Posted by:
John Hudson I have identified a bug in the implementation of the kerning* in the SBL Hebrew font. The kerning in the current beta version is implemented by applying a width adjustment (GPOS xAdv) to the second glyph in a pair. Since the width is measured from the left extreme of the glyph space, such an adjustment is an efficient way to kern right-to-left scripts. However, I've discovered that this results in the affected glyph being dropped from the run of glyphs being processed by the feature on the grounds that it has already been touched so can now be ignored. In some applications -- those that strictly conform to the OpenType spec -- this means that only every second glyph is kerned.
This bug will be fixed in the next release of the font. NB: this will result in document reflow in applications affected by the bug. Since this is a significant bug, a new beta version of the font will be made available, probably in late May. This new beta will also address some other, less critical issues.
The fix, for those who are interested in such things, is to implement kerning in right-to-left scripts by making an identical width (xAdv) and positioning (xPos) adjustment to the first glyph in a pair.
* Kerning is an adjustment to spacing for specific combinations of letters. Kerning information is stored in the font and applied by applications or rendering engines during line layout. The SBL Hebrew font uses the OpenType GPOS feature rather than the older, 'kern' table format.
