Question

Windows 7 SP1
MSVS 2010
Qt 4.8.4

Given this code:

#include <QTGui>

int main(int argc, char *argv[])
{
    QTextDocument* text_document = new QTextDocument("testing");
    QTextBlock text_block = text_document->begin();
    qDebug() << text_block.text() << text_block.blockFormat().lineHeight()
             << text_block.blockFormat().lineHeightType();
}

The console displays:

"testing" 0 0

Question: Why doesn’t lineHeight return “the LineHeight property for the paragraph”? The lineHeightType is set for single spacing.

I am clearly not understanding this. When I try to set the line height before outputting, nothing happens (lineHeight() is still zero):

text_block.blockFormat().setLineHeight(30,QTextBlockFormat::SingleHeight);

To be clear, in my application nothing happens when outputting to a GUI window.

Even trying:

 qDebug() << text_block.text() << text_block.layout()->boundingRect().height();

gives me zero.

Answer 1

I never got lineHeight to work but this does:

int CalculateLineHeight(QTextBlock text_block)
{
    int max_ascent  = 0;
    int max_descent = 0;
    int max_leading = 0;

    // A fragment is a piece of the text block with the the same format, such as font.
    for (QTextBlock::Iterator fragment_it = text_block.begin(); !(fragment_it.atEnd()); ++fragment_it)
    {
        QTextFragment fragment = fragment_it.fragment();
        QTextCharFormat fragment_format = fragment.charFormat();
        QFont fragment_font = fragment_format.font();
        QFontMetrics fragment_font_metrics (fragment_font);
        max_ascent  = std::max(fragment_font_metrics.ascent(), max_ascent);
        max_descent = std::max(fragment_font_metrics.descent(),max_descent);
        // Find the leading of the font with the maximum height. If more than
        // one, then find the largest lead among them.
        if ( current_height > max_height )
        {
            max_height = current_height;
            max_leading = current_leading;
        }
        else if ( current_height == max_height && current_leading > max_leading )
        {
            max_leading = current_leading;
        }
    }
    return max_ascent + max_descent + max_leading + 1; // + 1 for the baseline
}

You may think the answer is max height() but fonts with the same height may have different ascents, descents and leadings.