On 19/4/13 22:13, Behdad Esfahbod wrote:
Konstantin has brought up a few interesting issues regarding variation
selectors. I'll summarize issues that I think can be improved:
- Right now we don't handle a sequence of multiple variation selectors in a
row. What *is* the best way to handle that? Ideally the extra ones should
become visible. Even a box is better than silently not showing them, right?
No - silently not showing them should be the default behavior, IMO.
They're default-ignorable.
- Now, Unicode says only registered variation sequences are valid. I assume
we don't want to enforce that?
In principle, it would be correct to enforce it - e.g. by only passing
the VS to get_glyph if the <char,VS> pair is registered - but I'm not
sure it's worth the overhead of maintaining the list of registered
sequences and checking each VS occurrence we encounter.
- What should HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES do to variation
selectors? Right now they do not become visible no matter what. Is it
relevant whether the variation selector was actually used during glyph lookup
or not?
Yes, I think it's relevant. A variation selector that was used during
glyph lookup has (in a sense) become "visible", manifested as a
particular choice of glyph. I don't think it should -also- appear as a
separate glyph, even in a "show invisibles" mode. In effect, the <base,
VS> pair has been ligated - it's just a "ligation" that's handled by the
cmap instead of GSUB.
Any variation selector that was -not- part of a valid sequence - e.g. if
there are multiple VS chars after a base - should ideally be preserved
by PRESERVE_DEFAULT_IGNORABLES.
The remaining question is what to do about a VS that was part of a valid
sequence, but the font being used didn't handle it. That's a slightly
different case than a VS used in an invalid way, which represents an
encoding error in the data. I suppose PRESERVE_DEFAULT_IGNORABLES should
probably make this case visible as well, though.
- If the answer to the last question of the previous item is yes, then we
would new to change get_glyph() API to two separate get_glyph_variation() and
get_glyph() calls. But that would be an API and ABI break :(. Thinking about
it now, I wish I had done that from the beginning.
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