Whats wrong with my Matrix Rotation?

Question

I'm trying to rotate a model by

(float) Math.atan2(-camX.getXf() * padX, -camDir.getZf() * padY)

Y

and

-MathUtils.HALF_PI

Z

But

model.setRotation(new Matrix3(1,0,0,
                              0,(float) Math.atan2(-camX.getXf() * padX, -camDir.getZf() * padY),0,
                              0,0,-MathUtils.HALF_PI));

It rotates on the y axis (Though it's sideways because it's a md2 model) but rotating the Z axis doesn't make it right side up. Any idea why?

Each variable is in it's respective area of the matrix.

EDIT: alright, now I'm using this code:

float x = 0;
            float y = (float) Math.atan2(-camX.getXf() * padX, -camDir.getZf() * padY);
            float z = (float) -MathUtils.HALF_PI;

            float a = (float) Math.sin(x);
            float A = (float) Math.cos(x);
            float b = (float) Math.sin(y);
            float B = (float) Math.cos(y);
            float c = (float) Math.sin(z);
            float C = (float) Math.cos(z);

            Matrix3 m = new Matrix3(A*b, -(B*a),b,
                                   (C*a)+(A*b*c), (A*C)-(a*b*c), -(B*c),
                                   (a*c)-(A*C*b), (A*c)+(C*a*b), B*C); 

But now none of the axis are rotating correctly.

This is how the matrix is set up:

xx, xy, xz, 
yx, yy, yz, 
zx, zy, zz

Answer 1

Rotation matrices don't work this way. Angles don't go into matrices! Instead I assume that Java handles a rotation matrix just like any other transformation matrix in cartesian coordinates. Since I think you don't want to input the rotation matrix by hand, you are probably better off starting with a new Matrix3 (I hope it is automatically initialized at the identity matrix), and then successively rotating it using rotateX(float x), rotateY(float y) and rotateZ(float z), where x, y, z are the angles you want to rotate about. (In case you are using com.threed.jpct.Matrix, at least.) Note that the result does depend on the succession of the three rotations.

Answer 2

Here is a typical tutorial on how to use rotation matrices http://www.euclideanspace.com/maths/algebra/matrix/orthogonal/rotation/index.htm . The order of applying rotations round the three axes is critical. Alternatively you can rotate about an arbitrary axis. Also you may want to explore quaternions .

Answer 3

This is what a rotation matrix looks like in 2D; it rotates a point in (x,y) space about the z-axis in the counterclockwise direction.

http://en.wikipedia.org/wiki/Rotation_matrix