# Updates

1.2 A minor correction with the formula of converting from Quat to Axis. The scale is missing a square root. Thanks to Shi for pointing that out. From version 1.0 - 1.1 The norm of a quaternion should be the square root of the q.q. The mistake was brought to my attention by several kind readers and upon checking the definition of the Euclidean properties for complex numbers, I realize the norm property [bquote][color=#000080][font=courier new,courier,monospace]### || u+v || <= || u || + || v ||

[/font][/color][/bquote] is violated for the previous definition of the magnitude. The code in the samples are updated as well.# Foreword

To me, the term 'Quaternion' sounds out of this world, like some term from quantum theory about dark matter, having dark secret powers. If you, too, are enthralled by this dark power, this article will bring enlightenment (I hope). The article will show you how to do rotations using quaternions, and bring you closer to understanding quaternions (and their powers). If you do spot a mistake please email me at [email="robin@cyberversion.com"]robin@cyberversion.com[/email]. Also if you intend to put this on your site, please send me a mail. I like to know where this ends up.# Why use Quaternions?

To answer the question, let's first discuss some common orientation implementations.## Euler representation

This is by far the simplest method to implement orientation. For each axis, there is a value specifying the rotation around the axis. Therefore, we have 3 variables [bquote]### [font=Courier New][color=#000080]x, y, z <-- angle to rotate around global coordinate axis [/color][/font]

[/bquote] that vary between 0 and 360 degrees (or 0 - 2pi). They are the roll, pitch, and yaw (or pitch, roll, and yaw - whatever) representation. Orientation is obtained by multiplying the 3 rotation matrices generated from the 3 angles together (in a specific order that you define). Note: The rotations are specified with respect to the global coordinate axis frame. This means the first rotation does not change the axis of rotation for the second and third rotations. This causes a situation known as gimbal lock, which I will discuss later.## Angle Axis representation

This implementation method is better than the Euler representation as it avoids the gimbal lock problem. The representation consists of a unit vector representing an arbitrary axis of rotation, and another variable (0 - 360) representing the rotation around the vector: [bquote][font=Courier New][color=#000080]### x, y, z <-- unit vector representing arbitrary axis angle <-- angle to rotate around above axis

[/color][/font][/bquote] Why are these representations bad?## Gimbal Lock

As rotations in the Euler representation are done with respect to the global axis, a rotation in one axis could 'override' a rotation in another, making you lose a degree of freedom. This is gimbal lock. Say, if the rotation in the Y axis rotates a vector (parallel to the x axis) so that the vector is parallel to the z axis. Then, any rotations in the z axis would have no effect on the vector. Later, I will show you an example of gimbal lock and how you can use quaternions to overcome it.## Interpolation Problems

Though the angle axis representation does not suffer from gimbal lock, there are problems when you need to interpolate between two rotations. The calculated interpolated orientations may not be smooth, so you will get jerky rotation movements. Euler representation suffers from this problem as well.# Let's get started

Before we begin, let's establish some assumptions I'll be making. I hate the way many articles leave this important section out, causing a great deal of confusion when it comes to the mathematics. Coordinate System - This article assumes a right hand coordinate system, like OpenGL. If you are using a left handed coordinate system like Direct3D, you may need to transpose the matrices. Note that the Direct3D samples have a quaternion library already, though I recommend you check through their implementation before using it. Rotation Order - The sequence of rotations in the Euler representation is X, then Y, and then Z. In matrix form: [bquote][font=Courier New][color=#000080]### RotX * RotY * RotZ <-- Very Important

[/color][/font][/bquote] Matrix - Matrices are in column major format, like they are in OpenGL. [bquote]### [font=Courier New][color=#000080]Example[nbsp][[nbsp]0[nbsp][nbsp]4[nbsp][nbsp]8[nbsp][nbsp]12 [nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]1[nbsp][nbsp]5[nbsp][nbsp]9[nbsp][nbsp]13 [nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]2[nbsp][nbsp]6[nbsp][nbsp]10[nbsp]14 [nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]3[nbsp][nbsp]7[nbsp][nbsp]11[nbsp]15[nbsp]][/color][/font]

[/bquote] Vectors and Points - Implemented as a 4x1 matrix so applying a transformation is of the order [bquote]### [font=Courier New][color=#000080]Rotation[nbsp]Matrix*[nbsp][nbsp][[nbsp]vx [nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]vy [nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]vz [nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]1[nbsp][nbsp]][nbsp][nbsp]<--[nbsp]4x1[nbsp]vector[/color][/font]

[/bquote] This does not imply that I prefer OpenGL over Direct3D. It just happened that I learned OpenGL first, and so my quaternion knowledge was gained in OpenGL. Note: If you specify rotations in another order, certain quaternion functions will be implemented differently, especially those that deal with Euler representation.# What is a Quaternion?

