3D Math Review - Beginning Android 3D Game Development - page 73

The model matrix is called the m_OrientationMatrix in our code. The m_PositionMatrix is the
translation matrix; the m_RotationMatrix is our rotation matrix; and the m_ScaleMatrix is our scale
matrix. We also have a TempMatrix for use for the temporary storage of matrices.
The SetPositionMatrix() function creates the translation matrix by first initializing the matrix to
the identity matrix, by calling setIdentity() and then creating the translation matrix by calling
translateM() in the default Matrix class that is part of the standard Android library.
The SetScaleMatrix() function creates the scale matrix by first initializing the matrix to the identity
matrix and then calling scaleM() from the Matrix class library to create the scale matrix.
The UpdateOrientation() function actually builds the model matrix.

It first creates the translation matrix by calling SetPositionMatrix() .

1.

Next, SetScaleMatrix() is called to create the Scale Matrix. 2.

Then, the final model matrix starts to be built by calling Matrix.multiplyMM()
to multiply the translation matrix by the rotation matrix.

3.

4. Finally, the result matrix from step 3 is multiplied by the scale matrix and then

returned to the caller of the function. The net result is that a matrix is created
that first scales a 3D object, then rotates it around its axis of rotation, and
then finally puts it into the 3D world at a location specified by m_Position
(see Listing 3-9).

Listing 3-9. Building the Model Matrix in the Orientation Class

// Orientation Matrices
private float[] m_OrientationMatrix = new float[16];
private float[] m_PositionMatrix = new float[16];
private float[] m_RotationMatrix = new float[16];
private float[] m_ScaleMatrix = new float[16];
private float[] TempMatrix = new float[16];

// Set Orientation Matrices
void SetPositionMatrix(Vector3 position)
{
// Build Translation Matrix
Matrix.setIdentityM(m_PositionMatrix, 0);
Matrix.translateM(m_PositionMatrix, 0, position.x, position.y, position.z);
}

void SetScaleMatrix(Vector3 Scale)
{
// Build Scale Matrix
Matrix.setIdentityM(m_ScaleMatrix, 0);
Matrix.scaleM(m_ScaleMatrix, 0, Scale.x, Scale.y, Scale.z);
}