Re: How to determine if a font is a fallback font?

[Urs Liska]

Am 28.04.2015 um 17:53 schrieb Urs Liska:
> I think I have found something.
>
> After some experimenting with a copy of font-config-get-font-file and 
> trying different approaches I could guess from the documentations 
> available I found that replacing an item in the FcPatternGetString 
> call this function returns the font name and not the Postscript name 
> or the font file.
>
> The following preliminary function (if added to font-config-scheme.cc) 
> returns either the Font Name (if the font exists) or "Emmentaler-11" 
> (on my system) if it doesn't exist. I assume it would return other 
> names on other systems:
>
> LY_DEFINE (ly_font_config_font_exists, "ly:font-config-font-exists", 
> 1, 0, 0,
>            (SCM name),
>            "Determing if font @var{name} exists.")
> {
>   LY_ASSERT_TYPE (scm_is_string, name, 1);
>
>   FcPattern *pat = FcPatternCreate ();
>   FcValue val;
>
>   val.type = FcTypeString;
>   val.u.s = (const FcChar8 *)ly_scm2string (name).c_str (); // 
> FC_SLANT_ITALIC;
>   FcPatternAdd (pat, FC_FAMILY, val, FcFalse);
>
>   FcResult result;
>   SCM scm_result = SCM_BOOL_F;
>
>   FcConfigSubstitute (NULL, pat, FcMatchFont);
>   FcDefaultSubstitute (pat);
>
>   pat = FcFontMatch (NULL, pat, &result);
>   FcChar8 *str = 0;
>   if (FcPatternGetString (pat, FC_FAMILY, 0, &str) == FcResultMatch)
>     scm_result = scm_from_utf8_string ((char const *)str);
>
>   FcPatternDestroy (pat);
>   return scm_result;
> }
>
> Now I get stuck with C++ basics, namely types, references and 
> pointers, but it should be possible to achieve something with the 
> following approach:
> In the line
> scm_result = scm_from_utf ...
>
> "str" holds the result from this match.
> If this matches the originally passed "name" argument the return value 
> should be set to SCM_BOOL_T.
> This should be dead easy, but I don't know to perform this comparison.
>
> If someone gives me a helping hand here we would get a function 
> ly:font-config-font-exists that returns #t or #f depending on the 
> existence of the font.
>
> Please ...
>
> Best
> Urs

Wrapping this Scheme function around that:

fontExists =
#(define-scheme-function (parser location font-name)(string?)
   (string=? font-name (ly:font-config-font-exists font-name)))

shows that my idea is right: The C++ function returns the family name of 
either the given font or the (system-dependent) fallback font. Now all 
there would be to it is moving the comparison of the latter Scheme 
function to the C++ function .

Urs