A Guide to Creating Custom Graphics in Qt GUIs with Painting Classes

Core Class: QPainter

• QPainter also supports transformations for scaling, rotating, or translating your drawings (QTransform).
• You can define styles for lines and fills using QPen and QBrush classes respectively.
• It offers functions to draw various graphical elements like:
  • Lines, rectangles, polygons, ellipses
  • Complex shapes using QPainterPath
  • Text with font control (QFont)
  • Images (QPixmap)
• This is the workhorse for drawing on widgets.

• QFont
  Controls the appearance of text drawn by QPainter, specifying font family, size, and styles.
• QBrush
  Defines how the interior of shapes are filled, with options for solid colors, patterns, gradients, etc.
• QPen
  Controls the style of lines drawn by QPainter, including width, color, and end-cap styles.
• QPainterPath
  Defines a path for complex shapes that can be reused for efficient drawing.
• QPixmap
  Represents an image object for drawing raster graphics.

---

#include <QApplication>
#include <QWidget>
#include <QPainter>

class ScribbleArea : public QWidget {
  Q_OBJECT

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

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

    // Draw a blue rectangle
    painter.setPen(QPen(Qt::blue, 2));
    painter.drawRect(50, 50, 100, 150);

    // Draw a red circle
    painter.setPen(QPen(Qt::red, 5));
    painter.setBrush(Qt::red);
    painter.drawEllipse(200, 50, 80, 80);

    // Draw some text
    painter.setFont(QFont("Arial", 16));
    painter.drawText(QRect(100, 250, 200, 30), Qt::AlignCenter, "Qt Painting Example");
  }
};

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

  ScribbleArea scribble;
  scribble.resize(400, 300);
  scribble.show();

  return app.exec();
}

1. We define a ScribbleArea class that inherits from QWidget.
2. In the paintEvent function, we create a QPainter object to draw on the widget.
3. We use setPen and setBrush methods of QPainter to define the style of lines and fill for our shapes.
4. We draw a rectangle, a circle, and some text using various functionalities of QPainter.
5. In the main function, we create an instance of ScribbleArea, set its size, and show it on the screen.

---

Qt Quick with QQuickPaintedItem

• While it offers a different approach compared to C++ and QPainter, it allows for a more declarative and potentially faster development process, especially for simpler UI elements.
• QQuickPaintedItem is a QML type specifically designed for custom drawing within Qt Quick applications.
• Qt Quick is a declarative UI framework within Qt that allows you to define your UI using a JavaScript-like language called QML.

Third-party Libraries

• Examples include:
  • OpenGL: A low-level graphics API for high-performance and complex 2D/3D rendering.
  • SDL (Simple DirectMedia Layer): Another popular library for multimedia applications, including graphics.
  • Specialized libraries for specific needs like vector graphics (e.g., Cairo) or image processing (e.g., OpenCV).
• Several third-party libraries can be integrated with Qt for advanced graphics needs.

Hardware Acceleration

• Qt provides mechanisms to leverage these capabilities for improved performance, especially when dealing with complex graphics or animations.
• Modern GPUs offer powerful hardware acceleration capabilities.

Choosing the Right Option

The best alternative depends on your specific needs:

• High-performance Graphics
  For complex graphics or animations, consider third-party libraries like OpenGL or hardware acceleration.
• Declarative UI and Faster Development
  If you're using Qt Quick and need custom drawing elements, QQuickPaintedItem is a good alternative.
• Simple Drawing
  Qt's Painting Classes (particularly QPainter) are often the easiest and most efficient choice for basic drawing needs within your Qt GUI application.