[Fontforge-users] Some fundamental opentype tables questions

[Dov G. <dov...@gm...>]

I give up... I've been reading both the OpenType spec, and all the
relevant FontForge manuals, but I can still not figure out "how it
works". I guess I would really need a "FontForge and OpenType for
Dummies" introduction.

So let me describe my toy problem, and what I don't understand from
the FontForge tutorial and manual.

E.g. let's take the "5th" examples from the manual. Let's take two
examples. In the first we want to do a vertical displacement of t and
h, and in the second example we want to replace the th with a separate
glyph.

Example 1:

Q1. Which table should I use? "Context Pos" or "Chain Pos"?
Q2. I'm assuming Context Pos since I want to match a certain context.
I choose Contextual/Context Pos/New. What tag should I use? I'm trying
"kern". This is probably not ok. But how can I know what is allowed
here?
Q3. I choose "by coverage". I do Match/New and type "five t h"? Is this ok?
Q4. In the lookup I choose Sequence number 0 and type the label L001.
Is this ok?

If I now save the font as an otf font and reload it I get the
following error messages:

  Attempt to apply a lookup to a location out of the range of this
contextual lookup seq=1, max=0
  Bad format for coverage table 3
  Attempt to reference lookup 3 (within a contextual lookup), but
there are only 2 lookups in 'GPOS'
  :

So I erase the "Context Pos" entry and will instead try "Chain Pos".
Try reloading and I still get a similar error message.

Erase the context pos and save and reload without any gpos/gsub
tables. This works without any error message.

So I moved over to example 2, using "Chained Sub" and it worked! I.e.
I could save and reload a truetype font. Some progress. :-)

But now the question is, how do I use my newly created tag L001? What
do I now need to do to make it trigger the use of glyph "thlig" glyph
that I presumably defined?

(What I want to do later is to change the order of "B n m" to "B m n"
so that the "B m" ligature may be applied, where B is a base character
and m and n are marks. But I figured that I should first learn the
basics.)

Thank you very much in advance!
Dov