Developing OpenType Fonts
The features listed below have been defined to create the basic forms for the languages that are supported on Hebrew systems. Regardless of the model an application chooses for supporting layout of complex scripts, Uniscribe requires a fixed order for executing features within a run of text to consistently obtain the proper basic form. This is achieved by calling features one-by-one in the standard order listed below.
The order of the lookups within each feature is also very important. For more information on lookups and defining features in OpenType fonts, see the Encoding section of the OpenType Font Development document.
The standard order for applying Hebrew features encoded in OpenType fonts:
Feature Tag: "ccmp"
The 'ccmp' feature is used to compose a number of glyphs into one glyph, or decompose one glyph into a number of glyphs. This feature is implemented before any other features because there may be times when a font vender wants to control certain shaping of glyphs. An example of using this table is seen below. The 'ccmp' table maps default alphabetic forms to both a composed form (essentially a ligature, GSUB lookup type 4), and decomposed forms (GSUB lookup type 2).
Example; the 'ccmp' feature used to pre-compose the Yiddish ligature 'yod yod patah'. (uni05F2 + uni05B7 -> uniFB1F)
Feature Tag: "dlig"
The 'dlig' feature is also used to map glyphs to their optional ligated form. Font developers should use this table for all ligatures that they want the user to be able to control by user preference. Uniscribe has a flag that will allow this type of feature to be deactivated. The 'dlig' feature maps sequences of glyphs to corresponding ligatures (GSUB lookup type 4). Ligatures with more components must be stored ahead of those with fewer components in order to be found. See Ordering ligatures in the Encoding section of the OpenType Font Development document. The set of optional ligatures will vary by typeface design and script.
Example; the 'dlig' feature used to substitute the alef lamed ligature.
Feature Tag: "kern"
The 'kern' feature is used to adjust amount of space between glyphs, generally to provide optically consistent spacing between glyphs. Although a well-designed typeface has consistent inter-glyph spacing overall, some glyph combinations require adjustment for improved legibility. Besides standard adjustment in either horizontal or vertical direction, this feature can supply size-dependent kerning data via device tables, "cross-stream" kerning in the Y text direction, and adjustment of glyph placement independent of the advance adjustment. Note that this feature would not be used in monospaced fonts.
The font stores a set of adjustments for pairs of glyphs (GPOS lookup type 2 or 8). These may be stored as one or more tables matching left and right classes, and/or as individual pairs. If both forms are used, the classes should be listed last, so as to provide a means to replace any non-ideal values that may result from the class tables. Additional adjustments may be provided for larger sets of glyphs (e.g., triplets, quadruplets, etc.) to overwrite the results of pair kerns in particular combinations. These should precede the pairs.
It is unlikely that there are many needs for kerning in Hebrew fonts. A few exceptions may be the use of kerning for adding space to bases with diactitics (Nikud or Teamin) or for better spacing on handwriting style typefaces. However, this feature is being made available for the instances that a type designer might want to take advantage of kerning.
Feature Tag: "mark"
The 'mark' feature positions mark glyphs in relation to a base glyph, or a ligature glyph. This feature may be implemented as a MarkToBase Attachment lookup (GPOS LookupType = 4) or a MarkToLigature Attachment lookup (GPOS LookupType = 5).
Positioning mark to base using Microsoft VOLT