Jason Harrop created FOP-3343:
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Summary: A font configured with kerning="false" is still kerned
through GPOS; the flag disables only the legacy kern table
Key: FOP-3343
URL: https://issues.apache.org/jira/browse/FOP-3343
Project: FOP
Issue Type: Bug
Components: font/opentype
Affects Versions: 2.11
Reporter: Jason Harrop
The font configuration's kerning attribute is documented as enabling or
disabling kerning for the font (default true). It does so only for the legacy
kern table: OFFontLoader skips copyKerning when it is false, and
GlyphMapping.useKerningAdjustments then finds no kerning info. GPOS positioning
is independent of it: MultiByteFont.performPositioning runs whenever the font
has a GPOS table, and every script processor's positioning feature list carries
kern. So an OpenType font with a GPOS kern feature, which is nearly every
modern font, is kerned regardless of the flag.
Two things underneath: the loader never records the flag on the font object, so
CustomFont.isKerningEnabled() is true for every TrueType font whatever the
configuration says (only the XML-metrics path sets it); and a producer that
declares a font twice, once with kerning off and once on, to kern only where
its source document kerns, gets kerning in both.
Measured, 2.11 command line, Carlito, "AVATAR" at 14pt, language="en" (with the
lookup fallback fixed so that GPOS applies to this font at all; see FOP-3341):
{noformat}
kerning="true" AVATAR 43.19 pt wide
kerning="false" AVATAR 43.19 pt wide
{noformat}
h3. Fix
# OFFontLoader.buildFont: returnFont.setKerningEnabled(useKerning).
# MultiByteFont.performPositioning: when kerning is disabled, position with the
kern feature removed from the script processor's positioning list; mark and
mkmk still apply, since they are not kerning. Done as a feature delta ("-kern")
through new overloads of GlyphPositioningTable.position and
ScriptProcessor.position, the same mechanism as the gsub-features delta.
After the fix, same sample: kerning="true" 43.19 pt, kerning="false" 46.62 pt;
ligatures (GSUB) unaffected either way.
Tests: LanguageSystemTestCase (the flag reaches the font),
MultiByteFontTestCase (the delta reaches positioning), GsubDeltaTestCase (the
delta on a positioning list).
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