Re: Position of prime for derivatives

[Hans Hagen <pragma@wxs.nl>]

On 6/4/2017 11:06 PM, Otared Kavian wrote:
> Hi Mikael,
> 
> Indeed your observation is correct, but using \over instead of \frac yields the correct behavior. Please try the following sample:
> 
> %%% begin derivatives-prime-4.tex
> \setupbodyfont[lucidaot]
> 
> \starttext
> 
> Compare the position of the prime sign in $f'/g$ in these two expressions: the first one using \type{{... \over ...}}
> \startformula
> \left({f \over g}\right)' = {f' \over g} - {f \over g^2}g'
> \stopformula
> (which yields a correct position of the prime for $f'$ in $f'/g$), and the second using \type{\frac{...}{...}}
> \startformula
> \left(\frac{f}{g} \right)' = \frac{f'}{g} - \frac{f}{g^2} g'
> \stopformula
> which yields a prime sign for $f'$ a little bit lower.
> 
> \stoptext
> %%% end derivatives-prime-4.tex
The problem with primes is that it's a real mess, conceptually as well 
as in fonts.

- we need to deal with not only one prime but also with double, tripple 
and quadruple ones and they need to become proper unicode symbols too

- fonts have one, two or three sizes, often inconsistent and with funny 
dimensions, onlyt the text size can be trusted

- we cannot use prime as nuclues as it can be combined with a subscript

- but it isn't designed as a script symbol and is already raised (somewhat)

- so we have to deal with a good old tex / fonts / plain inheritance, 
unicode but also want to avoid active character and parsing mess etc.

I tried several approaches and it's hard to come up with a general 
solution that suits all fonts.

Now, the bad is that when we use a superscript with a smashed virtual 
characters we end up in script choice mess with primes being one step 
too small.

So, I decided to follow a different route and extend luatex with a 
sizeless super/subscript feature so that we can handle such weird cases 
(there might be more).

This also means that the primes get larger. I only use the text size 
prime and scale that down for the other sizes i.e. no stty for primes as 
they are such a mess) but we can live with that.

The good news is that it gets better but the bad news is that you have 
to wait till we release luatex 1.0.5 (planned for the ctx meeting but we 
might do an intermediate garden release anyway).

btw, with \frac you get cramped styles and these have different 
positioning. Such is life.

Hans

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                                           Hans Hagen | PRAGMA ADE
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