[Fontconfig] editing FcConfig in code

[Jean Abou Samra]

Le mercredi 12 avril 2023 à 01:12 +0100, Idriss Fekir a écrit :
> Hi,
> 
> Some fonts aren't matched by fontconfig correctly e.g: the Amber Whiskey  
> family has 4 faces Regular, Aged, Full Color, Shadow.  
> `fc-match "Amber Whiskey"` outputs "Aged" instead of "Regular", And  
> since Pango relies on fontconfig for that, it also exhibits the same  
> behavior (e.g: Inkscape an gimp, both rely on pango, would pick the  
> "Aged" variant). This could be worked around with a rule:
> 
> ```  
> <match target="pattern">  
>      <test name="family">  
>          <string>Amber Whiskey</string>  
>      </test>  
>      <edit name="style">  
>          <string>Regular</string>  
>      </edit>  
> </match>  
> ```
> 
> My question is how can i do that but in code (in C), that is  
> how can i check what fc-match would output, then if the output is wrong  
> add a rule like the following to the FcConfig?
> 
> I'm open to suggestions if there are better ways.
> 
> I did read the docs but i couldn't figure out how.
> 
> The man page for FcFontMatch says that it should only be called after  
> FcConfigSubstitute and FcDefaultSubstitute have been called for  
> the pattern, i don't understand what that means exactly.
> 
> Thanks a lot.


I'm not a Fontconfig developer, but I will try to answer your question anyway.

As far as I understand, the intent of Fontconfig is to make font configuration shared and static. So, if the font in question is installed through a package manager, the package should normally also include a conf file that makes Fontconfig handle the font correctly.

If you're not taking it from a package, I think you could try

```
FcConfigParseAndLoad(NULL, (FcChar8 *) "your-config-file.conf", FcTrue);
```

or

```
FcConfigParseAndLoadFromMemory(NULL, (FcChar8 *) "<the string of Fontconfig XML code that you would put in a file>", FcTrue);
```

Best,

Jean
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