I’ve found spatial vector algebra pretty confusing, as commonly found in most physics engines. So I wanted to derive a few properties here for my own sanity.

Spatial vector algebra combines linear and angular parts of motions, forces, and transformations into 6D spatial vectors (see Rigid Body Dynamics Algorithms by Featherstone for a more comprehensive explanation). In general, spatial vectors are more compact to code and they reduce the number of floating point operations when calculating rigid body dynamics.

### Background

Featherstone uses Plücker coordinates to represent force and motion. We’ll outline the basis vectors in a bit, but to put it into equations, a motion is

\[\vec{m} = \begin{bmatrix} \vec{v} \\ \vec{\omega} \end{bmatrix}\]and a force is

\[\vec{f} = \begin{bmatrix} \vec{F} \\ \vec{\tau} \end{bmatrix}\]A motion is comprised of a velocity $\vec{v} = [v_x \ v_y \ v_z]$ and an angular velocity $\vec{\omega} = [\omega_x \ \omega_y \ \omega_z]$. A force is comprised of a linear force $\vec{F} = [F_x \ F_y \ F_z]$ and a torque $\vec{\tau} = [\tau_x \ \tau_y \ \tau_z]$. I’m not using the same exact notation as Featherstone, but you get the idea.

Only certain scalar products are defined in this coordinate system. Given $\bf{m} \in M^6$ and $\bf{f} \in F^6$, $\bf{m} \cdot \bf{f}$ or $\bf{f} \cdot \bf{m}$ both mean the same thing, in this case the power delivered by a force to a body in motion. $\bf{m} \cdot \bf{m}$ and $\bf{f} \cdot \bf{f}$ are not defined. $M^6$ and $F^6$ are the space of all motions and forces, which form a dual vector space (see RBDA for more details on dual vector spaces). We define a basis in cartesian coordinates where the 6d basis vector is comprised of the x-y-z axes plus rotations about the x-y-z axes (Plücker basis). Dot products in dual coordinates work the same way as you’re normally used to (i.e. $\bf{m} \cdot \bf{f} = \vec{m}^T \vec{f}$).

One nice thing about spatial vectors is that we can now define transforms in $SE(3)$ that transform both the angular and linear components of motions/forces in one go! Let’s define a transformation $\bf{X}$ that operates on motions $\bf{m}$. The same transformation applied to a force $\bf{f}$ is $\bf{X^\ast} = \bf{X^{-T}}$ where the “-T” means $(X^-1)^T$. This is because transformations must respect the inner product, $\vec{m}^T \vec{f} = (\bf{X \vec m})^T ( \bf{X^\ast \vec f})$.

So what is the transformation of say a motion vector? A rotation is simply

\[\bf{X_{rot}} = \begin{bmatrix} E & 0\\ 0 & E \end{bmatrix}\](rotate both the angular and linear velocities by a rotation matrix $E$). A translation is

\[\bf{X_{trans}} = \begin{bmatrix} \bf{1} & 0\\ -\vec r_\times & \bf{1} \end{bmatrix}\], where $\vec r_\times$ is a cross product of $\vec r$ with another vector:

\[\begin{bmatrix} 0 & -z & y \\ z & 0 & -x \\ -y & x & 0 \end{bmatrix}\]The translation needs a bit more explaining. When we translate a motion, the angular velocity is untouched, hence the identity in the diagonal of $\bf{X_{trans}}$. A linear velocity however transforms as $\vec v^{\prime} = \vec v - \vec r \times \vec \omega$ where $\vec v^{\prime}$ is the translated velocity.

A translation followed by a rotation is:

\[\bf{X} = \begin{bmatrix} E & 0\\ 0 & E \end{bmatrix} \begin{bmatrix} \bf{1} & 0\\ -\vec r_\times & \bf{1} \end{bmatrix} = \begin{bmatrix} E & 0\\ -E \vec r_\times & E \end{bmatrix}\]Now to make use of these transformations in code, Featherstone introduces operator notations in Appendix A.2 of RBDA. Notably, $plx(E, r)$ is a Plücker transform where we apply a translation by $r$ and then a rotation by $E$, which we just derived above. This is really convenient since we can now implement a data structure called `Transform`

in code as having two attributes, a translation and rotation, rather than a 6x6 matrix. When we want to apply the transform, we can derive equations using the 6x6 spatial matrix, and then convert the 6x6 back to the $plx$ notation to write it in code.

Something very tricky just happend though! We just separated out the translation and rotation components of a transformation $\bf{X}$ into $plx$. As you may know, translations and rotations don’t commute, so the order of operations is crucially important when *applying* $plx$. While $plx$ is more compact compared to the 6x6 matrix, the developer needs to keep careful track of the order of operations when applying transformations.

