Subtables. Rules. Working with "chained context substitution"

[Torsten Ulv]

I'm training to work with an opentype designer.
The form used is "chained context substitution". Various conditions apply. Each condition is created in one rule. The rules are arranged in four subtables. The same effect is obtained by creating four rules in one subtable. Why is this happening? The glyph substitution effect is obtained, but I can not understand the intended purpose of the subtables, rules.

subtable and rules.PNG (6.21 KiB) Viewed 1522 times

How to interact with multiple items in any of the columns?

question1.PNG (16.44 KiB) Viewed 1522 times

How to apply conditions in the "includes" tab?

question.PNG (17.5 KiB) Viewed 1522 times

includes.PNG (14.32 KiB) Viewed 1522 times

What does this option mean?

sequenceindex.PNG (15.53 KiB) Viewed 1522 times

I don't speak English. To understand the above, a monosyllabic text is written. Sorry.

[Erwin Denissen]

I'm training to work with an opentype designer.
The form used is "chained context substitution". Various conditions apply. Each condition is created in one rule. The rules are arranged in four subtables. The same effect is obtained by creating four rules in one subtable. Why is this happening? The glyph substitution effect is obtained, but I can not understand the intended purpose of the subtables, rules.
subtable and rules.

They are basically the same, but the way they are stored inside the font binary is different.

How to apply conditions in the "includes" tab?

That is something very special which related to class0. It is hardly used.

With both chained context positioning and chained context substitution you can also make use of a special class known as class0. This class contains all glyphs except the ones used within the context. For example, if a font contains only these characters: a b c d e, then if Input uses a and d, class0 contains b c and e. The classes within the Includes section will all be treated as being not part of class0. If your rules do not make use of class0, then you can leave the Includes fields empty.

[Torsten Ulv]

That is something very special which related to class0. It is hardly used.

However, I'm interested in trying it out.
Created a simple design and nothing more. In the Lookups added a single substitution of glyphs. Glyphs grouped into three classes.

Class.PNG (55.07 KiB) Viewed 1464 times

Where:
@0 - digits
@A - all uppercase
@a - all lowercase

Rules.PNG (22.21 KiB) Viewed 1464 times

If you change the Input field in the Includes section by inserting classes one by one, then the effect of excluding glyphs from a particular class is obtained. It looks like this:

Includes1.PNG (51.46 KiB) Viewed 1464 times

Include_A.PNG (52.86 KiB) Viewed 1464 times

Includes_a.PNG (50.9 KiB) Viewed 1464 times

Includes_Aa.PNG (52.32 KiB) Viewed 1464 times

I can't figure out how the Backtrack and Lookahead fields in the Includes section will work. Please, tell me.

[Erwin Denissen]

Try this feature code. It is important to know it is not fea code but our own OTLFD syntax, as fea does not support this advanced stuff.

Also note, the "include input @three;" is not actually used in this example and can be left out.

You can test it in Word and in Edge, but somehow Firefox seems to have an issue with it. It might be a harfbuzz bug.

Code: Select all

script DFLT {
  feature RequiredLigatures;
}

class @one [one];
class @three [three];
class @two_three [two three];
class @digits [zero-nine];

feature RequiredLigatures rlig {
  lookup ChainingContextClass0;
}

lookup ChainingContextClass0 {
  include backtrack @one;
  include input @three;
  context (@class0 @two_three) @digits;
  sub 0 digittonumber;
}

lookup digittonumber {
  sub zero -> numbersign;
  sub one -> numbersign;
  sub two -> numbersign;
  sub three -> numbersign;
}

sampleclass0a.png (114.72 KiB) Viewed 1449 times

sampleclass0b.png (103.46 KiB) Viewed 1449 times

sampleclass0c.png (131.79 KiB) Viewed 1449 times