For Qt 4.6.x, how to auto-size text to fit in a specified width?

[JohannesMunk]

I needed this functionality myself - here is what I came up with:

qreal lod = option->levelOfDetailFromTransform(painter->worldTransform());
QRectF r = boundingRect();
QFont f = painter->font();
qreal aspectRatio = painter->fontMetrics().lineSpacing() / painter->fontMetrics().averageCharWidth();
int pixelsize = sqrt(r.width()*r.height()/aspectRatio/(stitle.length()*3))*aspectRatio;
f.setPixelSize(pixelsize);
int flags = Qt::AlignCenter|Qt::TextDontClip|Qt::TextWordWrap;
if ((pixelsize * lod) < 13)
    flags |= Qt::TextWrapAnywhere;
QFontMetricsF fm(f);
QRectF tbr = fm.boundingRect(r,flags,stitle);
pixelsize = f.pixelSize()*qMin(r.width()*0.95/tbr.width(),r.height()*0.95/tbr.height());
f.setPixelSize(pixelsize);
painter->setFont(f);
painter->drawText(r,flags,stitle);

qreal lod = option->levelOfDetailFromTransform(painter->worldTransform());
QRectF r = boundingRect();
QFont f = painter->font();
qreal aspectRatio = painter->fontMetrics().lineSpacing() / painter->fontMetrics().averageCharWidth();
int pixelsize = sqrt(r.width()*r.height()/aspectRatio/(stitle.length()*3))*aspectRatio;
f.setPixelSize(pixelsize);
int flags = Qt::AlignCenter|Qt::TextDontClip|Qt::TextWordWrap;
if ((pixelsize * lod) < 13)
    flags |= Qt::TextWrapAnywhere;
QFontMetricsF fm(f);
QRectF tbr = fm.boundingRect(r,flags,stitle);
pixelsize = f.pixelSize()*qMin(r.width()*0.95/tbr.width(),r.height()*0.95/tbr.height());
f.setPixelSize(pixelsize);
painter->setFont(f);
painter->drawText(r,flags,stitle);

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The idea of the first attempt to calculate a pixelsize is to calculate how big a letter can be if you want to distribute a certain amount of letters on the available space.

numberOfLetters = width / charWidth * height / (charWidth*aspectRatio)

=> pixelSize = charWidth * aspectRatio = sqrt ( width * height / numberOfLetters / aspectRatio) * aspectRatio;

For my scenario numberOfLetters is 3 times the amount of text I want to display, because I don't want to cover the whole item with text.

With this guess I call QFontMetricF::boundingRect and fine tune the result accordingly.

This works fine for my purpose.

HIH

Johannes

[corrado1972]

This works fine for my purpose.

I'm confirming you that this solution (i named it the 2nd) fit at the best my requirements too.
Here is my two solutions comparison:
solutioncomparison.jpg

My scene doesn't has too much items, so the drawing performances are acceptable.
Thank you guys for shared your work.

[victtim]

Hi Johannes / All

Could you please explain the math behind this?

"numberOfLetters = width / charWidth * height / (charWidth*aspectRatio)

=> pixelSize = charWidth * aspectRatio = sqrt ( width * height / numberOfLetters / aspectRatio) * aspectRatio;
"

I don't follow it.

Thank you

Tim