On 9/5/2016 5:34 PM, Charlotte Buff wrote:
It has just come to my attention that U+1D159 MUSICAL SYMBOL NULL NOTEHEAD is not default ignorable, even though it has no visible glyph appearance and no advance width in text, just like the various Hangul jamo fillers that *are* default ignorable. Is there a technical reason for this or is it just an oversight?
Well, the proximate reason is that it is General_Category=So, so that unless it were special-cased for the derivation of the Default_Ignorable property, it will end up Default_Ignorable=No in the UCD.
As to why it wouldn't be special-cased to force it to end up Default_Ignorable=Yes, I don't think there was a whole lot of special thinking that went into this when the musical symbols were first added in Unicode 3.1 way back in 2001. Default_Ignorable was not even a formal property as of Unicode 3.1. That property was added (and rationalized) rather later.
As to why Default_Ignorable=No is probably the correct value for U+1D159 anyway, think of it this way. The null notehead is essentially a musical notation specialized version of a non-breaking space -- it is essentially just a base for applying the various combining stems and flags for a display without showing a particular notehead, analogous to applying a generic combining mark to a NBSP to show that combining mark in isolation. It isn't clear that the null notehead should have no advance width, and in general, if you don't have a rendering system that displays such combinations correctly in context, it would arguably be better to show that there is some *thing* there, rather than to just omit any visible display at all. Such a situation is also roughly akin to the various synthetic virama characters in the standard, e.g., U+17D2 KHMER SIGN COENG, which is essentially a subscript consonant stacker. But if you can't display Khmer conjuncts correctly, it would be better to display a visible glyph at that point than to just ignore it for display altogether. So U+17D2 is also not Default_Ignorable, even though it has no well-defined glyph of its own (hence the dotted box shape shown in the code charts). And in the case of U+17D2, when correctly rendered, it definitely would *not* have its own advance width, yet it is still not Default_Ignorable=Yes.
--Ken
