Using QMatrix2x3 for Transformations in Custom Qt Painting

• Useful for custom painting
  If you're implementing custom painting in your Qt application, you can use QMatrix2x3 to define transformations for the shapes or graphics you draw.

• Not for direct GUI elements
  While it affects how things are drawn, QMatrix2x3 isn't used to modify properties of GUI elements like buttons or widgets.

• Represents a transformation
  It stores values that define how to transform points in a 2D space. This transformation can involve scaling, rotation, shearing, or a combination of these.

Here are some common functionalities of QMatrix2x3:

• Transposing
  The transposed() function creates a new matrix with rows and columns swapped.
• Checking identity
  You can check if the matrix represents an identity transformation (no change).
• Manipulating the matrix
  There are functions to perform various transformations like scaling, rotation, and shearing.
• Accessing elements
  You can access individual elements of the matrix using the operator() function.
• Creating a matrix
  You can create a QMatrix2x3 object by passing another matrix or a list of values.

In summary

• It doesn't directly modify visual properties of GUI widgets.
• It's used in conjunction with custom painting in Qt GUI to manipulate how you draw elements.
• QMatrix2x3 is a mathematical tool for defining 2D transformations.

---

#include <QtWidgets>
#include <QtGui>

class MyWidget : public QWidget {
  Q_OBJECT

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

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

    // Define a rotation angle (in radians)
    qreal rotationAngle = 0.5; // Adjust for desired rotation

    // Create a QMatrix2x3 with rotation
    QMatrix2x3 rotationMatrix;
    rotationMatrix.rotate(rotationAngle);

    // Apply the transformation to the painter
    painter.setTransform(rotationMatrix);

    // Draw a rectangle with transformed position
    painter.drawRect(50, 50, 100, 100);
  }
};

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

1. We define a MyWidget class inheriting from QWidget.
2. In the paintEvent method, we create a QPainter object to draw on the widget.
3. We define a rotationAngle variable (adjust the value for desired rotation).
4. We create a QMatrix2x3 object (rotationMatrix).
5. We use the rotate function of rotationMatrix to define the rotation in radians.
6. We call painter.setTransform(rotationMatrix) to apply the transformation to the painter object.
7. Finally, we draw a rectangle using drawRect with a specific position.

---

1. • You can use std::vector<std::vector<double>> to represent a 2D matrix. This approach offers flexibility in size and requires manual manipulation for transformations.

2. Eigen or Armadillo libraries
   

   • If you need more advanced functionalities for linear algebra operations beyond basic transformations, consider using external libraries like Eigen or Armadillo. These libraries are specifically designed for efficient matrix operations and offer a wider range of functions.

3. Custom class
   

   • For complete control and specific use cases, you can develop your own custom class representing a 2x3 matrix. This approach allows you to define custom functions for your specific transformations.

Choosing the best alternative depends on your project requirements:

• Customization
  If you need specific functionalities beyond what existing libraries provide, a custom class might be the way to go.
• Advanced functionalities
  When dealing with complex linear algebra operations, libraries like Eigen or Armadillo offer a more powerful and optimized solution.
• Simplicity
  If you only need basic 2x3 transformations and prefer a Qt-centric approach, using std::vector<std::vector<double>> with manual manipulation might suffice.

Option	Pros	Cons
std::vector<std::vector<double>>	Simple and familiar (C++ standard library)	Requires manual manipulation for transformations
Eigen/Armadillo libraries	Powerful, efficient, wide range of functions	External libraries add complexity to project setup
Custom class	Highly customizable	More development effort