[mpeg-OTspec] Composite Font, transformation matrix vs scaling

[Karsten Luecke]

Hello Mr Strebe,

yes, this is correct. As soon as rotation is applied to connecting 
glyphs (forming quasi-ligatures), this will result in what I vaguely 
called "nonsense results". I regard it as acceptable -- the designer 
needs to care which transformations he applies to which kind of font(s).

The question is, what are usual scenarios in which rotation is applied?

Best wishes,
Karsten




Daniel Strebe wrote:
> Karsten,
> 
> Thank you very much for calling attention to the mark attachment case. 
> To restate the problem, applying the x/y metric scaling to the anchor 
> positions does not yield correct results. Is that a correct statement of 
> your thesis? I agree this is a problem.
> 
> If I understand your solution correctly, you think we should apply the 
> glyph transformation matrix to the anchor points and not just to the 
> glyph outlines. I agree that solves the problem. Metric scaling is not 
> applicable to glyph construction, but only to placement and scaling of 
> the fully constructed glyph.
> 
> I have less of a feel for how burdensome this requirement is. One method 
> is to construct the glyph first, including all marks, and then to apply 
> the general transformation. The other is to work in transformed space. 
> They yield the same results.
> 
> If a ligature gets built up as a series of marks (does this happen?) 
> then obviously glyph rotation would be wrong and meaningless; it would 
> cause the elements of the ligature to move off the baseline. Indeed this 
> is true of normal ligatures as well, and so it seems that  the ligature 
> must be broken into component characters before applying a 
> transformation matrix that includes rotation. Shear (obliquing), on the 
> other hand, seems like it could apply to any character in any script, 
> and without harming ligatures.
> 
> Regards,
> 
> — Daniel “daan” Strebe
> Senior Computer Scientist
> Adobe Systems Incorporated
> 
> 
> 
> On 09/09/19 3:40, "Karsten Luecke" <karstenluecke at yahoo.de> wrote:
> 
> 
>      
>      
> 
>     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