Lookup Type 1 — Single Substitution

Single Substitution is a GSUB rule that replaces a single glyph with another single glyph.

sub a by a.sc;

Rules are written inside a feature block. Used in the smcp feature, for example:

feature smcp {
    sub a by a.sc;
} smcp;

Syntax

sub <glyph> by <glyph>;            # format A
sub <glyphclass> by <glyph>;       # format B
sub <glyphclass> by <glyphclass>;  # format C

<glyph> — a single glyph.
<glyphclass> — a glyph class.

Format B replaces every glyph in the target glyph class with the same single glyph. Format C replaces each glyph in the target glyph class with the glyph at the same position in the replacement glyph class. Format C requires the target and replacement glyph classes to have the same number of glyphs. When the replacement glyph class has only one glyph, it is treated the same as a format B rule.

A single format C line produces the same result in the font as writing out format A rules glyph by glyph. The following two are the same:

sub [a b c] by [a.sc b.sc c.sc];

sub a by a.sc;
sub b by b.sc;
sub c by c.sc;

Shorthand

sub is the short form of substitute. These two rules are the same:

substitute a by a.sc;

sub a by a.sc;

How the lookup type is determined

The syntax is similar to Lookup Type 3, but there is no need to state the Lookup Type. The compiler determines it from the shape of the rule.

# every glyph in the class → the same single glyph = Single Substitution (Lookup Type 1)
sub [ampersand.1 ampersand.2 ampersand.3] by ampersand;

# a single glyph → one candidate from the class = Alternate Substitution (Lookup Type 3)
sub ampersand from [ampersand.1 ampersand.2 ampersand.3];

Try it

Small capitals (smcp)

.fea source

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Small Caps

With rules

Small Caps

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