The Rigid Body Dynamics Library (RBDL) uses Featherstone’s convention and implements $plx$ as well as other spatial vector algebra. One key difference compared to other physics engines like TDS and brax, is that operations are *left-associative* in Featherstone/RBDL, and *right-associative* in TDS and brax. We’ll explain associativity in the next section.

### Associativity

Left-associative transforms, as seen in RBDA, applies transforms on the left. In other words, if we have two transforms $X_1 X_2$ applied to force/motion, first we apply $X_2$ to the force/motion, then we apply $X_1$. Right-associative transforms, as seen in TDS or Brax, apply transforms on the right. To spell it out, a transform $Y_1 Y_2$ applied to a force/motion, first applies $Y_1$ then $Y_2$. In the rest of the blog post, we’ll use $X$ to mean a transform that is left-associative and $Y$ to mean a transform that is right-associative. We’ll also use $plx$ as the Plücker transform for $X$ and $plxb$ as the Plücker transform for $Y$.

Another way to interpret associativity is to look at kinematic chains. For a left-associative transform, the order of transformations gets written as $X_{child} X_{parent}$ if we want to transform something from a parent-to-joint then a joint-to-child link (i.e. parent-to-child transformation). $X_{child}$ is the transform that goes from the joint to the child link position/orientation, and $X_{parent}$ is the transform that goes from the parent to the joint position/orientation. For a right-associative transform, the order is $Y_{parent} Y_{child}$ for the same transformation.

So how are $plx$ and $plxb$ now related? Let’s write out the 6x6 transform for $plxb(E, r)$:

\[plxb(E, r) = \begin{bmatrix} E^T & 0\\ -E^T \vec r_\times & E^T \end{bmatrix}\]It’s essentially the same as $plx(E, r)$ except the rotation matrix gets a transpose. Now we can derive useful formulas commonly found in physics engines!

### Deriving Transform * Transform

Let’s first derive the $plx$ operator for two combined transformations, $\textbf{X}_2 = plx(E_2, r_2)$ and $\textbf{X}_1 = plx(E_1, r_1)$. For the sake of clarity, let’s say $\bf{X}_2$ is the transformation the world to a parent link, and $\bf{X}_1$ is the transformation from a parent to a child link (assuming we have a kinematic chain). The transformation from the world to the child is then:

\[\bf{X}_1 \bf{X}_2 = \begin{bmatrix} E_1 & 0\\ -E_1 \vec r_{1\times} & E_1 \end{bmatrix} \begin{bmatrix} E_2 & 0\\ -E_2 \vec r_{2\times} & E_2 \end{bmatrix} = \begin{bmatrix} E_1 E_2 & 0\\ -E_1 \vec r_{1\times} E_2 - E_1 E_2 r_{2\times} & E_1 E_2 \end{bmatrix}\]Let’s simplify the lower left entry by using the identities

\[(\vec a + \vec b)_\times = \vec a_\times + \vec b_\times\]and

\[(E \vec a)_\times = E \vec a_\times E^{-1}\]where $(E \vec a)_\times$ is a cross product operator by the rotated $\vec a$. Now the lower left entry of $X_1 X_2$ can be written as:

\[\begin{multline} \begin{split} &-E_1 \vec r_{1\times} E_2 - E_1 E_2 \vec r_{2\times}\\ &= -E_1 E_2 (E_2^T \vec r_{1\times} E_2 + \vec r_{2\times})\\ &= -E_1 E_2 ( (E_2^T \vec r_1)_\times + \vec r_{2\times})\\ &= -E_1 E_2 ( E_2^T \vec r_1 + \vec r_{2})_\times \end{split} \end{multline}\]Notice we can now re-order $\bf{X}_1 \bf{X}_2$ to get

\[\bf{X}_1 \bf{X}_2 = \begin{bmatrix} E_1 E_2 & 0\\ 0 & E_1 E_2 \end{bmatrix} \begin{bmatrix} \bf{1} & 0\\ -(E_2^T \vec r_1)_\times - \vec r_{2\times} & \bf{1} \end{bmatrix}\]which is simply $plx(E_1 E_2, E_2^T r_1 + r_2)$. If you look at RBDL, this is exactly what you see! Yay.