A complex number is an imaginary number that is defined in terms of i, the imaginary number, which is defined such that i * i = -1. A quaternion is an extension of the complex number. Instead of just i, we have three numbers that are all square roots of -1, denoted by i, j, and k. This means that [bquote]j * j = -1
k * k = -1[/bquote]

So a quaternion can be represented as
[bquote]q = w + xi + yj + zk[/bquote]

where w is a real number, and x, y, and z are complex numbers.
Another common representation is
[bquote]q=[ w,v ][/bquote]

where v = (x, y, z) is called a "vector" and w is called a "scalar". Although the v is called a vector, don't think of it as a typical 3 dimensional vector. It is a vector in 4D space, which is totally unintuitive to visualize.
# Identity Quaternions

Unlike vectors, there are two identity quaternions. The multiplication identity quaternion is[bquote]q= [1,(0, 0, 0)][/bquote]

So any quaternion multiplied with this identity quaternion will not be changed.
The addition identity quaternion (which we do not use) is
[bquote]q= [0,(0, 0, 0)][/bquote]

# Using quaternions as orientations

The first thing I want to point out is that quaternions are not vectors, so please don't use your preconceived vector mathematics on what I'm going to show. This is going to get very mathematical, so please bear with me. We need to first define the magnitude of a quaternion.[bquote]|| q ||= Norm(q) =sqrt(w[sup]2[/sup] + x[sup]2[/sup] + y[sup]2[/sup] + z[sup]2[/sup])[/bquote]

A unit quaternion has the following property
[bquote]w[sup]2[/sup] + x[sup]2[/sup] + y[sup]2[/sup] + z[sup]2[/sup]=1[/bquote]

So to normalize a quaternion q, we do
[bquote]q = q / || q || = q / sqrt(w[sup]2[/sup] + x[sup]2[/sup] + y[sup]2[/sup] + z[sup]2[/sup])[/bquote]

What is so special about this unit quaternion is that it represents an orientation in 3D space. So you can use a unit quaternion to represent an orientation instead of the two methods discussed previously. To use them as orientations, you will need methods to convert them to other representations (e.g. matrices) and back, which will be discussed soon.
## Visualizing a unit quaternion

You can visualize unit quaternions as a rotation in 4D space where the (x,y,z) components form the arbitrary axis and the w forms the angle of rotation. All the unit quaternions form a sphere of unit length in the 4D space. Again, this is not very intuitive but what I'm getting at is that you can get a 180 degree rotation of a quaternion by simply inverting the scalar (w) component. Note: Only unit quaternions can be used for representing orientations. All discussions from here on will assume unit quaternions.# Conversion from Quaternions

To be able to use quaternions effectively, you will eventually need to convert them to some other representation. You cannot interpret keyboard presses as quaternions, can you? Well, not yet.## Quaternion to Matrix

Since OpenGL and Direct3D allow rotations to be specified as matrices, this is probably the most important conversion function, since homogenous matrices are the standard 3D representations. The equivalent rotation matrix representing a quaternion is [bquote]### [font=Courier New][color=#000080]Matrix[nbsp]=[nbsp][nbsp][[nbsp]w[sup]2[/sup][nbsp]+[nbsp]x[sup]2[/sup][nbsp]-[nbsp]y[sup]2[/sup][nbsp]-[nbsp]z[sup]2[/sup][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]2xy[nbsp]-[nbsp]2wz[nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]2xz[nbsp]+[nbsp]2wy [nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]2xy[nbsp]+[nbsp]2wz[nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]w[sup]2[/sup][nbsp]-[nbsp]x[sup]2[/sup][nbsp]+[nbsp]y[sup]2[/sup][nbsp]-[nbsp]z[sup]2[/sup][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]2yz[nbsp]-[nbsp]2wx [nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]2xz[nbsp]-[nbsp]2wy[nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]2yz[nbsp]+[nbsp]2wx[nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]w[sup]2[/sup][nbsp]-[nbsp]x[sup]2[/sup][nbsp]-[nbsp]y[sup]2[/sup][nbsp]+[nbsp]z[sup]2[/sup][nbsp]][/color][/font]

