Rotation Matrix Java

Question

I've been following a tutorial for creating a game engine and when I got to calculating the 3D Rotation matrix I ran into the problem that I believe the matrix isn't being calculated properly. When I try to rotate the matrix by 0 degrees on the x, y, and z axis, I should be getting an identity matrix for the rotation matrix that should look like this:

Rotation Matrix:

1.0 0.0 0.0 0.0 
0.0 1.0 0.0 0.0 
0.0 0.0 1.0 0.0 
0.0 0.0 0.0 1.0

but I end up with a matrix that looks like this:

Rotation Matrix:

1.0 1.0 1.0 1.0 
1.0 1.0 1.0 1.0 
1.0 1.0 1.0 1.0 
1.0 1.0 1.0 1.0

Here is the code for calculating the rotation matrix directly from the tutorial:

public class Matrix4f 
{
    private float[][] m;

    //generate a 4X4 array as my inital matrix which will represent homogeneous
    //coordinates: x, y, z, w
    public Matrix4f()
    {
        m = new float[4][4];
    }

    public Matrix4f initRotation(float x, float y, float z)
    {
        //generate rotation matrices for x, y, and z
        Matrix4f rx = new Matrix4f();
        Matrix4f ry = new Matrix4f();
        Matrix4f rz = new Matrix4f();

        //convert x,y, and z to radians for angle calculations
        x = (float)Math.toRadians(x);
        y = (float)Math.toRadians(y);
        z = (float)Math.toRadians(z);

        //calculate rotation matrices for x, y, z

        rz.m[0][0] = (float)Math.cos(z);rz.m[0][1] = (float)Math.sin(z);rz.m[0][2] = 0;             rz.m[0][3] = 0;
        rz.m[1][0] = -(float)Math.sin(z);rz.m[1][1] = (float)Math.cos(z);rz.m[1][2] = 0;                    rz.m[1][3] = 0;
        rz.m[2][0] = 0;                 rz.m[2][1] = 0;                 rz.m[2][2] = 1;                 rz.m[2][3] = 0;
        rz.m[3][0] = 0;                 rz.m[3][1] = 0;                 rz.m[3][2] = 0;                 rz.m[3][3] = 1;

        rx.m[0][0] = 1;                 rx.m[0][1] = 0;                 rx.m[0][2] = 0;                 rx.m[0][3] = 0;
        rx.m[1][0] = 0;                 rx.m[1][1] = (float)Math.cos(x);rx.m[1][2] = -(float)Math.sin(x);rx.m[1][3] = 0;
        rx.m[2][0] = 0;                 rx.m[2][1] = -(float)Math.sin(x);rx.m[2][2] = (float)Math.cos(x);rx.m[2][3] = 0;
        rx.m[3][0] = 0;                 rx.m[3][1] = 0;                 rx.m[3][2] = 0;                 rx.m[3][3] = 1;

        ry.m[0][0] = (float)Math.cos(y);ry.m[0][1] = 0;                 ry.m[0][2] = -(float)Math.sin(y);ry.m[0][3] = 0;
        ry.m[1][0] = 0;                 ry.m[1][1] = 1;                 ry.m[1][2] = 0;                 ry.m[1][3] = 0;
        ry.m[2][0] = (float)Math.sin(y);ry.m[2][1] = 0;                 ry.m[2][2] = (float)Math.cos(y);ry.m[2][3] = 0;
        ry.m[3][0] = 0;                 ry.m[3][1] = 0;                 ry.m[3][2] = 0;                 ry.m[3][3] = 1;

        //calculate the final rotation matrix by multiplying the 3 rotation matrices together
        m = rz.mul(ry.mul(rx)).getM();

        //a simple way to print out the full matrix
        System.out.println("Rotation Matrix:");
        for(int i = 0; i < 4; i++)
        {
            System.out.println("");
            for(int j = 0; j < 4; j++)
            {
                System.out.print(m[i][j] + " ");
            }
        }
        System.out.println("");

        return this;
    }

    //defining the multiplication operation for matrices
    public Matrix4f mul(Matrix4f r)
    {
        Matrix4f res = new Matrix4f();

        for(int i = 0; i < 4; i++)
        {
            for(int j = 0; j < 4; j++)
            {
                res.set(i, j, m[i][0]*r.get(0, i) + 
                              m[i][1]*r.get(1, i) +
                              m[i][2]*r.get(2, i) +
                              m[i][3]*r.get(3, i));
            }
        }

        return res;

