[smufl-discuss] Re: Glyph registration proposal

[dspreadbury]

Joe wrote:

> I have had two items on my SMuFL to-do list for a while: propose
> standardized glyph registration w/r/t left edge and baseline, and
> propose an external metadata file format. The nature of the external
> metadata depends on the registration, since it only need supply
> whatever information the registration lacks. So registration is the
> best place to begin.

Agreed. I had already drafted a basic set of guidelines for metrics for
fonts intended for use by scoring applications in the SMuFL 0.5 document,
so your contribution arrived just at the right time, and I was able to
supplement the guidelines I had started with your recommendations.

> I acknowledge that there are some differences between this proposal
> and the actual glyph registration found in Bravura today. However,
> in light of the lack of discussion so far on this subject I felt
> that it was best to begin by taking my best shot at a set of
> consistent principles even if this required some adjustments to
> Bravura.

That's fine by me. Our in-development application is only using tiny bits
and pieces of Bravura at the moment anyway, so it's not a major
inconvenience for us to rework our rudimentary drawing code to work with
different glyph registration at this early stage.

In general I am in agreement with all of the guidelines you have proposed,
and have incorporated them into the forthcoming SMuFL 0.5 documentation,
as well as modifying Bravura to match the changes. I have a few comments,
below.

> Currently, the registration roughly centers each staff vertically
> (within a half staff line) with respect to the center line of a 5-
> line staff, but takes an unclear approach for staves with an even
> number of lines that seems to round off the vertical placement. The
> glyphs for 2-, 4- and 6-line staves should be adjusted so the staff
> line symbols all have their vertical center aligned with the center
> of the middle line of the 5-line staff glyph.

I have adjusted Bravura to match this recommendation, but I'm not 100%
sure that e.g. a 2-line staff should conceptually be centered on the
middle line of a 5-line staff; there is presumably some argument about how
changes in the number of staff lines (e.g. midway through a system) should
be handled: should there be any continuity between the positions of staff
lines when switching between a 5-line and, say, a 2-line staff?

I imagine that few (if any) scoring applications will actually choose to
use these symbols for drawing staves anyway, so this is somewhat academic,
I suppose.

> NOTES (U+E100–U+E11F)
>
> These should be vertically centered on the baseline. Currently only
> the breve and semibreve are centered.

I had already added "duplicates" of the breve and semibreve notes into the
Noteheads range, which are centered on the baseline (I had already made
this change earlier in SMuFL 0.5's development, ditto for accidentals),
and had left all of the notes in the Notes range sitting on the baseline.
My justification for doing this was that precomposed notes would
presumably only be used for text-like purposes. However, since we all seem
to be agreed that we need separate fonts for text-based applications
anyway, that justification is no longer valid.

> MENSURAL NOTATION (U+E570–U+E59F)
> Gregorian notation (U+E5E0–U+E5FF)

I'm not actually sure how best to adjust the registration of the motley
collection of Gregorian notations inherited from the Unicode Musical
Symbols range, since several of them are complex ligatures encompassing
multiple pitches. So for now I have left these alone.

> - Should the vertical line at X=0 in design space be used to define
> the nominal alignment point of ornaments, articulations, stem
> decorations (such as single note tremolos), much as the baseline is
> used to align note heads etc.? Note that it's quite common to have
> glyphs that have bits to the left of X=0, such as the "combining
> diacritical marks" range of Unicode.

I wonder if this gains us anything? I guess that the most common kinds of
alignment for these kinds of objects is above/below notes; unless we want
to propose that noteheads should also be centered on x=0, then I'm not
sure centering things like ornaments and articulations on x=0 would help.
Would substantively worse results be had by centering the center of the
glyph's bounding box on the center of the notehead's bounding box, I
wonder?

Daniel

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