Feature 'kern' causes overflow in fontlab

Carlos Padilla

Hello everyone

When I try to export a font in FontLab I get this error:

"GPOS feature 'kern' causes overflow of offset to a subtable"

Someone can help me or guide me about this error and how can I fix.

Thank's

Adam Twardoch

The easiest way to optimize your kerning so it fits into the limitations of the OpenType font format is to use the KLTF Make Kern Feature script: http://kltf.de/kltf_otproduction.shtml

Christoph Koeberlin

Karsten’s macro is just fine, but the FDK script should solve the subtable overflow, too: WriteFeaturesKernFDK.py

Ramiro Espinoza

Every time I had this problem I solved it with KLTF's script. Only one thing: Don't forget to name your left and right kerning classes in a very consistent way so the script can recognize them, otherwise it will crash.

Kent Lew

The FDK script can work well if the issue is primarily related to large multi-script data and if you set up your classes to fit the recommended schema.

However, I think Karsten’s script (linked by Adam) is much more calculating and robust in its approach to assembling the subtables and optimizing PairPos1 and PairPos2 tables. As a result, it works more reliably with a broader range of kern data and approaches, especially when the underlying cause is not necessarily multi-script.

[Deleted User]

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Kent Lew

Indeed, he should make sure that all of his kerning data is necessary and important. In my experience, kern overflow issues are often just a symptom of bad data or inefficient structuring. It’s good to address the root cause first.

But wouldn’t duplicate glyphs in kerning classes yield a different error than “overflow”?

[Deleted User]

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Carlos Padilla

Hello everyone, thanks for your comments.

Download the "KLTF Check Kern Classes" plugin. I have actually duplicates kerning pairs.

The problem arose from doubling glyphs superscript letters A to Z, to make the subscript, my idea was to use the same glyphs and just move them down.

The ying yang symbol in "Kerning Assistance" says "No errors found".

I did an experiment, delete all kerning pairs of type and even so, the error continues.

Any idea why this happens?

Ramiro Espinoza

The Yin Yang algorithm is not perfect. As far as I can remember if you have a glyph repeated in the same class this issue is not detected and cause compilation problems.
Also sometimes the Yin Yang icon does not detect mistakes **if there are not kerning pairs using this class**. 
I usually check which is the glyph mentioned in the traceback message, save the classes, and inspect this class file with a good text editor, using the search function.

Ramiro Espinoza

@Christoph Koeberlin  Can you explain me how to use the FDK script 'WriteFeaturesKernFDK.py ?

Christoph Koeberlin

Sorry @Ramiro Espinoza , never used it. But surely @Frank Grießhammer can help!