[designspaceLib] "layer" attribute to sourcedescriptor (edited)

Following up on the pre-robothon 2018 talk for support layers in designspaces.

We discussed a couple of steps to enable supporting outlines to specific glyphs. The use case would be a variable font can have n masters for most glyphs, but one or more need some extra help and they can have n+i additional shapes. If you

• A new support attribute in the source element indicates the name of the UFO3 layer the glyph geometry is coming from. (does it need a naming scheme or convention?)
• The layer does not need to have a complete glyphset, only the glyphs that are present in the layer will be processed.
• The layer does not need to be in a specific UFO - it can be in the default UFO, but can be any ufo really.
• The designspace consumer should be able to handle a varying number of masters in each glyph.

To do:

1. I guess this requires a change to the specification: https://github.com/fonttools/fonttools/blob/master/Doc/source/designspaceLib/readme.rst#attributes
2. Add support to varlib (what else cu2qu ? ) to support a varying number of masters per glyph.

Kerning is specifically not a part of this proposal because it is difficult to store kerning tables with glyphs that are not present in the font.

[justvanrossum]

I thought the ability of specifying a layer name from which to take glyphs is more or less independent to the need to have “sparse” masters. If possible (and if it makes sense) I would suggest:

• add an optional layer attribute to the <source> element, meaning “take glyphs from this named layer”
• allow sparse glyph sets everywhere except in the neutral

[behdad]

Agree. The name “support” doesn’t make sense to me. Any source can come from another layer. Any source can be sparse. Initially, base source must have all glyphs.