Accurately Positioning Underlines in Qt GUIs with QFontMetrics::underlinePos()

Purpose

• In Qt GUI applications, QFontMetrics::underlinePos() is a function used to retrieve the baseline position of an underline decoration for a particular font.

Functionality

• This information is crucial for rendering text with underlines in your Qt applications, ensuring that the underline is positioned correctly relative to the characters.
• It returns an integer value representing the y-coordinate (in pixels) relative to the baseline of the font where an underline should be drawn.

Example

#include <QApplication>
#include <QLabel>
#include <QFontMetrics>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QFont font("Arial", 12); // Set the desired font

    QLabel label("This text is underlined");
    label.setFont(font);

    QFontMetrics fontMetrics(label.font()); // Get font metrics for the label's font

    int underlinePos = fontMetrics.underlinePos();

    // Use underlinePos to draw the underline:
    QPainter painter(&label);
    painter.setPen(Qt::red); // Set the underline color
    painter.drawLine(0, underlinePos, label.width(), underlinePos);

    label.show();

    return app.exec();
}

In this example:

1. The code creates a QLabel with some text and sets its font.
2. A QFontMetrics object is obtained using the label's font.
3. underlinePos() is called on the QFontMetrics object to get the baseline position for the underline.
4. A QPainter object is created to draw on the label.
5. The pen color is set to red for the underline.
6. A line is drawn using drawLine() with the starting y-coordinate set to underlinePos, ensuring the underline is positioned correctly relative to the text.

Key Points

• Consider using Qt's rich text formatting capabilities (e.g., QString::underline()) for simpler underline decoration.
• The actual thickness of the underline might vary depending on the font's design. You might need to adjust the y-coordinate slightly for a more visually appealing underline.
• The position returned by underlinePos() is relative to the font's baseline, not the top of the characters.

• For more advanced text rendering and decoration, explore Qt's text layout classes like QTextLayout and QTextDocument.
• If you're working with non-Latin character sets, the underline position might not be as consistent. You might need to experiment with different fonts or adjust the positioning based on the specific character set.

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Underline with Custom Thickness

#include <QApplication>
#include <QLabel>
#include <QFontMetrics>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QFont font("Arial", 12);
    QLabel label("This text is underlined");
    label.setFont(font);

    QFontMetrics fontMetrics(label.font());
    int underlinePos = fontMetrics.underlinePos();

    QPainter painter(&label);
    painter.setPen(QPen(Qt::red, 2)); // Set underline color and thickness (2px)
    painter.drawLine(0, underlinePos - 1, label.width(), underlinePos - 1); // Adjust y-coordinate for thickness

    label.show();

    return app.exec();
}

This code adjusts the y-coordinate by subtracting 1 to account for the desired underline thickness (2px) and positions the underline at the bottom of the line.

Underlining Specific Text Range

#include <QApplication>
#include <QLabel>
#include <QFontMetrics>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QFont font("Arial", 12);
    QLabel label("This is some underlined text");
    label.setFont(font);

    QFontMetrics fontMetrics(label.font());
    int underlinePos = fontMetrics.underlinePos();

    QPainter painter(&label);
    painter.setPen(Qt::blue);

    // Underline "underlined" only
    int startIndex = 8;
    int endIndex = startIndex + strlen("underlined");
    int textWidth = fontMetrics.width(label.text().mid(startIndex, endIndex - startIndex));

    painter.drawLine(startIndex, underlinePos - 1, startIndex + textWidth, underlinePos - 1);

    label.show();

    return app.exec();
}

This code calculates the width of the specific text ("underlined") and uses that to draw the underline only for that portion of the text.

Dynamic Underline Color Based on State

#include <QApplication>
#include <QLabel>
#include <QFontMetrics>
#include <QMouseEvent>

class UnderlineLabel : public QLabel
{
    Q_OBJECT

public:
    UnderlineLabel(QWidget *parent = nullptr) : QLabel(parent) {}

protected:
    void mouseEnterEvent(QMouseEvent *event) override
    {
        underlineColor = Qt::red;
        update(); // Trigger repaint
    }

    void mouseLeaveEvent(QMouseEvent *event) override
    {
        underlineColor = Qt::black;
        update();
    }

    void paintEvent(QPaintEvent *event) override
    {
        QLabel::paintEvent(event);

        QFontMetrics fontMetrics(font());
        int underlinePos = fontMetrics.underlinePos();

        QPainter painter(this);
        painter.setPen(underlineColor);
        painter.drawLine(0, underlinePos - 1, width(), underlinePos - 1);
    }

private:
    QColor underlineColor = Qt::black;
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    UnderlineLabel label("Hover over me to underline");
    label.show();

    return app.exec();
}

This code creates a custom UnderlineLabel class that changes the underline color based on mouse hover events. It demonstrates dynamic control over the underline appearance using paintEvent().

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Using Rich Text Formatting

• For simple underlining, you can directly apply the underline() modifier to a QString object:
• Qt provides rich text formatting capabilities through the QString class.

QString text = "This text is underlined";
text.underline();

• This approach avoids manual calculation of the underline position and integrates well with Qt's text handling mechanisms.

Custom Text Layout with QTextLayout

• You can set underline styles (e.g., single, double) and positions using QTextCharFormat.
• If you require more granular control over text layout and decoration, explore QTextLayout.

QTextLayout layout(text);
QTextCharFormat format;
format.setUnderlineStyle(QTextCharFormat::SingleUnderline);
layout.setCharFormat(startIndex, endIndex, format);

• This approach offers greater flexibility for complex text formatting scenarios.

Third-Party Libraries

• These libraries can simplify development and offer additional features.

• If you need more control over text layout or have complex formatting requirements, QTextLayout or third-party libraries might be better suited.
• For basic underlining, QString::underline() is a convenient option.