2011/8/21 Peter Constable <[email protected]>: >> Exactly, but mirroring data for remapping glyphs will not be be part of that >> font. > > Um... Why not? If the mirroring isn't in reflected in > http://www.unicode.org/Public/5.1.0/ucd/BidiMirroring.txt, then it must be > handled by glyph substitution in the font as a normal GSUB operation.
A GSUB operation will only be used if it is specified in the correct feature table. The problem here is which feature to use: "rtlm" or "ltrm" ? It's impossible to know because it first depend on the layout engine to KNOW exactly if the run of text is RTL or LTR. Without a font-level support of BiDi properties of PUAs (or unassigned characters), the layout engine will assume the wrong guess from the "default" property value. And then it won't find the expected GSUB operation, because it won't match it in the correct feature subtable.
