How to derive an Italic TTF from the current Slant axis?

I'm posting these notes that @dberlow made from the FB/Google meeting (with DB and DC only) Thursdays February 17, 2022.

> @davelab6 informed FB, developers of Roboto Flex, that CSS has still not implemented font-style property linking to the slant (slnt) axis (see [https://arrowtype.github.io/vf-slnt-test/slnt-ital-tests](https://arrowtype.github.io/vf-slnt-test/slnt-ital-tests/index.html) etc), and so FB should prepare source files or build code for being or outputting two variable fonts, one regular and one italic.
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> To review how the “Italic” is currently enabled, the max slant axis contains, at the default location, (opsz14-wght400-wdth100…), an obliqued from the regular source, that then serves as the default style from which all other slanted styles are derived.
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> This includes slanted sources for weights, widths, optical sizes and grades, all of which inherit deltas from their corresponding regular styles. So the largest and boldest style of the regular, and the slant axis, combine to form the largest boldest slanted style. This combination may then have additional deltas applied just for its design space requirements.
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> Separating the regular from the slant has the upside of being deployable soon, leaving the issue of whether the two will need to be rejoined later as the main issue .
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> If we separate the slant axis in our source files, and then we then make changes to the regular, they will no longer be transferred to the slant. Any of those changes that do need to transfer to the slant axis would need to be undone if the two were ever combined again. In addition, of course, any changes we make to slant sources after separation would need to be carefully handled in a later merge.
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> So, if this separation happens in source files, it would be better to do so after all the regular reviews have been completed. Then we can split the two and only make changes to the slant from there on. Santiago and I can look into, and provide the two new designspace files fairly easily, and make an example pair of vfs including the review edits on regular by 2/23/22.
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> I leave the reorganization of the repository up to Marc and Santiago.

I proposed an alternative, to keep the source files with a single design space, but adjust the build script to derive from the current single design space TTF two children "subspace" or "partial instance" TTFs, a Roman which is simply dropping the slant axis, and an Italic which is all the axes with a single fixed Slant axis value instantiated. I think the fonttools subsetter has been capable of outputting such subspaces for a few years and should be very reliable at doing this. Then, going forward as changes are made to the regular, they will be transferred to the full range of the slant axis as intended, and therefore to the Italic subspace output.

But I am not sure the fonttools subsetter can make a subspace for the Italic this way. @anthrotype wdyt?

Start with the build script and the fonttools subsetter, using the proposed Roman and Italic subspace outputs as the investigation target. Determine whether the current single design space can reliably produce both TTF children, and document whether that approach preserves the intended regular-to-slant changes.

Written by the indexing model from the issue text.