Dear ConTeXt users,

I have a question concerning font fallbacks in math fonts. I have not been able 
to find an option to specify optical sizes in fallbacks. I want to use Minion 
Math for the alphabetical symbols, and this font ships with single font files 
for each optical size. So I am searching for the right command to get for 
example the following behaviour:

Use the CambriaMath fonts for mathematical symbols and use Minion Math 
(Minion-Math-Regular.otf for standard size, Minion-Math-Caption.otf for indices 
and Minion-Math-Tiny for sub-indices) for alphabetical symbols.

Can anyone tell me which commands I have to use to achieve this?

So the general question is paraphrased as follows: Use some font as a general 
math font (here CambriaMath) and replace some symbols out of another fonts that 
ships with single files for each optical size.

One remark about that: Minion Math has supporting mkiv-files. So you can use 
this font (specifying all optical sizes) by writing \setupbodyfont[minion]. One 
could make use of that and maybe use a command like

\definefamilyfallback[MathFallback][family:minion][<range of symbols to be 
replaced>][force=yes].

As far as I know a command like that does not exist at present. Maybe it could 
be designed to take use of existing font definition files.

And I have some similar problem: I do not like the mathematical symbols of the 
STIX2 fonts that much (since they are mainly taken from STIX1) and let us 
suppose now that I want to use again CambriaMath instead for mathematical 
symbols. I was able to figure out the following commands that nearly achieve 
what I want to get:

\definefontfallback[MathFallback][file:STIX2Math.otf][lowercasescript,uppercasescript,lowercasefraktur,uppercasefraktur][force=yes]
%
\starttypescriptcollection[stix2own]
        %
        \starttypescript [\s!math][stix2own][\s!all]
        \loadfontgoodies[cambria-math]
        \definefontsynonym[\s!MathRoman] [Cambria-Math] 
[\s!features={\s!math\mathsizesuffix},\s!designsize=\s!auto,\s!fallbacks={MathFallback},\s!goodies=cambria-math]
         \stoptypescript
        %
        \starttypescript [stix2own]
                \definetypeface [\typescriptone] [\s!mm] [\s!math]  [stix2own] 
[\s!default]
        \stoptypescript
        %
\stoptypescriptcollection

This font can be used by the command \setupbodyfont[stix2own].

The only problem is that STIX2 has design sizes embedded in the file 
STIX2Math.otf that are used if I am using this file as whole for math, but that 
are not used any longer using the mechanism defined above. That means: If I use 
Cambria as Math font and replace the alphabetical symbols out of STIX2Math.otf 
the optical sizes for indices are not used any longer.

What can be done to solve this second problem?

One last question remains: I dislike the summation symbol of CambriaMath 
(because it is too large in my eyes) and this is why I want to replace it by 
the version of STIX2Math again (this symbols is indeed redesigned in STIX2Math 
and looks very nice in my eyes). So I could add a second font fallback that 
should be applied besides the one above. The unicode slot of the summation 
symbol is 0x2211in STIX2Math. So I could define

\definefontfallback[SummationFallback][file:STIX2Math.otf][0x2211][force=yes]

and pass it as a second fallback as follows (changing the following line in the 
example above):

\definefontsynonym[\s!MathRoman] [Cambria-Math] 
[\s!features={\s!math\mathsizesuffix},\s!designsize=\s!auto,\s!fallbacks={MathFallback,SummationFallback},\s!goodies=cambria-math]

The problem is that only one of those fallbacks is used if I pass two arguments 
for fallbacks. Of course this problem can be solved easily if I add the 
summation symbol to the list of replaced symbols in the first fallback 
definition. But if I want to use a summation symbol out of another font this 
trick does not work any longer.

Besides there is a second problem in replacing the summation symbol in the way 
described: Every time I am using $\sum$, ConTeXt uses the bigger symbol 
designed for displayed formulas and not the one designed for inline formulas. 
How can I solve this problem?

Thank you very much for your assistance! If running minimal examples are 
required I can of course provide them. But I hope someone understands the 
fallback mechanism better than I do and then the minimal examples written by me 
would be obsolete (besides the require some font definition files which makes 
them more of less a little complex). Besides because of Minion Math is 
commercial not everyone owns this font an thus is unable to run the minimal 
example.

Kind regards
Mathias

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