Need for means of distinguishing "interface default/representative instance" from format default

[skef]

A central aspect of support for variable fonts in OpenType is the default
location, which has a normalized value of 0 on every axis. Variable glyph
outline data and other values (e.g. for spacing and kerning) are specified as
deltas relative to the value at the default location.

Many TTF variable fonts, with glyf outlines and deltas stored in the gvar
table, can still be used on systems that have not yet been upgraded to support
variable font functionality. Indeed, the split between glyf and gvar was
specified to allow this. When designing a glyph for this sort of use, one
would typically position the most "general" font instance as the default. For
example, in a font with one wght axis, the "Regular" weight would be a good
choice of default.

However, not all fonts are designed this way. The CFF2 table is not backward
compatible, except in the sense that it is possible to include both a CFF and
a CFF2 table in the same font, with the former serving as a fallback if the
latter is not supported. And although one can choose to put the most general
instance of a glyf-based font at the default location, one may not want to.
For example, if the starting point in the design is two axes with four masters
at four "extreme" locations, it might be preferable to choose one of those
masters as the default, because doing so would substantially decrease the font
file size compared with adding an extra pre-generated master there.

It is already possible to build a self-consistent variable font in this way.
However, not putting the "most regular" instance at the default location
poses an additional problem, as the OpenType format default location is also,
at present, also the interface default location. That is, it is the location:

1. Rendered by default when no other location is specified.
2. Presented as the initial instance in a UI or font picker.

This hard link between the interface default and the format default is not
inherent to variable font technology; it is just how things work now because
there is currently no means of adjusting one without adjusting the other.
Because providing that option would allow some fonts to be significantly
smaller with no other drawback, we are proposing a format extension to do so.

A proposal

An InstanceRecord in an OpenType fvar table includes a uint16 called
flags, which is currently unused. Adobe proposes that Bit 0 of flags be used
to indicate that the instance is the interface default.

A conforming font must use this bit for at most once instance. If it is
present, that instance:

1. Will be presented initially in a UI or font picker,
2. Will be the location rendered if no other location is specified, and
3. Will provide the initial position of any axis not otherwise specified.

If the flag is not present the format default will be used for those purposes.

Alternatives

Obviously the currently available alternative is to interpolate a master
corresponding to the desired interface default and make that the format
default, at the expense of file size.

The proposed avar 2 specification possibly provides another alternative, in
that it has enough flexibility to remap the default location. Adobe sees no problem
with using the avar 2 facilities in this way but feel it is a complex and
potentially confusing solution to a simple problem. Even if a font compiler
were able to automate the translation we worry the result could be hard to
understand and debug, and mixing that convention with simple, "avar 1 style"
adjustments might be difficult.

It has also been suggested that modifications to the standard more in the
spirit of avar 1, and therefore without the complexities of avar 2, could be
sufficient.

References

• TypeDrawers discussion
• avar 2 option (and discussion of this proposal)