Composite Font, transformation matrix vs scaling

[Karsten Luecke]

Hello Mr Lunde and Mr Strebe,

discussion has moved on but I want to go back to:

 > Perhaps we should apply a transformation matrics to the glyph,
 > and a separate x/y scaling to the metrics.

It occurred to me that in some circumstances, marks may get positioned 
not at the desired place e.g. when using a transformation matrics to 
slant outlines but not to metrics: As soon as mark attachment involves 
an y-shift of the mark from its original position, the mark will be off 
its ideal horizontal position.

I made four screenshots for illustration. Each time with base glyph to 
the left, mark glyph in the middle, and to the right is the result of 
attachment. The little circles to the left and in the middle symbolize 
anchor/attachment points.

    1.a
The mark sits on destined height already, the y value of anchors in both 
base and mark glyph are 0:
http://www.kltf.de/OFF/1a_same_y_in_base_and_mark_anchors_not_slanted.jpg

    1.b
In this case, slanting outlines but not anchor points is no problem 
since both base and mark glyph would be slanted "in tune".
http://www.kltf.de/OFF/1b_same_y_in_base_and_mark_anchors_slanted.jpg

    2.a
The mark does not sit on destined height -- imagine lowercase marks used 
on uppercase letters. The y value of anchors in base and mark glyph differ.
http://www.kltf.de/OFF/2a_diff_y_in_base_and_mark_anchors_not_slanted.jpg

    2.b
In this case, slanting outlines but not anchor points results in 
mal-aligned marks.
http://www.kltf.de/OFF/2b_diff_y_in_base_and_mark_anchors_slanted.jpg

What do you think about a transformation matrics (rather than mere 
scaling) for metrics too? Or would this be too complicated to implement?
Really crazy transformations may lead to nonsense results anyway in 
terms of mark attachment, but slanting seems like something that people 
are likely to make use of. (And in my opinion, mark attachment will be 
"standard" in future fonts so should be taken into consideration.)

Best wishes,
Karsten