QFontDatabase/QFont question

Charles peacech at gmail.com
Fri Jan 10 05:02:06 GMT 2025 

To add from the documentation

> void QFont::setStyleName(const QString &styleName)
> Sets the style name of the font to styleName. When set, other style
properties like style() and weight() will be ignored for font matching,
though they may be simulated afterwards if supported by the platform's font
engine.
> 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

By using the "Bold" style, you are preventing the "Bold Italic" style to be
matched, instead the italic is simulated by making it oblique. So either
always replace the style name or only use style flags.


On Fri, Jan 10, 2025 at 11:55 AM Charles <peacech at gmail.com> wrote:

> > you're getting a named style from the variable font file
>
> You didn't mention that you are using the variable font. I didn't use the
> variable font instead I use the static fonts. I also got the thin oblique
> font with the variable fonts.
>
> These codes works for me:
>
> 1. f = QFont('Exo', 24, 700, True) # use QFont specifying weight and italic
> 2. f = QFont('Exo', 24); f.setWeight(700); f.setItalic(True)
> 3. f = QFontDatabase('Exo', 'Bold', 24); f.setStyleName('Bold Italic')
>
> Note that italic is part of the style name. If you call setItalic(True)
> there is a mismatch between the style flags and the style name. So if you
> use QFontDatabase and style name always use setStyleName to change the
> style, otherwise use QFont and setWeight and setItalic.
>
> On Fri, Jan 10, 2025 at 11:36 AM John Sturtz <john at sturtz.org> wrote:
>
>> 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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