QFont::fromString() change between Qt 4 and Qt 5

AndyBrice

When I convert this string:

MS Shell Dlg 2,36,-1,5,50,0,0,0,0,0

Using QFont::fromString() in Qt 4.8.6 I get font:

MS Shell Dlg 2/Normal/36pt

But when I convert the same string using QFont::fromString() in Qt 5.11.1 on the same Windows 7 development machine I get font:

MS Shell Dlg 2/Regular/36pt

They look quite different sizes, which is an issue for backward compatibility. In QFontDialog on Qt 5 'Normal' isn't listed as an option for MS Shell Dlg 2.

Similarly Arial/Normal/36pt gets converted to Arial/Regular/36pt.

Is this a bug or just change I am going to have to live with? Is there a workaround for backward compatibility?

--
Andy Brice
https://www.perfecttableplan.com

raven-worx

@AndyBrice
One thing that has drastically changed from Qt4 to Qt5 is the QPA (Qt Platform Abstraction).
Along with it a so called (internal) class QPlatformFontDatabase introduced. So it's very likely that some fundamentals regarding resolution of fonts also have changed.

Using your example MS Shell Dlg 2,36,-1,5,50,0,0,0,0,0 and comparing the implementations across Qt also shows not much differences:

Qt5:
MS Shell Dlg 2 -> font family
36 -> point size
-1 -> pixelsize
5 -> StyleHint
50 -> Font weight
0 -> style
0 -> underline
0 -> strikeout
0 -> fixed pitch
0 -> <not-used?>

Qt4:
MS Shell Dlg 2 -> font family
36 -> point size
-1 -> pixelsize
5 -> StyleHint
50 -> Font weight
0 -> style
0 -> underline
0 -> strikeout
0 -> fixed pitch
0 -> raw mode

The enum QFont::StyleHint hasn't changed. But the QFont::Weight was extended by various values.
I would try to play around with those values. Maybe a value of 50 (=Normal) in Qt4 can be easily mapped to a new value in Qt5.
Anyway i think the changes are height that there is no straight forward workaround for your issue.

AndyBrice

The differences are significant. For example look at the big text which is Arial Italic 72, on Qt 4 versions of my app:

And Arial Italic 72, on Qt 5 versions of my app:

It isn't a showstopper though. Users will just have to modify the fonts if they move a plan from PerfectTablePlan v5 to PerfectTablePlan v6. Just wanted to make sure I wasn't missing anything.

--
Andy Brice
https://www.perfecttableplan.com

raven-worx

@AndyBrice
but this rather looks like the font size is different, than the weight.
regular/normal specifies the weight of a font, not it's size.

Is this a QTextDocument (QTextEdit etc) you are using here?

AndyBrice

It is a QPainter::drawText() inside a QGraphicsItem derived object drawn on a QGraphicsCanvas.

I believe it is the same font in both cases, but I will have a double check.

--
Andy Brice
https://www.perfecttableplan.com

AndyBrice

I am using pixel size, instead of point size. IIRC because I had issues with exporting point size fonts to PDF.

On Qt 5.11.1:

qDebug() << p->font();

returns:

QFont( "Arial,-1,144,5,50,1,0,0,0,0" )

On Qt 4.8.5:

qDebug() << p->font();

returns:

QFont( "Arial,-1,96,5,50,1,0,0,0,0" )

They are both reading in the same font generated by QFont::toString(). So I am not sure why the pixel sizes are different. I need to dig further.

--
Andy Brice
https://www.perfecttableplan.com

AndyBrice

I investigated a bit further. This code:

QFont convertPointToPixelSize( const QFont& f )
{
    QFont ret( f );
    QFontInfo fi( ret );
    ret.setPixelSize( fi.pixelSize() );
    return ret;
}
qDebug() << "getFont()=" << getFont();
qDebug() << "convertPointToPixelSize( getFont() )=" << convertPointToPixelSize( getFont());

Returns this on Qt 4.8.5:

getFont()= QFont( "MS Shell Dlg 2,36,-1,5,50,0,0,0,0,0" ) 
convertPointToPixelSize( getFont() )= QFont( "MS Shell Dlg 2,-1,48,5,50,0,0,0,0,0" ) 
getFont()= QFont( "Arial,28,-1,5,50,0,0,0,0,0" ) 
convertPointToPixelSize( getFont() )= QFont( "Arial,-1,37,5,50,0,0,0,0,0" ) 
getFont()= QFont( "Times New Roman,72,-1,5,50,0,0,0,0,0" ) 
convertPointToPixelSize( getFont() )= QFont( "Times New Roman,-1,96,5,50,0,0,0,0,0" )

And this on Qt 5.11.1:

getFont()= QFont( "MS Shell Dlg 2,36,-1,5,50,0,0,0,0,0" )
convertPointToPixelSize( getFont() )= QFont( "MS Shell Dlg 2,-1,72,5,50,0,0,0,0,0" )
getFont()= QFont( "Arial,28,-1,5,50,0,0,0,0,0" )
convertPointToPixelSize( getFont() )= QFont( "Arial,-1,56,5,50,0,0,0,0,0" )
getFont()= QFont( "Times New Roman,72,-1,5,50,0,0,0,0,0" )
convertPointToPixelSize( getFont() )= QFont( "Times New Roman,-1,144,5,50,0,0,0,0,0" )

So QFontInfo::pixelSize() seems to have changed between Qt 4 and Qt 5. It isn't a showstopper. But it is a minor annoyance.

--
Andy Brice
https://www.perfecttableplan.com

raven-worx

@AndyBrice said in QFont::fromString() change between Qt 4 and Qt 5:

QFont convertPointToPixelSize( const QFont& f )
{
QFont ret( f );
QFontInfo fi( ret );
ret.setPixelSize( fi.pixelSize() );
return ret;
}

what is the purpose of this method in the first place?
Do you want to set it's default pixel size?

AndyBrice

@raven-worx IIRC it is because point font sizes don't work well when you export a QGraphicsCanvas to PDF. So I set pixel sizes (rather than point sizes) in the QGraphicsCanvas. To be honest, it is a bit lost in the mists of time and I am just trying to keep the Qt 5 version of my software backward compatible with the Qt 4 version.

--
Andy Brice
https://www.perfecttableplan.com

raven-worx

@AndyBrice
ok then in this case a workaround could be simply to calculate the point to pixel conversation yourself: 1 px = 0.75 pt
It may be that the newer (Qt5) implementation takes the current screen resolution into account, where the older (Qt4) didn't.

AndyBrice

@raven-worx Yes, it may be a related to the DPI of the device in Qt. I hadn't thought of that. In which case the ratio between pixels and points may vary between devices. I'll have to look through the QFontInfo::pixelSize() source. Thanks.

--
Andy Brice
https://www.perfecttableplan.com

AndyBrice

I have ended up multiplying the font point size by 0.66 when I read in plans created using Qt 4 on Windows. It is a bit of a hack, but it is good enough!

--
Andy Brice
https://www.perfecttableplan.com