Now let’s do the same thing for $plxb$, where a transformation $Y$ is right-associative. $Y_1$ or $plxb(E_1, r_1)$ is the world-to-parent transform and $Y_2$ or $plxb(E_2, r_2)$ is the parent-to-child transform:

\[\bf{Y}_1 \bf{Y}_2 = \begin{bmatrix} E_2^T & 0\\ -E_2^T \vec r_{2\times} & E_2^T \end{bmatrix} \begin{bmatrix} E_1^T & 0\\ -E_1^T \vec r_{1\times} & E_1^T \end{bmatrix} = \begin{bmatrix} E_2^T E_1^T & 0\\ -E_2^T \vec r_{2\times} E_1^T - E_2^T E_1^T r_{1\times} & E_2^T E_1^T \end{bmatrix}\]Notice that we apply $Y_1$ on the right because it is right associative. Again simplifying the lower left entry:

\[\begin{multline} \begin{split} &-E_2^T \vec r_{2\times} E_1^T - E_2^T E_1^T r_{1\times}\\ &= -E_2^T E_1^T (E_1 \vec r_{2\times} E_1^T - \vec r_{1\times})\\ &= -E_2^T E_1^T ((E_1 \vec r_2)_{\times} + \vec r_{1\times}) \\ &= -E_1^T E_2^T (E_1 \vec r_2 + \vec r_1)_{\times} \end{split} \end{multline}\]Re-writing $\bf{Y}_1 \bf{Y}_2$ we get

\[\bf{Y}_1 \bf{Y}_2 = \begin{bmatrix} E_2^T E_1^T & 0\\ -E_2^T E_1^T (E_1 \vec r_2 + \vec r_1)_{\times} & E_2^T E_1^T \end{bmatrix}\]which is just $plxb((E_2^T E_1^T)^T, \vec r_1 + E_1 \vec r_2)$ or $plxb(E_1 E_2, \vec r_1 + E_1 \vec r_2)$. This is exactly what we see in TDS here, and brax here. Yay again!

### Transform a Motion/Force

What about transforming a motion or force? We’ll finally get to apply our spatial vector algebra using the force/motion basis! Let’s say we have a transform $plx(E, r)$ (let’s call the transformation $\bf{X}$) applied to a motion $\vec m$:

\[\bf{X} \vec m\ = \begin{bmatrix} E & 0\\ -\vec r_\times E & E \end{bmatrix} \begin{bmatrix} \vec \omega \\ \vec v \end{bmatrix} = \begin{bmatrix} E \vec \omega \\ E(\vec v - \vec r \times \vec \omega) \end{bmatrix}\]which is exactly what we find in RLBD here. Let’s do the same thing for $plxb(E, r)$ (or a transform $\bf{Y}$) applied to a motion:

\[\bf{Y} \vec m\ = \begin{bmatrix} E^T & 0\\ -E^T \vec r_\times & E^T \end{bmatrix} \begin{bmatrix} \vec \omega \\ \vec v \end{bmatrix} = \begin{bmatrix} E^T \vec \omega \\ E^T (\vec v - \vec r \times \vec \omega) \end{bmatrix}\]That’s what we find in TDS and brax.

Getting the inverse transformations just amounts to taking the inverse of the 6x6, and following the same derivation as above. $X^-1$ for $plx(E, r)$ is:

\[\begin{multline} \begin{split} X^{-1} &= \begin{bmatrix} E & 0\\ -E \vec r_\times & E \end{bmatrix}^{-1} = \begin{bmatrix} \bf{1} & 0\\ -\vec r_\times & \bf{1} \end{bmatrix}^{-1} \begin{bmatrix} E & 0\\ 0 & E \end{bmatrix}^{-1}\\ &= \begin{bmatrix} \bf{1} & 0\\ \vec r_\times & \bf{1} \end{bmatrix} \begin{bmatrix} E^T & 0\\ 0 & E^T \end{bmatrix} = \begin{bmatrix} E^T & 0\\ \vec r_\times E^T & E^T \end{bmatrix} \end{split} \end{multline}\]And $plxb(E, r)$ is:

\[Y^{-1} = \begin{bmatrix} E^T & 0\\ -E^T \vec r_\times & E^T \end{bmatrix}^{-1} = \begin{bmatrix} E & 0\\ \vec r_\times E & E \end{bmatrix}\]To get transforms applied to forces, recall that the dual transform is $\bf{X^\ast} = \bf{X^{-T}}$, which we can now derive:

\[X^{-T} = \begin{bmatrix} E^T & -\vec r_\times E^T\\ 0 & E^T \end{bmatrix}\]and

\[Y^{-T} = \begin{bmatrix} E & \vec r_\times E\\ 0 & E \end{bmatrix}\]Now we can apply those transformations to forces to get the corresponding code. I’ll leave that to the reader!

### In conclusion

First we discussed spatial vector algebra, with some motivation based on heavy usage in physics simulators. Then we derived a few operators for applying two transforms and transforming motions/forces. Maybe one day I’ll extend this a bit to inertias. Hope this is helpful so far, as I couldn’t find clear derivations elsewhere on the interwebs.

Please let me know if you find errors or a more intuitive explanation of associativity!