[Fontconfig] Variable fonts redux

[Jerry Casiano]

The changes to FC_WEIGHT and FC_WIDTH will break anything currently
using those, correct?
Are the currently defined weights and widths sticking around or do they
become pointless to even have?
Also, does this mean that a font such as Voto Serif GX will list 500
different variations in a font selector dialog?
Sorry if these are silly questions. 

On Sat, 2017-09-16 at 10:49 -0700, Behdad Esfahbod wrote:
> Hi,
> Previously I have had implemented and merged support for named-
> instances in variable fonts, but the generic variation mechanism was
> not integrated.  I've finished the core of what was left of
> implementing variable fonts in fontconfig:
> 
>   https://github.com/behdad/fontconfig/commits/varfonts2
> 
> High-level changes are:
> 
>   - Add FC_FONT_VARIATIONS (string): this one is easy.  The library
> doesn't do anything about it.  It's like FC_FONT_FEATURES. Client
> sets and then retrieves. Primarily useful for non-standard variation
> axes,
> 
>   - Add FC_VARIABLE (bool), which is at the core of varfont support.
> 
> All font patterns that were discovered previously will get
> FC_VARIABLE=False When querying fonts with (the new)
> FcFreeTypeQueryFaceAll(), or when face index has top 16 bits
> (instance index) set to 0x8000, then a pattern is created for the
> variable font as a whole.  This pattern is only generated if there's
> actual variation in at least one of weight, width, and optical size
> axes. This pattern then might have ranges for FC_WEIGHT, FC_WIDTH,
> and FC_SIZE, and has FC_VARIABLE=True.
> 
>   - FC_WIDTH and FC_WEIGHT are now typed as ranges instead of double.
> 
>   - Fix matching and comparison logic with ranges to behave as we
> expect.
> 
>   - In FcDefaultSubstitute(), set FC_VARIABLE to false if it's
> unset.  FC_VARIABLE has very high match priority, so this takes care
> of backward compatibility with clients that do not support variable
> fonts by pushing them to the very back of fallback list.
> 
>   - In FcFontRenderPrepare, special-case ranges, such that if eg.
> request is for weight=47, and font has weight=[50,100], then rendered
> pattern will get weight=50.  For size, special case again, such that
> if request size=47 and font has size=[50,100], then rendered font
> will have size=47.
> 
>   - For clients that support variable font, eg. pango soon, they have
> to opt-in.  I could have special-cased matcher for FC_VARIABLE, such
> that if FC_VARIABLE=True, then both variable and non-variable fonts
> will be returned, but I didn't like a custom match.  Currently if
> FC_VARIABLE=True is set then variable fonts will dominate the
> results, even if they are not the best match.  To solve that:
> 
>   - To solve above problem, I added a new value for FcBool:
> FcDontCare=2.  Pango should set FC_VARIABLE=FcDontCare.  I was hoping
> to allow clients to tell fontconfig whether, everything else being
> equal, they prefer variable font or static font, but that needs a
> very low-priority element, or a hack, and I didn't like either, so I
> gave up on that.  It's not particularly useful anyway.
> 
> Here's what's left, which I'll do in the next few days.  I just
> wanted to get this out for now.
> 
>   - Config compare operations on "dontcare" are not implemented yet,
> 
>   - The variable-font pattern currently has FC_STYLE set.  I think I
> should completely leave out FC_STYLE,
> 
> With those fixed, I think this can be merged.  At some point I also
> like to:
> 
>   - Speed up scanning of varfonts that have many named instances. 
> Voto Serif GX has over 500!  Right now we open the face and do
> everything from scratch for each named instance.  Can be considerably
> sped up by opening once and switching coordinates around...
> 
>   - Expose slant & italic standard axes as well, but Fontconfig
> shoves FC_SLANT_ITALIC and FC_SLANT_OBLIQUE in the same element, with
> these values:
> 
> #define FC_SLANT_ROMAN              0
> #define FC_SLANT_ITALIC             100
> #define FC_SLANT_OBLIQUE            110
> 
> The varfonts axis for italic has a 0..1 range, 1 meaning italic.  And
> the slant axis uses slant angle.  We can:
> 
>   - Deprecate FC_SLANT element and add two new ones, eg FC_ITALIC and
> FC_SLANT_ANGLE.  I don't like this particularly,
> 
>   - Repurpose FC_SLANT for *italic*, since the 0..1 range maps nicely
> to FC_SLANT_ITALIC=100, and add FC_SLANT_ANGLE for the slant.  I
> don't particularly like to confusing names here either.  So I'm open
> to suggestions.
> 
> A point I'll raise before someone asks:
> 
>   - Exposing contents of STAT table or non-standard axes is out of
> scope for fontconfig.  The rule of thumb is: if it doesn't
> participate in font selection, it does not belong to fontconfig. 
> Things that a font dialog needs should be fetched directly using
> FreeType or HarfBuzz.
> 
> 
> Cheers,
> -- 
> behdad
> http://behdad.org/
> 
> 
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