[/bquote] Using the property of unit quaternions that w[sup]2[/sup] + x[sup]2[/sup] + y[sup]2[/sup] + z[sup]2[/sup] = 1, we can reduce the matrix to [bquote]### [font=Courier New][color=#000080]Matrix[nbsp]=[nbsp][nbsp][[nbsp]1[nbsp]-[nbsp]2y[sup]2[/sup][nbsp]-[nbsp]2z[sup]2[/sup][nbsp][nbsp][nbsp][nbsp]2xy[nbsp]-[nbsp]2wz[nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]2xz[nbsp]+[nbsp]2wy [nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]2xy[nbsp]+[nbsp]2wz[nbsp][nbsp][nbsp][nbsp]1[nbsp]-[nbsp]2x[sup]2[/sup][nbsp]-[nbsp]2z[sup]2[/sup][nbsp][nbsp][nbsp][nbsp]2yz[nbsp]-[nbsp]2wx [nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]2xz[nbsp]-[nbsp]2wy[nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]2yz[nbsp]+[nbsp]2wx[nbsp][nbsp][nbsp][nbsp]1[nbsp]-[nbsp]2x[sup]2[/sup][nbsp]-[nbsp]2y[sup]2[/sup][nbsp]][/color][/font]

[/bquote]## Quaternion to Axis Angle

To change a quaternion to a rotation around an arbitrary axis in 3D space, we do the following: [bquote]### [font=Courier New][color=#000080]If[nbsp]the[nbsp]axis[nbsp]of[nbsp]rotation[nbsp]is[nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp](ax,[nbsp]ay,[nbsp]az) and[nbsp]the[nbsp]angle[nbsp]is[nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]theta[nbsp](radians) then[nbsp]the[nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]angle=[nbsp]2[nbsp]*[nbsp]acos(w) [nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]ax=[nbsp]x[nbsp]/[nbsp]scale [nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]ay=[nbsp]y[nbsp]/[nbsp]scale [nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]az=[nbsp]z[nbsp]/[nbsp]scale where[nbsp]scale[nbsp]=[nbsp]sqrt[nbsp](x[sup]2[/sup][nbsp]+[nbsp]y[sup]2[/sup][nbsp]+[nbsp]z[sup]2[/sup])[/color][/font]

[/bquote] Another variation I have seen is that the scale = sin(acos(w)). They may be equivalent, though I didn't try to find the mathematical relationship behind them. Anyway if the scale is 0, it means there is no rotation so unless you do something, the axis will be infinite. So whenever the scale is 0, just set the rotation axis to any unit vector with a rotation angle of 0.# A Simple Example

In case you are getting confused with what I'm getting at, I will show you a simple example here. Say the camera orientation is represented as Euler angles. Then, in the rendering loop, we position the camera using [bquote][font=Courier New][color=#000080]### RotateX * RotateY * RotateZ * Translate

[/color][/font][/bquote] where each component is a 4x4 matrix. So if we are using a unit quaternion to represent the camera orientation, we have to convert the quaternion to a matrix first [bquote][font=Courier New][color=#000080]### Rotate (from Quaternion) * Translate

[/color][/font][/bquote] A more specific example in OpenGL:Euler | Quaternion |

[font=courier new,courier,monospace]## [color=#000080]glRotatef( [/font] | [color=#000080]## [font=courier new,courier,monospace]// convert Euler to quaternion[/font][/color][color=#000080][font=courier new,courier,monospace] ## // convert quaternion to axis angle[/font][/color][color=#000080] ## [font=courier new,courier,monospace]glRotate(theta, ax, ay, az)[/font][/color][color=#000080] ## [font=courier new,courier,monospace]// translate[/font][/color] |

# Multiplying Quaternions

Since a unit quaternion represents an orientation in 3D space, the multiplication of two unit quaternions will result in another unit quaternion that represents the combined rotation. Amazing, but it's true. Given two unit quaternions[bquote]Q1=(w1, x1, y1, z1);
Q2=(w2, x2, y2, z2);[/bquote]

