Cross-script OpenType feature?

Adam Jagosz

I've been wondering if it is possible to define a substitution rule for a letterpair consisting of two letters from different scripts, say, Latin and Greek? From what I tried, it seems impossible, I created some classes containing glyphs from mixed scripts and I defined lookups for them which I included in features for both scripts, and it didn't work.
I don't like the idea of scripts and language systems in OpenType scripting, I've always thought it was only limiting the designer. The only useful purpose of this system seems to be the localization feature. Am I wrong?

Kent Lew

I should think it would work fine if the rule is registered under DFLT dflt.

Erwin Denissen

No, you can't mix scripts, as such text will be split up in separate segments.

OpenType layout engines should work the same, but don't. For example a kern pair which includes a space character will work in Word, but fails in Firefox.

Bahman Eslami

This matter becomes more complex when the direction of the script changes. So it becomes very hard to process the shaping on both scripts in a same segment. Take the following sequence of Unicode string:

Latin فارسی

This sequence is stored in the memory in this order:

L
a
t
i
n
 
ف‍
ا
ر
س
ی

You see how the order is different from what you see in the top sequence. Because the order of the string in memory is in the order which was typed by the user. In the final display the first word is written left to right and second word is written right to left. Before applying the OpenType features every segment of scripts will be separated to avoid problems with directions. I'm not sure why this would happen with scripts with same direction though. This behavior could defer in different shaping engines.

John Hudson

Shaping engines segment text strings based on Unicode script property, so the Latin and Greek glyph end up in separate glyph runs. This means that they're never processed together, and hence any lookup that requires them to be processed together will fail.

You could try Kent's suggestion (putting the relevant lookup under the DFLT script tag), but I'm not hopeful that it will work reliably. It may work if the DFLT script is the only one in the font, but if the font also contains latn and grek scripts, these will probably override anything in DFLT.

Thomas Phinney

Agree with John. I will be surprised if you find (m)any layout engines that do not break the text run depending on writing system (script), regardless of how the features are set up in the font.

Khaled Hosny

For example a kern pair which includes a space character will work in Word, but fails in Firefox.

Firefox is able to handle OpenType features across space for a few years now (though fonts having such features will not benefit from shaped work caching in Firefox and Chrome which might negatively affect text rendering speed, especially for large amounts of text).

Erwin Denissen

@Khaled, you are right as it now seems to work.

It is still good to be aware the layout engines don't produce 100% identical output, so better test specific features within web browsers, word processors, and type design software.