[XeTeX] Support of variable OpenType fonts

[Bruno Voisin]

> On 13 Jul 2022, at 19:05, Bruno Voisin <bvoisin at icloud.com> wrote:
> 
> Overleaf has a very nice technical article on TeX's support of emojis
> 
> https://www.overleaf.com/learn/latex/Articles/An_overview_of_technologies_supporting_the_use_of_colour_emoji_fonts_in_LaTeX
> 
> discussing in detail why XeTeX doesn't support them (ie lack of support of raster-based OpenType colour fonts).

Possibly the following, taken from the Overleaf article, will challenge programmers more directly:

> The real cause of XeTeX’s failure
> 
> The error messages provided by XeTeX partially obscure the actual cause of the problem: OpenType colour fonts, particularly raster-based variants, are not supported by XeTeX. In reality, XeTeX (Kpathsea) can find the Noto Color Emoji font but XeTeX cannot load that font and is unable to initialize its internal font-data tables which are required to use it for typesetting. Internally, XeTeX starts the process of loading the font and tests it for “scalability” (using FreeType’s “definition” of “scalability”) but that test fails and XeTeX issues a standard, arguably misleading, TeX engine error message.
> 
> TeXnical note
> 
> XeTeX’s processing of NotoColorEmoji.ttf was investigated by compiling a debug version of the XeTeX executable. The Eclipse IDE was used to set a breakpoint on the XeTeX function creatFontFromFile(filename, index, pointsize) to step through the code and observe the subsequent processing.

Bruno