Resetting Transformations with QMatrix4x4::setToIdentity() in Qt

Purpose

• This function in Qt's QMatrix4x4 class resets a 4x4 transformation matrix to the identity matrix.

Identity Matrix

• In 3D graphics, it represents no transformation (rotation, scaling, or translation) applied to an object.
• The identity matrix is a special matrix where all diagonal elements (from top-left to bottom-right) are 1, and all other elements are 0.

How it Works

1. When you call setToIdentity(), the function iterates through the elements of the QMatrix4x4 object.
2. It sets all diagonal elements (m[0][0], m[1][1], m[2][2], and m[3][3]) to 1.
3. It sets all non-diagonal elements to 0.

Code Example (C++)

#include <QMatrix4x4>

int main() {
  QMatrix4x4 myMatrix;  // Initially, the matrix might have non-identity values

  // Apply some transformations (optional)
  // ...

  // Reset the matrix to identity
  myMatrix.setToIdentity();

  // Now, myMatrix represents no transformation
}

Use Cases in Qt GUI

• Combining Transformations
  You might use setToIdentity() to ensure a clean starting point before applying a sequence of transformations in a specific order.
• Resetting Transformations
  If you've applied various transformations (rotation, scaling, translation) to an object using other QMatrix4x4 functions, you can use setToIdentity() to clear those transformations and start over.
• Initializing Objects
  You can use setToIdentity() to create a QMatrix4x4 object that represents no initial transformation for a 3D object in your Qt GUI scene.

• You can check if a matrix is the identity using isIdentity().
• QMatrix4x4 provides other functions for various transformations like translate(), rotate(), and scale(). These functions typically multiply the existing matrix by a new transformation matrix, effectively combining transformations.

---

Initializing a 3D Object with Identity Transformation

#include <QMatrix4x4>
#include <QOpenGLWidget>

class MyWidget : public QOpenGLWidget {
  Q_OBJECT

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

protected:
  void paintGL() override {
    // ... (set up OpenGL rendering context)

    // Create a QMatrix4x4 representing no initial transformation
    QMatrix4x4 modelMatrix;
    modelMatrix.setToIdentity();

    // Apply other transformations (e.g., translation, rotation)
    modelMatrix.translate(1.0f, 0.0f, 0.0f);  // Move object 1 unit to the right
    modelMatrix.rotate(45.0f, 0.0f, 1.0f, 0.0f); // Rotate object 45 degrees around Y-axis

    // Use modelMatrix for rendering your 3D object (specific code depends on your rendering approach)
  }
};

Resetting Transformations

#include <QMatrix4x4>
#include <QPushButton>

class MyWidget : public QWidget {
  Q_OBJECT

public:
  MyWidget(QWidget *parent = nullptr) : QWidget(parent) {
    // ... (create a button and connect its clicked signal)

    modelMatrix.translate(0.5f, 0.5f, 0.0f);  // Initial transformation: move object to center
  }

private slots:
  void onButtonClicked() {
    // Reset the transformation matrix to identity
    modelMatrix.setToIdentity();
  }

private:
  QMatrix4x4 modelMatrix;
};

#include <QMatrix4x4>

// ... (other code)

// Function to rotate and scale an object
void applyTransformations(QMatrix4x4& matrix) {
  QMatrix4x4 rotationMatrix;
  rotationMatrix.rotate(angle, rotationAxis);

  QMatrix4x4 scaleMatrix;
  scaleMatrix.scale(scaleX, scaleY, scaleZ);

  // Combine rotation and scaling into a temporary matrix
  QMatrix4x4 combinedMatrix = rotationMatrix * scaleMatrix;

  // Apply the combined transformation to the main matrix
  matrix *= combinedMatrix;
}

// ... (later)

// Reset the main matrix and apply combined transformation
modelMatrix.setToIdentity();
applyTransformations(modelMatrix);

---

Constructor with Identity Values

• You can directly create a QMatrix4x4 object initialized with identity values using the constructor:

QMatrix4x4 identityMatrix(1.0f, 0.0f, 0.0f, 0.0f,
                           0.0f, 1.0f, 0.0f, 0.0f,
                           0.0f, 0.0f, 1.0f, 0.0f,
                           0.0f, 0.0f, 0.0f, 1.0f);

This is more concise for initializing a new identity matrix.

Assigning Identity Values Manually

• You can iterate through the matrix elements and assign 1 to diagonal elements and 0 to others:

QMatrix4x4 matrix;
for (int row = 0; row < 4; ++row) {
  for (int col = 0; col < 4; ++col) {
    matrix(row, col) = (row == col) ? 1.0f : 0.0f;
  }
}

This approach offers more control but can be less readable.

setIdentity() from Older Qt Versions

• If you're using an older version of Qt (pre-5.0), there might have been a setIdentity() function. Check your specific Qt documentation for compatibility.

• If you need to create a new identity matrix frequently, consider the constructor approach.
• For clarity and efficiency, using setToIdentity() is generally recommended.