Vietnamese diacritics

Vietnamese diacritics are an interesting challenge in a vf with broad opsz, wght and wdth.

Vietanh Nguyen Has kindly reviewed the Roboto (classic), and also advised us on issues with Amstelvar we’ve been working on.

Here is my summary of some of that advice:
KERNING HORNS, a. The horn and diacritics should be close to the same amount of visual impact on the reader, not a horn more prominent than diacritics above, and b. That horns should attach close to, or at the square height, (H or x), not lower where they interfere with adjacent following glyphs.
PAIRING HORNS, shows that horns of glyphs within each case should top-align commonly. I also see that Plex uses one form for both upper and lowercase, and has wider and narrower versions.

SIZE & WEIGHT, are I fact a large challenge in themselves, so in a design space with optical sizes, widths and weights I am expecting Roboto Extremo to set a good example.

The glyph architecture of extremo intended so far to provide flexibility for different cases, and for being stacked, and we need to proof that.

But the requirement for horns in a broad design space, to both top align and connect well aesthetically to round and square-topped glyphs, as it relates e.g. to design space at opsz-max, wght-min, implies horns that are glyph-specific.

We can get all this right in the default of Roboto extreme out before we do any further work to make sure that every composite and glyph gets a correct architecture and variation to allow for correctly positioned, weight and widthed diacritics for each language, Including Vietnamese.