Description
The section “Split vowel handling” does not define “split vowel”. The closest there is to a definition is “the split vowels, VOWEL_PRE_ABOVE, VOWEL_PRE_ABOVE_POST, and VOWEL_PRE_POST”, which appears in another section.
The section says “USE decomposes split vowel characters belonging to UISC = Vowel_Dependent”. However, the USE class VOWEL includes more than just UISC = Vowel_Dependent. Currently, the only decomposable VOWEL characters do have UISC = Vowel_Dependent; but if a decomposable character were added to Unicode with UISC = Pure_Killer, is the spec really saying that that character would not necessarily be decomposed?
It describes what to do for split vowels without canonical decompositions, but says “There are no characters of this category in the currently supported scripts”. That is not true. U+115B9 SIDDHAM VOWEL SIGN AI is in the subclass VOWEL_PRE_ABOVE but it has no canonical decomposition.
It twice mentions required decompositions but does not explain which decompositions are required. All of them? Just the ones from UnicodeData.txt? Whatever the font developer deems appropriate?
It says “Font developers must include glyphs for all required decompositions.” That is overstating it. If a font developer is not making a Sinhala font, for example, there is no need for them to include glyphs for U+0DD9 and U+0DCF. The intended meaning is probably that if a font supports character X, and X decomposes into <Y, Z>, then the font must include glyphs for Y and Z. It is not clear whether the font must also include a glyph for X, or what happens if the font has a glyph only for X and not Y or Z.
Document Details
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- ID: 194a6d3c-4137-46e9-3a4b-44b990200986
- Version Independent ID: a0c8e788-5228-aa28-670e-3ba1ac3faecd
- Content: Creating and supporting OpenType fonts for the Universal Shaping Engine - Typography
- Content Source: typographydocs/script-development/use.md
- Product: typography
- GitHub Login: @Anglass
- Microsoft Alias: anglass