# What I know/have:

• A sphere S with radius $r_0=1$ centered in Cartesian space at $c_0=(0,0,0)$
• Three points on the sphere $S$: $p_1$, $p_2$, and $p_3$
• A plane $P$ through the points $p_1$, $p_2$, and $p_3$
• A circle $C$, which is produced by intersecting plane $P$ with sphere $S$. All points $p_1$, $p_2$, and $p_3$ are on the circle $C$
• The arc $A$ when going from $p_1$ over circle $C$ to $p_3$ has the interesting property that at fraction $0.5$ of its total distance, we find $p_2$
• Hence, at fraction $0$ of the total distance of arc $A$ we find $p_1$, and at fraction $1$ of the total distance of arc $A$ we find $p_3$

# What I want:

A point $p_4$ in Cartesian space ($x$,$y$,$z$ with respect to origin $c_0$), which lies on the arc $A$ at an arbitrary fraction $X$ of the total distance that arc $A$ spans from $p_1$ to $p_3$

From a computational perspective this would mean:

• input coordinates of 3 points and a fraction
• output coordinates of 1 point

Unfortunately all the steps in between are a black box to me in terms of the mathematical/computational steps involved and I would appreciate a solution ... or help with finding the solution myself.

# Visualization of the Problem

The visualization was drawn by hand using geogebra so please just assume the assumptions above hold true (although it might look different in the visualization) # Previous research:

• The Great Circle is the circle that comes from intersecting a sphere and a plane that contains the origin of the sphere as a point
• There are computations for finding intermediate points along the Great Circle Distance in Ed William's Aviation Formulary
• In my case, the plane does NOT contain the origin of the sphere but three other points on the sphere
• What I am looking for are intermediate points on a "Small Circle Distance"

# Appendix (not required for answering the question) - Real-Life application:

When measuring EEG data, there is a conventional set of rules how to place electrodes (white buttons in image below) on the scalp of a human. Usually this is measured with a measuring tape but of course we can also model the human head by a geometrical sphere and compute the relative electrode positions on that sphere (based only on rules and the initial, arbitrary placement of one electrode ... no measurements involved then). Looking at figure B of the image above, and relating to this question, let's assume I know the positions $F7$, $F8$, and $Fz$. Knowing the rules, $F3$ and $F4$ will lie at fractions on the contour connecting $F7$ and $F8$ through $Fz$.

• You know it's a very hard question, I can't do it – Abhas Kumar Sinha Jun 2 '18 at 9:21

## 3 Answers

We have a sphere of radius $R$ centered at origin, and three points $$\vec{p}_1 = \left [ \begin{matrix} x_1 \\ y_1 \\ z_1 \end{matrix} \right ], \quad \vec{p}_2 = \left [ \begin{matrix} x_2 \\ y_2 \\ z_2 \end{matrix} \right ], \quad \vec{p}_3 = \left [ \begin{matrix} x_3 \\ y_3 \\ z_3 \end{matrix} \right ]$$ on the sphere, i.e. $\lVert\vec{p}_1\rVert = R$, $\lVert\vec{p}_2\rVert = R$, and $\lVert\vec{p}_3\rVert = R$.

The three points form a plane with normal $\vec{n}$, $$\vec{n} = (\vec{p}_2 - \vec{p}_1) \times (\vec{p}_3 - \vec{p}_1) \tag{1}\label{NA1}$$ where $\times$ represents cross product. For simplicity, we'll use an unit normal vector $\hat{n}$, $\lVert\hat{n}\rVert = 1$: $$\hat{n} = \left [ \begin{matrix} x_n \\ y_n \\ z_n \end{matrix} \right ] = \frac{\vec{n}}{\left\lVert\vec{n}\right\rVert} \tag{2}\label{NA2}$$ Note that if $\lVert\vec{n}\rVert = 0$, the three points are either collinear, or two or more have the same coordinates.

The plane is at distance $d$ from origin: $$d = \hat{n} \cdot \vec{p}_1 = \hat{n} \cdot \vec{p}_2 = \hat{n} \cdot \vec{p}_3 \tag{3}\label{NA3}$$ where $\cdot$ represents dot product. (In a computer program, you can use any one of the three; they might differ by rounding error.)

The intersection between the plane and the sphere is a circle, with center $\vec{c}$ and radius $r$, $$\vec{c} = \left [ \begin{matrix} x_c \\ y_c \\ z_c \end{matrix} \right ] = d \hat{n}, \quad r = \sqrt{R^2 - d^2} = \left\lVert\vec{p}_1 - \vec{c}\right\rVert = \left\lVert\vec{p}_2 - \vec{c} \right\rVert = \left\lVert \vec{p}_3 - \vec{c} \right\rVert \tag{4}\label{NA4}$$

My suggested approach would be to construct a 2D coordinate system, where the unit circle corresponds to the above circle, with the first axis towards $\vec{p}_1$, and the second axis so that $\vec{p}_2$ is in the positive half-plane.

