Jason Harrop created FOP-3342:
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Summary: A script's default language system is dropped when its
table is shared with a named language system, so DejaVu Sans is never kerned
under a default language
Key: FOP-3342
URL: https://issues.apache.org/jira/browse/FOP-3342
Project: FOP
Issue Type: Bug
Components: font/opentype
Affects Versions: 2.11
Reporter: Jason Harrop
OTFAdvancedTypographicTableReader.readScriptTable reads a script's LangSys
records and, for each one, compares its offset with the default LangSys offset:
{noformat}
if (dl == lo) {
dl = 0;
dt = lt;
}
{noformat}
When they are equal the default is not read; the script's default-language tag
is set to that named language instead. constructLookups then registers the
features under (script, that language) only, and nothing under (script, "dflt").
Font builders share the table on purpose. FontForge writes one LangSys table
for the default and for every language whose feature list equals the default's,
and it is the default LangSys offset that points at it. On this machine 252 of
1356 installed OpenType fonts have the shape (every DejaVu face, Cousine,
Inconsolata, Adwaita Sans among them), the GPOS table more often than GSUB
because kerning is rarely language-specific.
What is lost depends on the font's DFLT script, where the request lands after
the fallback:
{noformat}
DejaVu Sans, GPOS latn default = latn/ROM = the eight Sami systems: kern
lookups 14 and 15
DFLT/dflt: kern
lookup 15 only
Lookup 14 is the Latin class kerning (97 first glyphs);
lookup 15
covers 20 glyphs and no Latin letter. So under any
default language
DejaVu Sans has no Latin kerning at all.
DejaVu Serif, GPOS latn default shared likewise, but DFLT/dflt lists the
same lookup
as latn, so nothing is lost.
DejaVuLGCSerif, GPOS (the font in FOP's own test tree) latn default =
latn/AZE, and GPOS
has no DFLT script at all: no kerning under a default
language.
{noformat}
Measured on the 2.11 command line, DejaVu Sans, kerning="true", 14pt "AVATAR To
Ye", line width from mutool stext:
{noformat}
language="en" 99.68 pt (falls to DFLT)
no language 99.68 pt (script default, not found; falls to DFLT)
language="ro" 91.55 pt (latn/ROM, which is the shared table: kerned)
language="se" 91.55 pt (latn/NSM, likewise)
{noformat}
and on the same font loaded through FontLoader, matchLookupSpecs("latn",
"dflt", "kern") is empty while ("latn", "ROM", "kern") is not. The
pair-positioning subtables themselves work: performPositioning("AV", "latn",
"ROM") gives -63/1000 em.
This is the mechanism behind "FOP does not kern DejaVu Sans", which is easy to
misread as a PairPos format 2 problem, since the two subtables involved are
class-based.
h3. Fix
Read the default LangSys table and register it under "dflt" whether or not a
named record points at the same table: delete the three lines above. Nothing
else changes; a named language sharing the table is still read under its own
tag.
Test: SharedDefaultLanguageSystemTestCase, on the DejaVuLGCSerif already in the
tree: matchLookupSpecs("latn", "dflt", "kern") must match, hasFeature(GPOS,
"latn", "dflt", "kern") must be true, and performPositioning("AV", "latn",
"dflt") must return a negative x-advance. The first assertion fails on the
current code.
h3. Related
The lookup fallback report (FOP-3341): with the language code never translated
and (script, dflt) never tried, a run with language="en" reaches DFLT/dflt
whatever this report's fix does; both are needed before DejaVu Sans kerns
through a producer that sets the language, as docx4j does.
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