Right to Left Kerning

bahman

Hi,

Please correct me if I'm wrong but I have worked with RTL kerning before in other softwares/platforms (e.g fontlab) but I don't think "right to left kerning" is working as supposed to be in Robofont 1.4. For one reason when I type رب the order of letters is right before I turn the "Right to left" option on. After turning the option on, order of the letters becomes reversed and when I put kerning between them, kerning is implemented in reverse order. And what is the purpose of this option if I generate the font and the kerning is working in LTR behavior. I checked the generated OTF file and in opentype code kerning code is like this:

pos uni0631 uni0628 -225;

But RTL kerning code should be like this:

pos uni0631 uni0628 <-225 0 -225 0>;

I think Robofont should have other method of storing and generating RTL kerning, Am I wrong?

Thanks,
B

bahman

I don't know the difference! But in fontlab when I open a font, in the kern feature I can see that LTR kerning is interpreted like this:

pos uni0631 uni0628 -225;

But RTL kerning is interpreted like this:

pos uni0631 uni0628 <-225 0 -225 0>;

So I suppose RTL kerning should be implemented with adjustments applied to position of the glyph too, maybe I'm wrong; I'm not a pro. (see AFDKO OpenType Feature File Specification)
About the kerning in RoboFont, I think there is a malfunction in the "Right to left" Option in the kerning panel. Here is what I do:

I open a font which already has RTL kerning, Robofont interprets kerning correctly. I open the kerning panel and click on a class pair, the kerning shows up but order of glyphs is incorrect, I turn the "Right to Left" option on; Nothing Happens! I turn it off, now order of glyphs becomes right, even so the kerning has been implemented in RTL order.

frederik

Hi

UFO3 doesn't store the kerning any different. What really has changed is the naming form left/right to first/second.

see http://unifiedfontobject.org/versions/ufo3/kerning.html

The kerning data is writing direction neutral. For text written left-to-right, the left-most glyph is the key in the top level dictionary. For text written right-to-left, the right-most glyph is the key in the top level dictionary. For example, given the pair LG, written left-to-right, the L is the key in the top dictionary and the G is the sub-dictionary. Given the pair GL, written right-to-left, the G is the key in the top dictionary and the L is the key in the sub-dictionary.

can you explain me the difference between:

pos uni0631 uni0628 -225;
pos uni0631 uni0628 <-225 0 -225 0>;
(adjustment for <X-placement Y-placement X-advance Y-advance>)

thanks

bahman

I just red that UFO 3 stores RTL kerning in a different way, maybe that could help in next versions to avoid problems with it?

bahman

Oh thanks Jens, I was starting to think that my arguments were so stupid, so nobody wants to answer me!

Yes, This is the case every time I'm implementing Kerning in Farsi/Arabic fonts. I always have to rely on external python scripts to generate the RTL kerning (except in the Glyphs app that handles it natively and brilliantly intuitive). I really wished that an app could do this without any inconvenient scripting. I can write simple tools but I would rather to concentrate more on design than writing programs. When I saw Robofont 1.4 enhancements and the feature list, I thought finally we have it in Robofont, bummer!

jens

Perhaps in this case it would be easier to write the feature code for the kern feature directly instead of using space center?

bahman

Anyone?! Fredrickkk!!