Recently I posted a blog about my first foray into 3D game development using LWJGL. Since then, I've worked on adding simple physics to the game using JBullet (a Java port of the Bullet physics library for C++). However, I've been having difficulty with the angular measurement system it uses. It seems to use quaternion rotation, with which I am quite unfamiliar. Actually, that's a bit of an understatement -- I hadn't even heard of quaternion rotation until a few hours ago. This is especially problematic when it comes to rendering the object, as to render stuff in LWJGL I need to pass vectors in pitch/yaw/roll format. The Javadoc doesn't include any information on the subject, and while there seems to be no shortage of forum threads with similar questions, none of the code they use has actually worked for me. Does anyone know how to convert from JBullet's quaternion-based rotation system to the pitch/yaw/roll one usable in LWJGL?

For conversion from quaternion to Euler angles you can use my implementation of quaternion :

public class Quat {
public float x, y, z, w;


public Quat() {
// x = y = z = 0.0f;
w = 1.0f;
}


public Quat(float x, float y, float z, float w) {
this.x = x;
this.y = y;
this.z = z;
this.w = w;
}


public float pitch() {
return (float) Math.atan2(2.0 * (y * z + w * x), w * w - x * x - y * y
+ z * z);
}


public float yaw() {
return (float) Math.asin(-2.0 * (x * z - w * y));
}


public float roll() {
return (float) Math.atan2(2.0 * (x * y + w * z), w * w + x * x - y * y
- z * z);
}
// [...]
}

import javax.vecmath.Quat4f;

public class QuatHelper {
	public static float getPitch(Quat4f q){
		return (float)(Math.atan2(2.0 * (q.y * q.z + q.w * q.x), q.w * q.w - q.x * q.x - q.y * q.y + q.z * q.z));
	}
	
	public static float getYaw(Quat4f q){
		return (float)(Math.asin(-2.0 * (q.x * q.z - q.w * q.y)));
	}

	public static float getRoll(Quat4f q){
		return (float)(Math.atan2(2.0 * (q.x * q.y + q.w * q.z), q.w * q.w + q.x * q.x - q.y * q.y - q.z * q.z));
	}
}

public void render(Transform transform){
				
     GL11.glPushMatrix();

     Vector3f position = new Vector3f(transform.origin.x, transform.origin.y, transform.origin.z);
     GL11.glTranslatef(position.x, position.y, position.z);

     //Get the Quaternion angles from JBullet
     Quat4f a = new Quat4f();
     transform.getRotation(a);
     //Get Euler angles from class above
     Vector3f v = new Vector3f(QuatHelper.getPitch(a), QuatHelper.getYaw(a), QuatHelper.getRoll(a));

     //Print pitch, yaw, roll so I can see what's going on
     System.out.println((float)Math.toDegrees(v.x) + " | " + (float)Math.toDegrees(v.y) + " | " + (float)Math.toDegrees(v.z));
     //My rendering method. Arguments are X, Y, Z, Pitch, Yaw, Roll; Yaw value is applied before pitch and roll
     hull.render(position.x, position.y, position.z, (float)Math.toDegrees(v.x), (float)Math.toDegrees(v.y), (float)Math.toDegrees(v.z));
     GL11.glPopMatrix();
}

I can't say I figured out the problem exactly, but it turned out to be related to my rendering method and not your quaternion conversions. I couldn't fix it exactly, but I found another formula to convert to an axis-angle representation that worked more natively in LWJGL. So, problem solved... I guess.