[mpeg-OTspec] Composite Font Standard 2009/12/16 DRAFT

[John Hudson]

Ken wrote:

> In other words, while I am clearly in unknown territory due to my area of expertise, these transformation must be implemented somewhere, and thus it should be possible to test these ideas to determine to what extent these transformations have an effect on mark attachment.

> I am in the process of checking whether any of my colleagues are able to prepare a test font with mark attachment, but I am again at a loss as how to test these transformations.

I'm likewise at a loss to know how this might be tested in a 'live' 
transformation of the kind proposed for CFS. I don't know very much 
about existing composite font implementations, so am not sure what might 
be possible.

I do know, however, pretty intimately what happens when glyphs are 
transformed independently of GPOS. Let's say we're making an oblique 
version of a roman face, by mechanically slanting the upright outlines 
to a given value, e.g. 12 degrees. Presuming that all outlines are 
slanted relative to the 0,0 origin point, then all x-direction GPOS 
attachment positions will remain valid so long as there are no 
y-direction adjustments to the same attachments. Now lets consider that 
this font might use the same combining mark glyphs in both lowercase an 
uppercase letters (never an optimal design solution, but it happens). 
This means that the marks need to be raised over the uppercase letters, 
say 180 units. Let's say that we're looking at the uppercase I as a 
base: the anchor attachment for above marks is something like x=344 
y=180, on a 1000 UPM font. If we apply the unadjusted GPOS data from the 
roman font to the oblique, the mark will be positioned too far to the 
left (as in the attached image; the pale blue outline shows the desired 
position). In order to correctly position the mark, the transformation 
has to take into calculate the run-over-rise for the slant angle, and 
add this to the GPOS x-direction attachment.

Regards, John


-- 

Tiro Typeworks        www.tiro.com
Gulf Islands, BC      tiro at tiro.com

Car le chant bien plus que l'association d'un texte
et d'une mélodie, est d'abord un acte dans lequel
le son devient l'expression d'une mémoire, mémoire
d'un corps immergé dans le mouvement d'un geste
ancestral.  - Marcel Pérès
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