Optically-tuned font size in composite fonts

[John Hudson]

Dear All,

So far, we've been looking at composite fonts primarily in terms of 
overcoming the 64K glyph set limit and providing multi-script support 
from different source fonts within a single composite font. I'd like to 
consider the possibility of using composite fonts to support multiple 
optically-tuned fonts for different sizes of text. I presume the 
participants of this group are familiar with the desirability of 
optically-tuned fonts (also referred to as 'optical scaled' fonts or 
'optical size' fonts), i.e. designs that are intended to be used at 
specific sizes or ranges of sizes, often within a coordinated family of 
optically-tuned fonts for e.g. display, subhead, text, captions/notes or 
more precisely tuned fonts for individual sizes. [Anyone seeking a 
better understanding of these things should order a copy of Tim Ahren's 
very good book _Size-specific adjustments to type designs_.]

In metal type, type was by its nature size-specific and usually 
optically-tuned, either in the letter design stage or during 
punchcutting. In digital type, the most common mechanism to date has 
been the production of independent fonts for different size ranges. 
Conceivably, the optically tuned glyphs for different sizes could be 
contained within a single font, addressed through a substitution 
mechanism, or a single set out outlines could be parametrically or 
individually manipulated for different sizes (or a combination of these 
approaches could be used, e.g. applying substitution for basic sizing 
and then hinting for device dependent adjustments).

Attendant on any implementation of optically-tuned fonts is the problem 
of letting txt layout engines and applications know which font (or which 
set of glyphs within a font) to use for which size(s). Leaving aside, 
for now, models in which all the optically-tuned glyphs reside in the 
same source font (since this implies possible impact of the 64K TT/OT 
glyph set limit, and hence cannot be considered a scaleable solution (no 
pun intended)), there are a number of possible ways in which fonts might 
contain information usable by engines and applications to select an 
appropriately tuned design for a particular size. [I'm also leaving 
aside, for now, the tricky issue of nominal (pt) sizes in a variety of 
low resolutions, implying cruder scaling factors that accounted for in 
outline designs for higher resolution output.]

1. There exists in the current registered OpenType Layout feature set a 
nominal GPOS feature <size> that can be used to store information 
indicating at what sizes the font is intended to be used. I consider 
this a hack. It isn't really a GPOS feature; rather, the GPOS lookup 
structure is hijacked to store some basic size information. Since 
selecting an optically-tuned font is a pre-layout requirement, one is 
left with a mechanism in which this one GPOS feature needs to be 
consulted by an engine or app before any other features are applied, 
contrary to all other GPOS features, which are normally processed 
down-stream of GSUB features. There are also tool issues surrounding 
this feature, which means that only Adobe, to my knowledge, have 
consistently included support for this feature in their optically tuned 
fonts.

2. The idea has been put forward (on the OpenType developer list, 
Typophile forums, and during development of the OT 1.6 spec) of a 
dedicated 'size' table, which would contain information about both the 
size(s) at which the individual font should be used and also its 
relationship to other fonts in the same family. [In theory, this 
information could also be put into an extension of an existing table, 
e.g. OS/2.] Specification of such a table seems stalled by disagreement 
over what it should contain.

I would like to consider the possibility of building optically-tuned 
font selection information into the headers of the composite font, 
alongside the script selection information that it, in many ways, 
parallels (with the obvious implication that there may be quite complex 
interaction between these sets of information, e.g. a composite font may 
contain source fonts for different scripts with different sets of 
optically-tuned designs, or might provide optically-tuned designs for 
some scripts but not for others.

There are two ways in which this might be considered:

a) as a higher-level mechanism for optically-tuned font selection, 
overriding whatever mechanism might be defined within the source fonts 
('size' table or <size> feature); this offers users creating composite 
fonts the possibility to assert their own preferences against the 
decisions of the font developers;

b) as *the* mechanism for optically-tuned font selection, i.e. we give 
up on the idea of including size selection information in the source 
fonts and say that the way to define use of optically-tuned fonts is 
through their inclusion in a composite font.


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