The origin is obviously at $\vec{c}$. The $u$ axis unit vector is $$\hat{u} = \left [ \begin{matrix} x_u \\ y_u \\ z_u \end{matrix} \right ] = \vec{p}_1 - \vec{c} \tag{5}\label{NA5}$$ Note that by definition, $\left\lVert\hat{u}\right\rVert = r$.

The $v$ axis unit vector is perpendicular to both the plane normal $\vec{n}$ and the $u$ axis unit vector; we also need to select its sign (handedness) so that $\vec{p}_2$ has a positive $v$ coordinate. So, first we calculate the cross product, and scale it to the proper length: $$\hat{q} = r \frac{\vec{n} \times \hat{u}}{\left\lVert\vec{n}\times\hat{u}\right\rVert} = \hat{n} \times \hat{u}\tag{6}\label{NA6}$$ Then, we pick its direction (handedness, or sign for $v$ coordinates) so that $\vec{p}_2$ has a positive $v$ coordinate: $$\hat{v} = \left [ \begin{matrix} x_v \\ y_v \\ z_v \end{matrix} \right ] = \begin{cases} \hat{q}, & (\vec{p}_2 - \vec{c}) \cdot \hat{q} \gt 0 \\ -\hat{q}, & \text{otherwise} \end{cases} \tag{7}\label{NA7}$$ Note that $\vec{p}_2$ can have a zero $v$ coordinate only if it is opposite to $\vec{p}_1$ on the circle; and by OP's definition, that would require $\vec{p}_3 = \vec{p}_1$, in which case we would not have a plane at all.

At this point, we have a 2D coordinate system $(u, v)$: $$\vec{p} = \vec{c} + u \hat{u} + v \hat{v} \tag{8a}\label{NA8a}$$ and inversely \left\lbrace\begin{aligned} u &= \frac{(\vec{p} - \vec{c}) \cdot \hat{u}}{r^2} \\ v &= \frac{(\vec{p} - \vec{c}) \cdot \hat{v}}{r^2} \\ \end{aligned}\right.\tag{8b}\label{NA8b} We need to calculate the direction of $\vec{p}_3$ on this circle. Most programming languages provide an atan2(y, x) function, which computes $\arctan(y/x)$ except including the quadrant of $(x, y)$; i.e. with the result covering full 360°. Essentially, $$\theta^\prime = \operatorname{atan2}\bigl( (\vec{p}_3 - \vec{c})\cdot\hat{v} ,\, (\vec{p}_3 -\vec{c})\cdot\hat{u} \bigr) \tag{9}\label{NA9}$$ noting that since both components are divided by $r^2$, we can omit both divisions.

Because we have defined the 2D coordinate system so that counterclockwise rotation starts from $\vec{p}_1$, then reaches $\vec{p}_2$, and finally $\vec{p}_3$, we need a positive $\theta$. Therefore,

$$\theta = \begin{cases} \theta^\prime + 360°, & \theta^\prime \lt 0 \\ \theta^\prime, & \text{otherwise} \end{cases} \tag{10}\label{NA10}$$

If we now want to parametrise the arc using $t$ as the arc length parameter, $0 \le t \le 1$, so that $\vec{P}(0) = \vec{p}_1$, $\vec{P}(0.5) = \vec{p}_2$, $\vec{P}(1) = \vec{p}_3$, we simply use $u = \cos(t\theta)$, $v = \sin(t\theta)$. In summary, $$\vec{P}(t) = \vec{c} + \hat{u}\cos(t\theta) + \hat{v}\sin(t\theta) \tag{11}\label{NA11}$$

Note that the approach works for any $\vec{p}_2$ that is on the circular arc between $\vec{p}_1$ and $\vec{p}_3$; there is no need to require $\vec{p}_2 = \vec{P}(0.5)$. (Having $\vec{p}_2$ away from $\vec{p}_1$ and $\vec{p}_3$ means the triangle they form is as "wide" as possible, reducing the rounding error in $\hat{n}$, so it is a very good choice; but other than that, this algorithm only expcets $\vec{p}_2$ to be somewhere on the arc between $\vec{p}_1$ and $\vec{p}_2$, i.e. at $\vec{P}(\tau)$ with $0 < \tau < 1$.)

