF` Phonetic Extensions Supplement
715 `0x1DC0` - `0x1DFF` Combining Diacritical Marks Supplement
716 `0x1E00` - `0x1EFF` Latin Extended Additional
717 `0x1F00` - `0x1FFF` Greek Extended
718 `0x2000` - `0x206F` General Punctuation
719 `0x2070` - `0x209F` Superscripts and Subscripts
720 `0x20A0` - `0x20CF` Currency Symbols
721 `0x2150` - `0x218F` Number Forms
722 `0x2460` - `0x24FF` Enclosed Alphanumerics
723 `0x2C60` - `0x2C7F` Latin Extended-C
724 `0x2DE0` - `0x2DFF` Cyrillic Extended-A
725 `0x2E00` - `0x2E7F` Supplemental Punctuation
726 `0xA640` - `0xA69F` Cyrillic Extended-B
727 `0xA720` - `0xA7FF` Latin Extended-D
728 `0xFB00` - `0xFB06` Alphabetical Presentation Forms (Latin Ligatures)
729 `0x1D400` - `0x1D7FF` Mathematical Alphanumeric Symbols
730 `0x1F100` - `0x1F1FF` Enclosed Alphanumeric Supplement
732 If a glyph's character code is not covered by a script range, it is not
733 hinted (or rather, it gets hinted by the 'dummy' auto-hinting module which
734 essentially does nothing). This can be changed by specifying a *fallback
735 script* with [option `--latin-fallback`](#fallback-script).
737 It is planned to extend ttfautohint so that the `GSUB` OpenType table gets
738 analyzed, mapping character codes to all glyph indices which can be reached
739 by switching on or off various OpenType features.
745 ttfautohint touches almost all SFNT tables within a TrueType or OpenType
746 font. Note that only OpenType fonts with TrueType outlines are supported.
747 OpenType fonts with a `CFF` table (this is, with PostScript outlines) won't
750 * `glyf`: One glyph gets added (namely the '\.ttfautohint' glyph); all
751 composites get an additional component; all hints in the table are
752 replaced with new ones.
754 * `cvt`, `prep`, and `fpgm`: These tables get replaced with data
755 necessary for the new hinting bytecode.
757 * `gasp`: Set up to always use grayscale rendering with grid-fitting
758 for standard hinting, and symmetric grid-fitting and symmetric
759 smoothing for horizontal subpixel hinting (ClearType).
761 * `DSIG`: If it exists, it gets replaced with a dummy version.
762 ttfautohint can't digitally sign a font; you have to do that afterwards.
764 * `name`: The 'version' entries are modified to add information about the
765 parameters which have been used for calling ttfautohint. This can be
766 controlled with the [`--no-info`](#ttfautohint-info) option.
768 * `GPOS`, `hmtx`, `loca`, `head`, `maxp`, `post`: Updated to fit the
769 additional '\.ttfautohint' glyph, the additional subglyphs in
770 composites, and the new hinting bytecode.
772 * `LTSH`, `hdmx`: Since ttfautohint doesn't do any horizontal hinting,
773 those tables are superfluous and thus removed.
775 * `VDMX`: Removed, since it depends on the original bytecode which
776 ttfautohint removes. A font editor might recompute the necessary data