components in Glyphs3 sources can have a 'slant' attribute, in addition to 'pos', 'scale' and 'angle'

[anthrotype]

As seen in googlefonts/glyphsLib#1047

apparently this wasn't supported in glyphsLib either (and thus fontmake)

fontc should match Glyphs.app and support slanted components (and glyphsLib will need fixing at the same time).

The above linked issue documents the expected order of the partial transformations comprising the full affine:

translate * slant * rotate * scale

Currently fontc (and glyphLib) only does:

translate * rotate * scale

here:

fontc/glyphs-reader/src/font.rs

Lines 1821 to 1843 in 645cdff

	// Glyphs 3 gives us {angle, pos, scale}. Glyphs 2 gives us the standard 2x3 matrix.
	// The matrix is more general and less ambiguous (what order do you apply the angle, pos, scale?)
	// so convert Glyphs 3 to that. Order based on saving the same transformed comonent as
	// Glyphs 2 and Glyphs 3 then trying to convert one to the other.
	if !from.pos.is_empty() {
	if from.pos.len() != 2 {
	return Err(Error::StructuralError(format!("Bad pos: {:?}", from.pos)));
	}
	transform *= Affine::translate((from.pos[0], from.pos[1]));
	}
	if let Some(angle) = from.angle {
	transform *= Affine::rotate(angle.to_radians());
	}
	if !from.scale.is_empty() {
	if from.scale.len() != 2 {
	return Err(Error::StructuralError(format!(
	"Bad scale: {:?}",
	from.scale
	)));
	}
	transform *= Affine::scale_non_uniform(from.scale[0], from.scale[1]);
	}

[anthrotype]

the test file from the above linked issue example.glyphs can be used to reproduce this.

Exporting a static TTF directly from Glyphs.app (v3.3 3326) and building one with fontc, then comparing the result shows that ours is misisng the (back)slant

[rsheeter]

Good catch!