A combined rotation of unit two quaternions is achieved by
[bquote]Q1 * Q2 =( w1.w2 - v1.v2, w1.v2 + w2.v1 + v1*v2)[/bquote]

where
[bquote]v1= (x1, y1, z1)
v2 = (x2, y2, z2)[/bquote]

and both . and * are the standard vector dot and cross product.
However an optimization can be made by rearranging the terms to produce
[bquote]w=w1w2 - x1x2 - y1y2 - z1z2
x = w1x2 + x1w2 + y1z2 - z1y2
y = w1y2 + y1w2 + z1x2 - x1z2
z = w1z2 + z1w2 + x1y2 - y1x2[/bquote]

Of course, the resultant unit quaternion can be converted to other representations just like the two original unit quaternions. This is the real beauty of quaternions - the multiplication of two unit quaternions in 4D space solves gimbal lock because the unit quaternions lie on a sphere.
Be aware that the order of multiplication is important. Quaternion multiplication is not commutative, meaning
[bquote]q1 * q2 does not equal q2 * q1[/bquote]

Note: Both quaternions must refer to the same coordinate axis. I made the mistake of combining two quaternions from different coordinate axes, and I had a very hard time wondering why the result quaternion fails in certain angles only.
# Conversion To Quaternions

Now we learn how to convert other representations to quaternions. Although I do not use all the conversions in the sample program, there are times when you'll need them when you want to use quaternion orientation for more advanced stuff like inverse kinematics.## Axis Angle to Quaternion

A rotation around an arbitrary axis in 3D space can be converted to a quaternion as follows [bquote]### [font=Courier New][color=#000080]If[nbsp]the[nbsp]axis[nbsp]of[nbsp]rotation[nbsp]is[nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp](ax,[nbsp]ay,[nbsp]az)-[nbsp]must[nbsp]be[nbsp]a[nbsp]unit[nbsp]vector and[nbsp]the[nbsp]angle[nbsp]is[nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp][nbsp]theta[nbsp](radians) [nbsp][nbsp][nbsp][nbsp]w[nbsp][nbsp][nbsp]=[nbsp][nbsp][nbsp]cos(theta/2) [nbsp][nbsp][nbsp][nbsp]x[nbsp][nbsp][nbsp]=[nbsp][nbsp][nbsp]ax[nbsp]*[nbsp]sin(theta/2) [nbsp][nbsp][nbsp][nbsp]y[nbsp][nbsp][nbsp]=[nbsp][nbsp][nbsp]ay[nbsp]*[nbsp]sin(theta/2) [nbsp][nbsp][nbsp][nbsp]z[nbsp][nbsp][nbsp]=[nbsp][nbsp][nbsp]az[nbsp]*[nbsp]sin(theta/2)[/color][/font]

[/bquote] The axis must first be normalized. If the axis is a zero vector (meaning there is no rotation), the quaternion should be set to the rotation identity quaternion.## Euler to Quaternion

Converting from Euler angles to a quaternion is slightly more tricky, as the order of operations must be correct. Since you can convert the Euler angles to three independent quaternions by setting the arbitrary axis to the coordinate axes, you can then multiply the three quaternions together to obtain the final quaternion. So if you have three Euler angles (a, b, c), then you can form three independent quaternions [bquote]Qx = [ cos(a/2), (sin(a/2), 0, 0)] Qy = [ cos(b/2), (0, sin(b/2), 0)] Qz = [ cos(c/2), (0, 0, sin(c/2))][/bquote] And the final quaternion is obtained by Qx * Qy * Qz.# Demo - Avoiding Gimbal Lock

Finally, we've reached what you all been waiting for: "How can quaternions avoid gimbal lock.?" The basic idea is- Use a quaternion to represent the rotation.
- Generate a temporary quaternion for the change from the current orientation to the new orientation.
- PostMultiply the temp quaternion with the original quaternion. This results in a new orientation that combines both rotations.
- Convert the quaternion to a matrix and use matrix multiplication as normal.

- There are 3 angles I keep track of for rotation in the X, Y, and Z axis.
- With every key press, I adjust the corresponding rotation variable.
- In the while loop, I translate and then just convert the 3 Euler angles to rotation matrices and multiply them into the final transformation matrix.

- The orientation of the camera is a quaternion.
- There are 3 angles corresponding to the keypress. Note the angles are meant to be an on/off switch (not accumulative). I reset them inside the while loop. Of course this is not the best way to do it but as I said, it is a quick job.
- I convert the 3 angles to a temporary quaternion.
- I multiply the temporary quaternion to the camera quaternion to obtain the combined orientation. Note the order of multiplication.
- The camera rotation is then converted to the Axis Angle representation for transforming the final matrix.

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