Beyond Characters: Exploring Glyph Mapping with QRawFont::glyphIndexesForChars()

Purpose

• Glyphs are the visual representations of characters used for rendering text.
• This function in Qt's QRawFont class maps a sequence of Unicode characters to their corresponding glyph indices within a loaded font.

Functionality

1. • chars (const QChar*): A pointer to a null-terminated array of Unicode characters.

2. Output
   

   • glyphIndexes (QVector<quint32>*): An output pointer to a QVector that will be filled with the glyph indices for each character in the input chars array.

3. Mapping Process
   

   • The function iterates through the chars array.
   • For each character, it attempts to find its corresponding glyph index in the font's glyph table.
   • If the character is found in the font, its glyph index is added to the glyphIndexes vector.
   • If the character is not found (e.g., not supported by the font), a special "not found" glyph index (usually 0) might be added to the vector.

Return Value

• None. The results are written to the provided glyphIndexes vector.

Applications

• By obtaining the glyph indices, you can use other Qt functions like QGlyphRun to perform advanced text rendering, such as custom positioning, applying effects, or working with low-level drawing APIs.
• This function is useful when you need to render text in a Qt GUI application and want to have more control over the drawing process.

Example Usage

#include <QtGui/QRawFont>
#include <QtCore/QVector>

// ... (other code)

QRawFont font;
// Load a font from a file or data

const QChar* text = "Hello, world!";
QVector<quint32> glyphIndexes;

font.glyphIndexesForChars(text, glyphIndexes);

// Use the glyphIndexes vector for further text rendering operations

Additional Considerations

• For more complex text rendering tasks, Qt provides higher-level classes like QFont and QPainter that handle glyph mapping and drawing internally. Consider using these if you don't need the low-level control offered by QRawFont.
• Not all characters may be supported by a particular font. In such cases, the function might return a "not found" glyph index.

---

#include <QApplication>
#include <QtGui/QWidget>
#include <QtGui/QPainter>
#include <QtGui/QFont>
#include <QtGui/QRawFont>

class TextWidget : public QWidget {
    Q_OBJECT

public:
    TextWidget(QWidget *parent = nullptr) : QWidget(parent) {}

protected:
    void paintEvent(QPaintEvent *event) override {
        QPainter painter(this);

        // Set font properties
        QFont font("Arial", 20);
        painter.setFont(font);

        // Get the raw font for glyph access
        QRawFont rawFont = font.rawFont();

        // Text to render
        QString text = "This is an example text.";

        // Get glyph indices for each character
        QVector<quint32> glyphIndexes;
        rawFont.glyphIndexesForChars(text.data(), glyphIndexes);

        // Calculate baseline for text rendering
        int baseline = painter.fontMetrics().ascent();

        // Render text using glyph indices
        QPoint penPos(10, baseline);
        for (int i = 0; i < text.length(); ++i) {
            quint32 glyphIndex = glyphIndexes[i];
            QGlyphRun glyphRun(rawFont, text.at(i), false, penPos);
            glyphRun.paint(&painter);
            penPos.setX(penPos.x() + glyphRun.characterWidth()); // Advance pen position
        }
    }
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    TextWidget widget;
    widget.show();
    return app.exec();
}

1. • A simple TextWidget class inherits from QWidget.
   • It overrides the paintEvent method to handle text rendering.

2. paintEvent Implementation
   

   • A QPainter object is created to draw on the widget.
   • A font ("Arial", 20) is set on the painter.
   • QRawFont is obtained from the QFont object for glyph access.
   • Sample text is defined as a QString.
   • glyphIndexesForChars is used to get glyph indices for each character.
   • Baseline is calculated using font metrics.
   • A loop iterates through the characters:
     • Glyph index for the current character is retrieved.
     • A QGlyphRun object is created with the raw font, character, and pen position.
     • glyphRun.paint(&painter) draws the character glyph.
     • The pen position is advanced based on the character width.

3. main Function
   

   • A simple Qt application is created.
   • A TextWidget instance is created and shown.

Running the Code

1. Save the code as a .cpp file (e.g., text_widget.cpp).
2. Compile it with a Qt compiler, linking against the necessary Qt libraries.
3. Run the executable to see a window displaying the example text.

---

1. • The combination of QFont and QPainter is the most common and recommended approach for basic text rendering in Qt GUI applications.
   • QFont manages font properties and provides methods like drawtext() or drawText(const QString &text, const QPoint &pos) on the QPainter object.
   • These methods handle glyph mapping and drawing internally, simplifying your code.

2. QLabel Widget
   

   • For displaying simple text labels, QLabel is a convenient widget.
   • It sets the font, text content, and handles basic text rendering automatically.
   • It offers various properties for customization, like text alignment, margins, and color.

3. QTextDocument and QTextLayout
   

   • If you need more advanced text formatting capabilities like rich text (bold, italics, underline), line breaks, or complex layouts, consider using QTextDocument and QTextLayout.
   • These classes handle text parsing, formatting, and layout, providing detailed control over the presentation.

Approach	Level of Control	Ease of Use	Use Cases
QRawFont::glyphIndexesForChars()	Low-level (glyph indices)	More complex	Custom text rendering with precise control
QFont with QPainter	Medium (font properties)	Easier	Basic text rendering, common use case
QLabel	High-level (properties)	Easiest	Simple text labels, common use case
QTextDocument and QTextLayout	High-level (formatting)	More complex	Rich text formatting, complex layouts

Choosing the Right Option

• QTextDocument and QTextLayout become valuable for advanced text formatting tasks.
• If you need custom glyph rendering or fine-grained control over text layout, QRawFont::glyphIndexesForChars() might be necessary.
• For most GUI applications, QFont with QPainter or QLabel will be sufficient for text rendering.