Fonts with complex OpenType tables Karel Píška Institute of Physics of the ASCR, v. v. i. piska (at) fzu dot cz August 30, 2010 Abstract experiments and experiences for dialogs and future collaboration with involved people. My main di- The paper presents development of complex Open- rection prefers the usage of fonts within TEX based Type fonts. The sample fonts cover Czech and software providing Unicode and OpenType support Georgian handwriting with numerous letter connec- —X TE EX [9] and ConTEXt/LuaTEX[11]. For Open- tions. Type the abbreviation “OT” will also be used in the At the beginning, we show general principles article. of “advanced typography” — complex metric data represented by OpenType tables (GSUB and GPOS) — and compare them with the ligature and kerning 2 Advanced typography tables in METAFONT. Then we describe a history of the OpenType Under “advanced typography” we can assume not font production — approaches, tools and techniques. only so called OpenType font technologies but also We discuss crucial problems, critical barriers, at- our good “old” TEX&METAFONT capability provid- tempts and ways how to reach successful solutions. ing sophisticated word-processing. We demonstrate several tools for font creating, test- ing, debugging and conversions between various text 2.1 TEX& METAFONT – clear and and binary formats. Among these tools are, for clean example, AFDKO, VOLT, FontForge, TTX, Font- TTF. We illustrate their features, advantages, dis- In fact, advanced typography with METAFONT and advantages, and also cases of possible incompatibil- TEX has been available for TEX users for many ities (or maybe bugs). years. METAFONT contains powerful tools like gen- Finally, we present using the OpenType fonts in eralized ligatures together with boundary charac- the TeX world applications: X TE EX and LuaTEX ters [1]: ( MkIV), the programs allowing to read ConTEXt .mf: ligtable % produces % .tfm/.pl and process OpenType fonts directly. a : b |=:| c ;% acb /LIG/ 1 Key words: font, font production, Unicode, Open- a : b |=:|> c ;% acb /LIG/> 2 Type, GSUB, GPOS; AFDKO, VOLT, FontForge, a : b |=:|>> c ;% acb /LIG/>> 3 TTX, Font-TTF; T X, METAFONT, TFM, X T X, E E E a : b =:| c ;% cb LIG/ 4 , LuaT X. ConTEXt E a : b =:|> c ;% cb LIG/> 5 a : b |=: c ;% ac /LIG 6 1 Introduction a : b |=:> c ;% ac /LIG> 7 a : b =: c ;% c LIG 8 The presented fonts are successors of the META- where FONT fonts designed in 1997–98 by Petr Olšák [2] and Karel Píška [3]. Last year (2009) I was not able 1. retains both a and b, inserts c between: acb to create a complete font with OpenType tables working properly. This year I have finally reached 2. retains both a and b, inserts c between; the positive results. The current article can be con- the processing continues after a: acb sidered as a report summarizing my recent studies, 101 3. retains both a and b, inserts c between; draw (-4,0){right}..{sklon2}(3,6); the processing continues after c: acb endchar; % right end of character 4. retains b, inserts c before b: cb beginchar(1, .7u#, 7u#, 0); 5. retains b, inserts c before b; draw (0,6)..(.7,7); the processing continues after c: cb endchar; 6. retains a, inserts c after a: ac and the ligtable instructions 7. retains a, inserts c after a; ligtable ||: "e" |=:|> 3; % ... the processing continues after a: ac ligtable "s": "t" |=:| 6; % ... boundarychar:=1; 8. substitutes both a and b by c. ligtable "a": rightboundaries; def rightboundaries = Boundary characters. The METAFONT and TEX concept of the “word boundary” (the left and right 1 |=:> 1, boundary characters) allows to “implicit” process- % ........ ing of the beginning and the end of the word, i.e. enddef; a substitution or adjustment of the letters in the invoke inserting the requested strokes in the left “initial” and the “final” position of the word. In boundary point (3), between ‘s’ and ‘t’ (6), and in METAFONT sources the left boundary characters the right boundary position (1). is denoted by "||:", the right boundary character must be introduced as the “real” character using the "boundarychar code";" assignment. These facilities allow to apply substitution and positioning rules with some restrictions: only the e s t a pair of two adjacent characters can be processed, it is impossible to look ahead for longer sequence in a simple way; the maximal of glyphs in one font is e s t a 256. Anyway, definitions of ligatures and kernings in METAFONT and then in TFM and also the pro- cessing algorithm in TEX are clear and clean. We esta always know the actual position in the input stream METAFONT cannot process in a single and natu- and how to find the next rule from TEX metrics ta- ral way any character sequences consisting of three bles having to be applied. or more characters, e.g. triplets like "UN-KAN-AN" Abilities of METAFONT and TEX will be demon- and "UN-KAN-EN" in Georgian handwriting. strated by two short samples. Primarily, by