[HarfBuzz] handling of variation selectors

[Jonathan Kew]

On 3/7/13 00:51, Behdad Esfahbod wrote:
> On 13-07-02 09:58 AM, Jonathan Kew wrote:
>>
>> This will work fine for fonts that actually expect to handle VS codes in this
>> way. The trouble is that in many cases, the VS codepoint will -not- be mapped
>> to a suitable glyph code by the cmap; it will simply map to the .notdef glyph,
>> which will then appear as a box or similar unwanted artifact in the output.
>> (It would also -block- GSUB/GPOS rules from applying as expected to the base
>> and any following glyphs.)
>>
>> So in the case where the font does not actually map the VS codepoint to a
>> glyph, we should simply delete it from the buffer.
>
> My expectation was that even such .notdef will be marked default-ignorable and
> hence 1) not block GSUB/GPOS, and 2) be removed after shaping is done.  So,
> why is that not working?
>

Argh -- I figured it out, after looking in all the wrong places for the 
problem.....

The issue arose in a testcase we have in the Gecko tree that uses a 
"toy" test font that happens to lack a <space> glyph. And therefore 
hb_ot_hide_default_ignorables(), which relies on having a <space> glyph 
available, doesn't do anything to hide the VS in this testcase, and so 
our test fails.

So I think we can safely ignore this, given that it's such an artificial 
testcase; no need to hack around it in harfbuzz, we'll just fix our test.

Thanks,

JK