How to modify transform matrix get from getTransformMatrix()

Question

I could get the transform matrix(4x4) from surface by getTransformMatrix(), but I don't know how to set scale x,y or set pivot x,y of it. I don't know the matrix structure in this case.

Help me. Here is the code:

private SurfaceTexture mSurface;
private float[] mTransformMatrix = new float[16];
private int mTransformMatrixHandle;
public void onSurfaceCreated(GL10 glUnused, EGLConfig config) {
    ...
    mTransformMatrixHandle = GLES20.glGetUniformLocation(mProgram, "textureTransform");

}
public void onDrawFrame(GL10 glUnused) {
    ...
    mSurface.getTransformMatrix(mTransformMatrix);
    //how to set pivot/scale mTransformMatrix
    ...

    GLES20.glUniformMatrix4fv(mTransformMatrixHandle, 1, false, mTransformMatrix, 0);
    ...
}

Answer 1

Short answer :

If you don't want to bother too much with the maths, which is understandable and will save you a lot of time, you could use an existing matrix library such as android.opengl.Matrix

Math answer :

OpenGL matrix writing convention is column major order. So your 16 floats array should corresponds to this matrix :

0 4  8 12
1 5  9 13
2 6 10 14
3 7 11 15

Now to perform rotations and scaling, you have to multiply this matrix by corresponding rotation and scaling matrix. This will recquire some basic knowledge of matrix calculus and projection geometry.