QFontDatabase/QFont question

Fri Jan 10 04:36:01 GMT 2025

Hi again Maurizio.

First and foremost, thanks again for your input; I always appreciate it. 
  (I checked back in my mail archive, and I have responses from you 
dating back to Feb 2019; you've been helping me for a long time. 😊)  
Also thanks to Charles, who chimed in as well.

Apparently, the code I posted worked for Charles.  He didn't add Exo to 
the Windows system, but added it to the PyQt app with 
QFontDatabase.addApplicationFont().  I tried it that way, but it still 
behaved oddly for me.  We are using the same Qt version (6.8.1).  He 
said he has Win10, while I have Win11.  Not sure whether that would 
matter.

I've been playing around with this for a few days now, trying to get my 
head around it.  The post you linked regarding the changes in 6.7 helped 
with my understanding a lot.  I think I get where QFontDatabase gets its 
style names now.  As Charles pointed out, when you request an italic 
font style (as reported by QFontDatabase), you're getting a named style 
from the variable font file, whereas if you use .setItalic(), you're 
getting something else (Charles called it 'synthesized italic').  There 
is even a comment in the Qt documentation for QFont.setStyleName() that 
seems to explicitly discourage mixing the two:

https://doc.qt.io/qt-6/qfont.html#setStyleName
Due to the lower quality of artificially simulated styles, and the lack 
of full cross platform support, it is not recommended to use matching by 
style name together with matching by style properties

The setup I have is about as 'clean' as it can get.  I got this laptop 
just about a month ago, and it doesn't have a lot on it that didn't ship 
with it.  Exo is literally the only font I have installed beyond what 
came with Win11.

I see the problem with other fonts as well -- Bahnschrift, for example, 
which comes with Windows.  If I create a QFont object with 
QFontDatabase.font():

         f = QFontDatabase.font('Bahnschrift', 'Condensed', 24)

and then apply .setItalic(True) to it, it's italic, but no longer 
Condensed.

And actually, the same thing happens if I create the font with QFont() 
directly:

         f = QFont(
             'Bahnschrift Condensed',
             pointSize=24
         )

Here also, the font is fine initially, but after .setItalic(True) it is 
no longer Condensed.

The Bahnschrift file doesn't appear to have any italic styles defined, 
so there's no way to ask for italic by style name.  
Simulated/synthesized seems to be the only way to get italic Bahnschrift 
Condensed.  With the setup I have, it doesn't seem to be possible at 
all.  (The code above works as expected in PyQt5).

Anyway, probably a Qt issue, as you say, and one I'll just have to work 
around ...

Thanks again!

/John

------ Original Message ------
>From "Maurizio Berti" <maurizio.berti at gmail.com>
To "John Sturtz" <john at sturtz.org>
Cc pyqt at riverbankcomputing.com
Date 1/5/2025 10:11:01 PM
Subject Re: QFontDatabase/QFont question

>First of all, this almost certainly is a Qt issue, not a PyQt one.
>Then, you have to consider at least the following aspects:
>
>- font management and rendering is a quite complex aspect, and its 
>results may change a lot depending on the platform;
>- fonts are not always consistent, depending on many aspects;
>- font matching follows arbitrary rules (some of which are based on 
>text matching of families or properties);
>- the [infamous] 6.7 version introduced many changes and related 
>bugs/inconsistencies, including font management, which got important 
>changes (see this related post 
><https://www.qt.io/blog/text-improvements-in-qt-6.7>);
>- if you have many fonts installed, the font matching algorithm may 
>have issues especially if you installed different versions of the 
>"same" font (start by checking how many fonts you've installed that 
>match the "exo" family name); using installed fonts along with ones 
>added through QFontDatabase may further complicate things;
>
>My suggestion is to check again with a *clean* setup (possibly through 
>multiple VMs, OSs and configurations) and eventually consider filing a 
>detailed report on https://bugreports.qt.io.
>
>Regards,
>MaurizioB
>
>Il giorno lun 6 gen 2025 alle ore 00:09 John Sturtz <john at sturtz.org> 
>ha scritto:
>>Hi again.
>>
>>Wondering if anyone can help me understand this behavior that, to me, 
>>is unexpected.
>>
>>I'm using QFontDatabase to manipulate system fonts.  Mostly it seems 
>>to work great, and behaves as I expect.  I especially like its handy 
>>.font() method, that returns a QFont object.
>>
>>But in this instance, it's behaving in a way I don't understand.  I'm 
>>working with a Google font named Exo 
>>(https://fonts.google.com/specimen/Exo), which I've installed on my 
>>system.  QFontDatabase.styles('Exo') produces several (18, to be 
>>exact) styles.  Screenshot from Windows Font Settings below:
>>
>>
>>Now what I want to do is create a QFont object of the 'Bold' style, 
>>and then just make it Italic.  For example:
>>
>>>font = QFontDatabase.font('Exo', 'Bold', 24)
>>>font.setItalic(True)
>>
>>But after the .setItalic() call, the font object is no longer Bold 
>>anymore (it's Thin, the lightest weight of the Exo fonts).
>>
>>Sample code is attached (I guess one would have to install the Exo 
>>font to test it).  Basically, I get the font with 
>>QFontDatabase.font(), get and display QFontInfo data for it, and it's 
>>what I expect.  Then I get the same font again, call .setItalic(True) 
>>on it, display QFontInfo, and it's not what I expect:
>>
>>
>>I do (sort of) understand that the object returned by QFont is 
>>requested, and QFontInfo tells you what you actually got.  Apparently 
>>I didn't get what I requested, but I'm not quite sure why.
>>
>>As always, thanks in advance if you can shed any light on this!
>>
>>/John
>
>
>--
>È difficile avere una convinzione precisa quando si parla delle ragioni 
>del cuore. - "Sostiene Pereira", Antonio Tabucchi
>http://www.jidesk.net
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