[mpeg-OTspec] RE: Soliciting comments on draft OFF amendment

Bob Hallissy bobh528 at yahoo.com
Thu Sep 3 05:44:52 CEST 2015

On 2015-08-28 at 11:25 Greg Hitchcock gregh at microsoft.com [mpeg-OTspec] 
> Re: Shape and mapping of Glyph ID 0 (the .notdef glyph)
> This sounds reasonable. I suspect that color support would also be 
> reasonable for Glyph ID 0. I agree that no mappings should point to 
> this glyph.

Perhaps I misunderstand what is meant by "no mappings should point to" 
glyph ID 0, but it seems to me that the spec in some cases requires and 
in other cases allows this.

In format 4 cmaps (Segment mapping to delta values) we have the 
requirement that character 0xFFFF be included to terminate the search, 
with the suggestion -- only reasonable -- that it point to glyph ID 0.

Also, in format 4 cmaps, is this tricky phrase:
> If the value obtained from the indexing operation is not 0 (which 
> indicates missingGlyph), idDelta[i] is added to it to get the glyph index

This sounds like the authors expected in some cases that the indexing 
operation will return 0, resulting in a character code mapping to glyph 
ID 0.

Similarly for format 2 cmaps:
> Finally, if the value obtained from the subarray is not 0 (which 
> indicates the missing glyph), you should ...

And in format 0, the only way to indicate a character is not supported 
by the font is by pointing to glyph ID 0.

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