You have a number of options depending on your skill level.

Option 1 is to use Apache commons maths, but this is not straightforward.

Option 2 download the Shapes3D library and use the Rot (uses quaternions) and RotOrder classes which are convertions of the Apache commons maths classes that make use of the PVector class provided by Processing.

The advantage of the second option is that some of the library examples will demonstrate the use of these classes.

If you want to just use quaternions and PVectors rather than the whole library, copy the Rot.java and RotOrder.java files (do not rename the files) to your sketch folder, then restart your sketch and in each of these files remove the statements
package shapes3d.utils;
and change the
import processing.core.PVector;
to
import processing.core.*;

Hi Quarks, 

The Rot is the main class and implements the quaternion maths, to use this class you first create a Rot object using one of the constructors.

The ones I most commonly use are
public Rot(RotOrder order, float alpha1, float alpha2, float alpha3)
public Rot(PVector axis, float angle)
public Rot(PVector u, PVector v)
 
Step 1 creating the Rot object:-

Constructor 1 is used when you know the angles of rotation about each of the three axises, XYZ. The RotOrder object is used to specify the order of rotations in the parameters.
Rot r = new Rot(RotOrder.XYZ, angleX, angleY, angleZ);

Constructor 2 is used to apply a rotation about an arbitary axis specified by the PVector e.g. 1,0,0 is the x axis but 1,1,1 would be the diagonal in a cube.
Rot r = new Rot(axis, angle);

Constructor 3 is useful if you know that you want to rotate a vector from u to v then apply this to other vectors. For instance you want to rotate so that the Y axis is along the diagonal of a cube then u=(0,1,0) and v=(1,1,1)
Rot r = new Rot(u, v);

Step 2 using the roattion
Now that you have a Rot object lets assume you have an array of PVector(s) called lines that you want to apply this rotation to.

for(int i = 0; i < lines.length; i++){
    r.applyTo(lines[i]);
}

The RotOrder class is a utility class when you are working with Euler angles and is used with the method

public float[] getAngles(RotOrder order)

which returns an array of the XYZ rotations represented by the quaternion int the prder specified by the RotOrder object.

Note all angles are in radians.

HTH

Hi Quarks,