[Originally posted earlier today on Typedrawers. This information seems significant enough to warrant cross-posting. Comment in the forum of your choice.]
Matthew Carter's new Sitka* typeface family ships as 24 fonts with Windows 8.1. The standard Windows 4-style family is implemented in six different size-specific designs (Small, Text, Subhead, Heading, Display, Banner). After Ross and I got involved in the project, we spent a lot of time at Microsoft discussing options for software selection of size-specific designs. I think we probably hit all the possibilities in one way or another -- everything from new GSUB features to Composite Font Representation wrapper --, and some of the funkier options were quite attractive. In the end, Microsoft opted for a very simple mechanism: two new data values in the OS/2 table, one indicating the lower size range limit for the individual font and one the upper size range limit. The relationship of the fonts is established via existing name table entries.
A formal spec for the new OS/2 table version should be forthcoming from Microsoft in the next few weeks, and will be submitted for inclusion in the ISO MPEG Open Font Format. Here are the basics:
The OS/2 table is updated to version 5.
The two new data fields are
usLowerPointSize
usUpperPointSize
The unit used in defining these values is a twip, i.e. a twentieth of a point. This enables size use ranges to break at fractional point sizes without getting unreasonably fiddly.
The usLowerPointSize value is inclusive; the usUpperPointSize value is exclusive. In other words, the lower value indicates the size at which the font starts to be used, and the upper value indicates the size at which it is no longer used.
EXAMPLE: The Sitka Text font is intended to be used from 9.7 point up to any size below 13.5 point. Therefore, the usLowerPointSize value is set to 194 twips (9.7×20) and the usUpperPointSize value is set to 270 twips (13.5×20).
NOTES:
1. The size selection mechanism is based on the computer point (1/72 inch). This means that selection is made relative to nominal point size, and is not device dependent. This may be controversial (hello, David), and while I think it is reasonable in the long-term -- given device resolution increases, pinch-zoom behaviour, etc. -- there are current situations in which one might well want device dependent size selection that calls for different outlines for the same nominal point size at different resolutions. Folks who want to join in making a case for such should engage with the ISO MPEG specification process once MS submit the draft spec.
2. It should go without saying that this new mechanism is intended to replace the existing GPOS {size} feature mechanism, which has mostly languished unimplemented since it was first specified about 15 years ago. Hacking the GPOS data structure to put font selection information in a table normally accessed only at the end of glyph processing was always counter-intuitive, and I'm not surprised it turned out to be pretty much dead-on-arrival.
3. In order to implement the new data in the Sitka fonts, we wanted to be able to integrate the new OS/2 table version support in our existing workflow. Thanks to Frank Blokland at DTL and the OTMaster developers at URW for quickly adding this functionality for us, which I believe will be available in the next version of OTMaster, which Frank anounced at ATypI.
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* At ATypI Amsterdam, Matthew gave a presentation with Kevin Larson on the Sitka design process, which used iterative legibility testing to refine letter shapes.
