adjust Font size to a given rect when drawText

[Micawber]

You could also do something like this...

QFont serifont( "Times" );
qreal size1_x, size2_x, size1_y, size2_y;
QMatrix xform = painter.combinedMatrix();
QMatrix invertedxform = xform.inverted();
 
// calculate and set pixel height for font to be 3% of the display area size
invertedxform.map( qreal( width() * 0.10 ), qreal( height() * 0.10 ), &size1_x, &size1_y );
invertedxform.map( qreal( width() * 0.13 ), qreal( height() * 0.13 ), &size2_x, &size2_y );
serifFont.setPixelSize( static_cast< int >( qAbs( size2_x - size1_x ) ) );
painter.setFont( serifFont );

QFont serifont( "Times" );
qreal size1_x, size2_x, size1_y, size2_y;
QMatrix xform = painter.combinedMatrix();
QMatrix invertedxform = xform.inverted();

// calculate and set pixel height for font to be 3% of the display area size
invertedxform.map( qreal( width() * 0.10 ), qreal( height() * 0.10 ), &size1_x, &size1_y );
invertedxform.map( qreal( width() * 0.13 ), qreal( height() * 0.13 ), &size2_x, &size2_y );
serifFont.setPixelSize( static_cast< int >( qAbs( size2_x - size1_x ) ) );
painter.setFont( serifFont );

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This will ensure that the font size will stay the same if you scale the coordinates of the widget. I'm sure there's a more elegant way to accomplish it though.