[OpenJDK 2D-Dev] RFR: 8169202: [macos] Font substitution does not work for supplementary characters

[Dmitry Batrak]

Hello,

I'd like to propose a patch for
https://bugs.openjdk.java.net/browse/JDK-8169202.
I have a Contributor status via agreement signed by JetBrains, hope someone
can sponsor the patch.
This repeats the proposal I've sent earlier to this mailing list,
adding now a reference to the created OpenJDK issue.

Currently, if requested font cannot render a Unicode character represented
by a surrogate pair,
no substitution is performed - Font.canDisplay will return false, and the
character will be
rendered as a 'missing' glyph. This behaviour doesn't violate any
specification, but it looks like
it can be easily improved, as underlying OS framework used under the hood
does support surrogate pairs.

The proposed change consists of two parts. First part is adjusting the code
in CoreTextSupport.m
to handle surrogate pairs while performing char-to-glyph mapping, by
encoding non-displayable
surrogate pairs using negative values of the codepoint, similar to how
non-displayable BMP characters
are encoded. Second part is fixing the rendering code (in CGGlyphImages.m),
where wrong type was used
to pass character values around, so that code for surrogate pairs handling,
already present there,
could work.

Webrev for the fix is available at
http://cr.openjdk.java.net/~avu/JDK-8169202/webrev.02/
(kindly posted by my colleague, having access to cr.openjdk.java.net).

Best regards,
Dmitry Batrak
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