    }

    //get the matrix in array form
    public float[][] getM() 
    {
        return m;
    }

    //get an individual element of the matrix
    public float get(int x, int y)
    {
        return m[x][y];
    }

    //set the whole matrix equal to a matrix m
    public void setM(float[][] m) 
    {
        this.m = m;
    }

    //set an individual element of the matrix to a value
    public void set(int x, int y, float value)
    {
        m[x][y] = value;
    }

}

Then basically when I try to run it in the main method like so:

(new Matrix4f()).initRotation(0, 0, 0);

I get

Rotation Matrix:

1.0 1.0 1.0 1.0 
1.0 1.0 1.0 1.0 
1.0 1.0 1.0 1.0 
1.0 1.0 1.0 1.0

Even though according to rotation matrix operations I should be getting:

Rotation Matrix:

1.0 0.0 0.0 0.0 
0.0 1.0 0.0 0.0 
0.0 0.0 1.0 0.0 
0.0 0.0 0.0 1.0

EDIT: After some more debugging, I've resolved the problem after more debugging to be a problem in the matrix mul method. I apologize for my laziness. I've been programming for a while and I really don't do this sort of thing often, but I just got really frustrated with my program.

The correction for the matrix multiplication algorithm is:

public Matrix4f mul(Matrix4f r)
    {
        Matrix4f res = new Matrix4f();

        for(int i = 0; i < 4; i++)
        {
            for(int j = 0; j < 4; j++)
            {
                res.set(i, j, m[i][0]*r.get(0, j) + 
                              m[i][1]*r.get(1, j) +
                              m[i][2]*r.get(2, j) +
                              m[i][3]*r.get(3, j));
            }
        }

        return res;

    }

instead of:

public Matrix4f mul(Matrix4f r) { Matrix4f res = new Matrix4f();

    for(int i = 0; i < 4; i++)
    {
        for(int j = 0; j < 4; j++)
        {
            res.set(i, j, m[i][0]*r.get(0, i) + 
                          m[i][1]*r.get(1, i) +
                          m[i][2]*r.get(2, i) +
                          m[i][3]*r.get(3, i));
        }
    }

    return res;

}

I did a ton of debugging. What more information do you want? I would be happy to supply it. — Orren Ravid, Apr 19 '16 at 09:10
You should be able to identify the calculation/line where the behaviour diverged from expectations, and then construct a [minimal test-case](http://stackoverflow.com/help/mcve). — Oliver Charlesworth, Apr 19 '16 at 09:12
Again as I wrote in my edit above, I apologize for my lack of effort on the question, but I ran into the pitfall of being frustrated with my code. Hopefully my edit clears things up. — Orren Ravid, Apr 19 '16 at 09:30

Mrunal Pagnis · Accepted Answer · 2016-07-22T09:54:46.430

I think your multiplication function is incorrect logically. In your multiplication function you end up multiplying the first raw and column for every cell. changing the i to j as shown below will calculate the multiplication correctly.

public Matrix4f mul(Matrix4f r) {
Matrix4f res = new Matrix4f();
for(int i = 0; i < 4; i++)
{
    for(int j = 0; j < 4; j++)
    {
        res.set(i, j, m[i][0]*r.get(0, j) + 
                      m[i][1]*r.get(1, j) +
                      m[i][2]*r.get(2, j) +
                      m[i][3]*r.get(3, j));
    }
}

return res;

}

Secondly, you can try to debug your code using breakpoints in multiplication function. You can do the debug using any standard IDE (Eclipse) debug feature. You will get a correct idea of execution step by step and then you will be able to correct the code yourself.

Hope it helps.

score 2 · Answer 2 · answered Apr 19 '16 at 09:26

I did a lot of debugging, and I figured out you have bad indexes:

res.set(i, j, m[i][0]*r.get(0, i) + 
    m[i][1]*r.get(1, i) +
    m[i][2]*r.get(2, i) +
    m[i][3]*r.get(3, i));

You have index i on both matrices - you should fix it this way:

res.set(i, j, m[i][0]*r.get(0, j) + 
    m[i][1]*r.get(1, j) +
    m[i][2]*r.get(2, j) +
    m[i][3]*r.get(3, j));

In case you receive transposed matrix in next calculations (on identity matrix there is no way to figure it out) - swap i and j Hope it helps ;)

Rotation Matrix Java

2 Answers2