# Find the rotation between set of points

I have two sets (sourc and target) of points (x,y) that I would like to align. What I did so far is:

• find the centroid of each set of points
• use the difference between the centroids translations the point in x and y

What I would like is to find the best rotation (in degrees) to align the points.

Any idea?

M code is below (with plots to visualize the changes):

# Raw data
## Source data
sourc = matrix(
c(712,960,968,1200,360,644,84,360), # the data elements
nrow=2, byrow = TRUE)

## Target data
target = matrix(
c(744,996,980,1220,364,644,68,336), # the data elements
nrow=2, byrow = TRUE)

# Get the centroids
sCentroid <- c(mean(sourc[1,]), mean(sourc[2,])) # Source centroid
tCentroid <- c(mean(target[1,]), mean(target[2,])) # Target centroid

# Visualize the points
par(mfrow=c(2,2))
plot(sourc[1,], sourc[2,], col="green", pch=20, main="Raw Data",
lwd=5, xlim=range(sourceX, targetX),
ylim=range(sourceY, targetY))
points(target[1,], target[2,], col="red", pch=20, lwd=5)
points(sCentroid[1], sCentroid[2], col="green", pch=4, lwd=2)
points(tCentroid[1], tCentroid[2], col="red", pch=4, lwd=2)

# Find the translation
translation <- tCentroid - sCentroid
target[1,] <- target[1,] - translation[1]
target[2,] <- target[2,] - translation[2]

# Get the translated centroids
tCentroid <- c(mean(target[1,]), mean(target[2,])) # Target centroid

# Visualize the translation
plot(sourc[1,], sourc[2,], col="green", pch=20, main="After Translation",
lwd=5, xlim=range(sourceX, targetX),
ylim=range(sourceY, targetY))
points(target[1,], target[2,], col="red", pch=20, lwd=5)
points(sCentroid[1], sCentroid[2], col="green", pch=4, lwd=2)
points(tCentroid[1], tCentroid[2], col="red", pch=4, lwd=2)

• I cannot read your code, but the operation you need is called Procrustes rotation. Have you heard of it? It works when points are already paired ($x_i,y_i$). Pre-rotation optional operations include translation and scaling, and optional post-rotational isoscaling. – ttnphns Dec 10 '15 at 17:13
• A complex regression will do the job. – whuber Dec 10 '15 at 17:23
• I've seen, that, rotating the system about 180 degrees, then the pairs $(a,C),(b,D),(c,A),(d,B)$ become neighbours - and this is even a better fit than the best fit of the original $(a,A),(b,B),(c,C),(d,D)$ (where the small letters stand for vector source and capital letters for vector target) I've not seen this possibility mentioned and explicitely allowed or disallowed. Are you sure that you don't want that better fit? – Gottfried Helms Sep 4 '16 at 14:05

This can be done using the Kabsch Algorithm. The algorithm finds the best least-squares estimate for rotation of $RX-Y$ where $R$ is rotation matrix, $X$ and $Y$ are your target and source matrices with 2 rows and n columns.

In [1] it is shown that this problem can be solved using singular value decomposition. The algorithm is as follows:

1. Center the datasets so their centroids are on origin.
2. Compute the "covariance" matrix $C$=$XY^T$.
3. Obtain the Singular Value Decomposition of $C=UDV^T$.
4. Direction adjustment $d=sign(det(C))$.
5. Then the optimal rotation $R=V\left( \begin{array}{ccc} 1 & 0 \\ 0 & d \\ \end{array} \right)U^T$

I don't know of any implementation in R so wrote a small function below.

src <- matrix(c(712,960,968,1200,360,644,84,360), nrow=2, byrow=TRUE)
trg <- matrix(c(744,996,980,1220,364,644,68,336), nrow=2, byrow=TRUE)


Kabsch algorithm in an R funtion:

kabsch2d <- function(Y, X) {
X   <- X-rowMeans(X)
Y   <- Y-rowMeans(Y)
C   <- X %*% t(Y)
SVD <- svd(C)
D   <- diag(c(1, sign(det(C))))
t(SVD$v) %*% D %*% t(SVD$u)
}


Center the points:

src <- src-rowMeans(src)
trg <- trg-rowMeans(trg)


Obtain rotation:

rot <- kabsch2d(src, trg)


Result (black - original source, red - original target, green - rotated target)

plot(t(src), col="black", pch=19)
points(t(trg), col="red", pch=19)
points(t(rot %*% trg), col="green", pch=19)


• +1. However, the answer could be further much better if you included discourse about how the algo is related to well-known Procrustes rotation problem. – ttnphns Mar 19 '16 at 14:03

I've done this with an iterative optimum-search, and tested 2 versions.
I've taken the original arrays and centered them calling this arrays cSRC and cTAR . Then I've done a loop with angles $\varphi$ between $0$ and $2 \pi$ , and for each angle I computed the error-criterion using the difference between the rotated $\small D=rot(\text{cSRC} ,\varphi)- \text{cTAR}$.

1. In version 1) I took as criterion the sum-of-squares of all entries in $\small D$ as $$err_1 = \small \sum_{k=1}^4 \small((D_{k,1})^2+(D_{k,2})^2)$$and the angle $\small \varphi$ at which the minimal error occured is equivalent the kabsch2d-procedure in @Karolis' answer.

2. In version 2) I took as criterion the sum of the absolute distances, that means, the sum $$err_2=\small \sum_{k=1}^4 \small\sqrt{(D_{k,1})^2+(D_{k,2})^2}$$ and got a slightly different rotation angle $\small \varphi$ for the smallest error.

I don't know, which criterion fits your needs better.

Here are some results from the protocol.

$$\small \begin{array} {r|cc} & \text{version } 1 & \text{version } 2\\ \hline \varphi & -0.04895304& -0.05093647 \\ \text{rotation} & \begin{bmatrix} 0.99880204& -0.04893349\\ 0.04893349& 0.99880204\\ \end{bmatrix} & \begin{bmatrix} 0.99870302 & -0.05091444\\ 0.05091444 & 0.99870302\\ \end{bmatrix} \\ \text{distances} & \begin{bmatrix} -6.80077266 & -0.86209739\\ 2.79924551 & -9.33782500\\ -0.61309522 & 6.94156520\\ 4.61462237 & 3.25835719\\ \end{bmatrix} & \begin{bmatrix} -6.78017751& -0.37062404 \\ 3.35787307 & -9.36574874 \\ -1.16459115 & 6.95324527 \\ 4.58689559 & 2.78312752 \\ \end{bmatrix} \end{array}$$