bug#64013: [PATCH] macfont.m: Fix values for font widths and weights on macOS

[Po Lu]

Stanislav Yaglo <yaglo@me.com> writes:

> Hi everyone,
>
> Currently, on macOS, font weights are not handled correctly, which causes 
> Emacs to choose different font weights and widths than what you
> request. As one example, if you want to choose "Cascadia 
> Code:weight=semi-light", you will get Cascadia Code Light instead. Same for 
> font
> widths, what you specify is not always what you get.
>
> There's some interpolation going on which I didn't remove as it's potentially 
> handy for in-between values, but the problem itself is that the
> corresponding values are not correct for the majority of fonts, for example, 
> kCTFontWeightTrait is specified as -0.24 corresponding to 87.5 , but
> in reality, it is usually 50 (light), and the same for the other values. As 
> you can see here, it won't work as we have 50 followed by 87.5 and then
> (a lower) 80, and then 140:
>
> {-0.4, 50},      /* light */
> {-0.24, 87.5},   /* (semi-light + normal) / 2 */
> {0, 80},         /* normal */
> {0.24, 140},     /* (semi-bold + normal) / 2 */
>
> It probably is a typo, as in the second (duplicate) table of values in the 
> code, 100 is treated as normal instead of 80, which also isn't correct, as
> 100 is medium, not normal:
>
> {-0.4, 50},      /* light */
> {-0.24, 87.5},   /* (semi-light + normal) / 2 */
> {0, 100},        /* normal */
> {0.24, 140},     /* (semi-bold + normal) / 2 */
>
> For font widths, there's only two kCTFontWidthTrait values in the range that 
> are handled currently — from 0 to 1, and they are handled as
> linear font values from 100 to 200, which isn't correct either. For condensed 
> fonts, kCTFontWidthTrait is negative, which is not accounted for,
> and the slope is linear only from -0.4 (50, ultra-condensed) until 0.2 (125, 
> expanded), and from there until ultra-expanded is much steeper.
>
> I've included all values that are listed in font.c for explicitness even if 
> some values can be calculated with the code that interpolates the values.
> Also, I haven't refactored the duplication of the structs and code as I think 
> this is a different issue and should be done separately to keep this
> patch on point of the issue being fixed, and I will probably submit a patch 
> for this later to avoid code duplication and potential issues in the
> future as with the current 80/100 problem mentioned above.
>
> Kind regards,
> Stanislav Yaglo

Thanks.  Have you signed copyright papers for this change?
I've also copied in YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>,
who might have additional comments on this code.