Logic and the order in kerning tables

mauro sacchetto

Which is the logic and the order in kerning tables?
Let's make a hypothesis. I intend to create three different kerning tables inside the same Lookup, the first one for the letters (pairs as `AV`, `To`, `Fr` etc.), the second one for the punctuation (pairs as `W.` `T-` `F,` etc), the third one for the numbers. Should they be placed in a predefined order?
My problem is that if I create a `'kern' Horizontal Kernin lookup 1` and then two subtables, i.e. `'kern' Horizontal Kerning lookup 1 kerning class 1` and then `'kern' Horizontal Kerning lookup 1 kerning class 2`, only the first is applied. If it is reversed, same thing. The first one is applied, the second one ignored.
Have I to create different lookups (and not different subtables in the same lookup)?
Or there is a conflict if in the first subtable I have `WA` and in the second `W.' ? But are they summative?
Now I ask: what's wrong in my procedure? How can the problem be solved, perhaps keeping the various subtables distinct?
Thank you

mauro sacchetto

I solved alone, by trial and error because I did not understand the logic.
It was necessary to place both the lookups of the kerning of the apostrophe before the general kerning of the letters.
I'd like to understand why ...

mauro sacchetto

If I create a only kerning subtable and put <L'> and <'A> with a particular kerning, the <'A> works and the <L'> don't. Why? Any suggestion?

Kent Lew

If you are interested in designing fonts, then you should probably seek a tool that allows you to design kerning in a user interface and then manages the tables for you upon compilation.

I imagine that nearly everyone on this forum would never attempt to write kerning in the manner in which you appear to be doing, so you probably won’t get a lot of practical advice on this.

However, if you are interested in understanding and engineering fonts at the level of the actual tables and lookups, then you should probably study the OpenType specification to learn about these things.

For instance, regarding table formats in general: https://docs.microsoft.com/en-us/typography/opentype/spec/chapter2

and, with regard to kerning (aka GPOS tables): https://docs.microsoft.com/en-us/typography/opentype/spec/gpos

Erwin Denissen

Concerning your first issue, I did explain it in a previous post, but in case you missed it:

If you have more than one class based kern sub table, do ensure they don't overlap. E.g. individual glyphs used within the first of the pair should not be used in more than one class based sub table within the same lookup.

About the other issue; some smart word processing applications might change the input while you type.

For example in Word 'A (quote A) will be changed to
'A (quoteleft A)

while L' (L quote) is changed to
L' (L quoteright)

mauro sacchetto

Thank you for your direction. Obviously I'm a beginner: only a month ago I had never tried to edit a font...

But, pratically, for instance, I write:
l'America : kerning between ' and A
L'automobile: kerning between L and '
L'AMERICA. requested double kerning between L' and 'A
How can I set the table?
In GaramondPremierePro there is a Lookup with a first subtable (lookup 1 kerning class 1) with kerning for quoteright (left column) and letters as A, F and L (right column). A second subtable  (lookup 1 kerning class 4) with kerning for letters as A, L and F (left column) and quoteright (right column). Is this the correct approach?

Other question
Some fonts have a kerning table for letters, another one for numbers.
Is there any other substantial reason besides a simple principle of order?

Erwin Denissen

mauro sacchetto said:

Thank you for your direction. Obviously I'm a beginner: only a month ago I had never tried to edit a font...

But, pratically, for instance, I write:
l'America : kerning between ' and A
L'automobile: kerning between L and '
L'AMERICA. requested double kerning between L' and 'A
How can I set the table?

Whatever you prefer. If you provide better examples or a font file to test, then we might be able to better assist you.

mauro sacchetto said:

In GaramondPremierePro there is a Lookup with a first subtable (lookup 1 kerning class 1) with kerning for quoteright (left column) and letters as A, F and L (right column). A second subtable  (lookup 1 kerning class 4) with kerning for letters as A, L and F (left column) and quoteright (right column). Is this the correct approach?

There is no single correct approach, but this looks fine to me.

mauro sacchetto said:

Other question
Some fonts have a kerning table for letters, another one for numbers.
Is there any other substantial reason besides a simple principle of order?

Yes. If there are no pairs between letters and numbers than it will save space if you split them into subtables.

mauro sacchetto

Thank you, I solved analyzing some Adobe fonts...