default, Depending on the following character ("AN", "EN" the Latin (Czech) letters are in the “medial” form, or another) the original (“isolated” by default) glyph without connecting strokes. Then the TEX&MF "KAN" is replaced by its modified form or not: “machinery” joins the adjacent letters in words and adjusts the letters in the initial and final positions. ligtable GR_KAN: GR_AN =:| GR_kan_; The letter ‘e’ is preceded by one of the front-end and then it may be joined to the next character by strokes, ‘s’ and ‘t’ are joined by the correspondent the connecting stroke: inter-letter connecting stroke, and, finally, the last letter in the word is closed by the ending stroke. ligtable GR_kan_: GR_AN |=:| gr_en__an; METAFONT defines several initial, medial and final strokes (depending on concerned letters), for exam- ple: % left deflected end of character უა უკე beginchar(3, 6u#, 7u#, 0); draw (0,0){(3,2)}..{sklon2}(6,6); But no substitution and no kerning is defined for endchar; the pairs "UN-KAN" and "KAN-EN". After the pro- % shorter convex stroke for the pairs st,... cessing of the pair consisting of the 2nd and 3rd beginchar(6, 3u#, 7u#, 0); character and substituting of the second glyph we 102 უა უკე უკა უკე have no chance to return before the first character At first, we have to create an OpenType font and we are not able to adjust the kerning between properly, using some suitable tools. And, at second, the first and the second, modified, glyph (left) — the font must be in agreement with corresponding while "UN-KAN-EN" (right) needs no substitution or software to execute adequate operations according positioning changes. Two triplets above should be the rules (instruction) defined in the font. processed differently. It may be probably possible but the solution with METAFONT would not be triv- ial. 3 Tools to produce OpenType fonts 2.2 OpenType technology 3.1 Creating OpenType fonts 2.3 Advanced typography with Open- Type We assume that we already have Unicode encoded outline fonts but without OpenType features. We “Old TrueType fonts” can be “enriched” by adding have generated them earlier with FontForge. And “Advanced OpenType Typographic Tables” to pro- our aim and task is to produce OpenType, i.e. to duce fonts in OpenType format. We will not dis- enrich the fonts with OT tables. cuss two different format versions: “new” TTF and My fonts cover Czech, Georgian and also Arme- OTF, because the additional OT tables are com- nian handwriting letter repertoire (taught in pri- mon. mary/elementary schools). Opposite to Czech and Each feature is defined as a system of subsys- Georgian writing, the Armenian letters are designed tems called lookups. Any lookup is described as a and written in a simple way without no special join- subsystem consisted of substitution and positioning ers and there is no necessity to build any OpenType rules. Depending on script and language, a feature support/facility for Armenian. may be enabled or disabled. If the feature is enabled Sample of Armenian: and some lookup, contained in this feature, fulfil the given conditions, then the execution of correspond- ս ա տ ր ազատ ւ հավասար ing operations should be invoked. It is a signal and the real application must be executed by an applica- There is another problem – to distinguish the adja- tion program or operating system, e.g. by means of cent letters. a special library. OpenType introduces substitution In the following paragraphs we show the con- (GSUB), positioning (GPOS), and several other ta- struction of OT fonts using various tools, namely bles. VOLT, FontForge and AFDKO. These tables define the set of rules of several The specification of (binary) OT tables, data types specifying [from OpenType specification [4, formats of the VOLT project files, variants of fea- 5]]: ture files accepted by AFDKO, FontLab, FontForge Glyph substitution (GSUB) rules may all be different. Single, Multiple, Alternate, Ligature, Contextual, Chaining contextual, Extension, and Reverse Chain- 3.2 Managing OpenType with VOLT ing Single Substitution; Glyph positioning (GPOS) rules VOLT (Visual OpenType Layout Tool) [6], free prod- Single adjustment, Pair adjustment, Cursive attach- uct developed by Microsoft and running only un- ment, Mark-to-Base attachment, Mark-to-Ligature der MS Windows, offers an interactive approach to attachment, Mark-to-Mark attachment, Contextual, fill input areas with appropriate parameter values Chaining contextual, and Extension positioning. manually in the VOLT project window. Other pos- From METAFONT we inherited the entire let- sibility is writing and modifying source textual files ters with accents. Therefore we have not needed to in the VOLT project language. In the VOLT in- use marks and anchors and operations with them put area we can enter or in a text editor we have to assembly the complete letters from components to define glyphs (their names, types and code num- (accents, signs, marks, .