add support for glyph shape variation along the spacing axis

[gferreira]

as described in the original proposal by Frank Blokland:

Glyph side-bearing distances (with ensuing effect on advance widths) are typically the primary aspect of design that varies, though details such as serifs or spacing within ligatures may also be encompassed in this variation.

how to implement glyph shape variation in the current VariableSpacing workflow:

• store glyph variations in optional additional layers – SPAC_tight , SPAC_loose
• switch glyphs when loading spacing state

open questions:

• how to express glyph shape variation in the SpacingStates format?
• where to store the default glyph when another one is loaded? SPAC_default

[dberlow]

Frank's original proposal was for Glyph specific changes.

This comes from a traditional view where certain letters with multiple widths were added to a job case, to be used to change a line length.

We could easily simulate this with XTRA, which we use evenly across all letters except I, i and j, to change line lengths. That simulation could be directed only to use XTRA on m,h,n,u,r, v and w, e.g.

So, storing want, Frank is talking about for the Fonts were working with wood, mean storing the subset of cliffs that you wish to use in a particular style to accomplish justification.

Hope that helps? Let me know we can talk later this week.

[gferreira]

hello David, thanks for your feedback

the kind of adjustments I have in mind are related to glyphs which have some kind of internal spacing, like ligatures, digraphs, fractions, etc – see the list for Roboto Flex SPAC

in the default style this looks like this:

before: tight side-bearings, default internal spacing

after: tight side-bearings, tight internal spacing

I think this is different from making glyphs narrower with XTRA?

happy to discuss this and everything else, whenever you can.

[dberlow]

In most cases of these glyphs what you are doing is not different from what is going on an XTRA.