Foundation Concepts - when a lookup ends

[bobh0303]

The Foundation Concepts page says, for substitution lookups :

If a rule replaces the current glyph, the following rules are skipped for the current glyph.

and for positioning lookups:

The process is the same for positioning features, except that instead of rule evaluation stopping when a glyph is replaced, the evaluation is stopped when a glyph’s position is changed.

These statements are slightly misleading (though perhaps intentionally so for simplicity?).

When processing a lookup, the lookup ends (i.e. any subsequent rules are skipped) when a rule matches the glyph sequence at the current point of processing -- whether or not any substitutions or positioning takes place.

The OT spec says for substitution:

A lookup is finished for a glyph after the client locates the target glyph or glyph context and performs a substitution, if specified.

and similarly for positioning:

A lookup is finished for a glyph after the client locates the target glyph or glyph context and performs a positioning, if specified.

(italics mine)

Thus in both cases, there may not be any actual action done, but when the client locates the target glyph or glyph context, any subsequent rules are skipped.

[typesupply]

When processing a lookup, the lookup ends (i.e. any subsequent rules are skipped) when a rule matches the glyph sequence at the current point of processing -- whether or not any substitutions or positioning takes place.

Right. I didn't want to get into matches that don't result in a change at this point in this book. The ignore keyword uses this trick to generate a sub/pos subtable that will not change anything. I thought about explaining how this works in the ignore section but I couldn't come up with a way to do it that didn't get too deep for the audience this info is intended for. I'll think about it a bit more.