In pseudocode, the function could be written as

Globals: xc, yc, zc, xu, yu, zu, xv, yv, zv, theta

Function x(t) = xc + xu*cos(t*theta) + xv*sin(t*theta)
Function y(t) = yc + yu*cos(t*theta) + yv*sin(t*theta)
Function z(t) = zc + zu*cos(t*theta) + zv*sin(t*theta)

Function Setup(x1, y1, z1, x2, y2, z2, x3, y3, z3):
# Note: This assumes
#           x1*x1 + y1*y1 + z1*z1 == R*R
#           x2*x2 + y2*y2 + z2*z2 == R*R
#           x3*x3 + y3*y3 + z3*z3 == R*R
#       although the exact value of R is irrelevant.

# Find plane unit normal vector (xn, yn, zn):
Let  x12 = x2 - x1
Let  y12 = y2 - y1
Let  z12 = z2 - z1
Let  x13 = x3 - x1
Let  y13 = y3 - y1
Let  z13 = z3 - z1

Let  xn = y12*z13 - z12*y13
Let  yn = z12*x13 - x12*z13
Let  zn = x12*y13 - y12*x13
Let  n = sqrt(xn*xn + yn*yn + zn*zn)
If (n <= 0.0) Then
Fail: "The three points are not in general position."
End If
Let  xn = xn / n
Let  yn = yn / n
Let  zn = zn / n

# Signed distance from plane to origin
Let  d = xn*x1 + yn*y1 + zn*z1

# Center of circle
Let  xc = d*xn
Let  yc = d*yn
Let  zc = d*zn

# 2D U axis unit vector
Let  xu = x1 - xc
Let  yu = y1 - yc
Let  zu = z1 - zc

# 2D V axis unit vector
Let  xv = yn*zu - zn*yu
Let  yv = zn*xu - xn*zu
Let  zv = xn*yu - yn*xu

# Choose V axis towards (x2,y2,z2)
Let  v2 = (x2 - xc)*xv + (y2 - yc)*yv + (z2 - zc)*zv
If (v2 < 0.0) Then
Let  xv = -xv
Let  yv = -yv
Let  zv = -zv
End If

# Note: If needed, you can calculate
#           r = sqrt(xu*xu + yu*yu + zu*zu)
#             = sqrt(xv*xv + yv*yv + zv*zv)
#             = sqrt(R*R - d*d)
#       All three match to within rounding error,
#       iff the initial assumption of all three
#       points being at the same distance from
#       origin is true.

# Find theta, the positive plane angle towards (x3,y3,z3).
Let  xt = x3 - xc
Let  yt = y3 - yc
Let  zt = z3 - zt
Let  thetau = xt*xu + yt*yu + zt*zu
Let  thetav = xt*xv + yt*yv + zt*zv
Let  theta = atan2(thetav, thetau)
If (theta < 0.0) Then
# Add 360 degrees, or 2*Pi in radians, to make it positive
Let  theta = theta + 6.2831853071795864769252867665590
End If

# Done.
End Function


After calling Setup(), x(0) == x1, y(0) == y1, z(0) == z1; x(0.5) == x2, y(0.5) == y2, z(0.5) == z2; and x(1) == x3, y(1) == y3, z(1) == z3. The argument is relative arc length, or equivalently angle, from the first point to the third point (via the second point).

(I've tested this with an awk snippet and some random points. Do note that the initial assumption of the three points being equidistant from origin is necessary for the results to make sense; the pseudocode function above does not verify this is true. Obviously, small errors (rounding or measurement error) is allowed, and only causes an error of similar magnitude in the result; the function is surprisingly stable, numerically.)

Another possible approach would be to simply consider $\vec{p}_1$, $\vec{p}_2$, and $\vec{p}_3$ to be points on a circle, and solve $\hat{n}$, $\vec{c}$, and $r$ from them, ignoring the original sphere completely. Whether that approach produces more accurate results than the one shown above, I do not know, as it depends on the relative precision of the points; essentially, on how the actual measurements are done, and how accurate the 3D origin is to the measurement sphere.

• pseudocode would indeed substantially improve the (already good) answer! – S.A. Jun 2 '18 at 10:16
• @S.A.: Added. Let me know in a comment if you find anything odd. Note that although I did verify this using an awk script, some typos and copy-paste errors might still lurk around. – Nominal Animal Jun 2 '18 at 11:58
• Your answer is basically the illustration of why I was too lazy to go into full details. I was considering giving out more details if no one stepped up, so let me (+1) you for sparing me from that. – N.Bach Jun 2 '18 at 12:45
• @N.Bach: Thanks! I did intend to comment your answer that I deliberately stole from your answer (down to $u$ and $v$!) because I was hoping mine is like a step-by-step guide to the algorithm, while yours is the succinct description of the algorithm.. but I forgot. Sorry! :) – Nominal Animal Jun 2 '18 at 13:23
• Just tried it out, that's exactly what I needed! Thanks @NominalAnimal – S.A. Jun 4 '18 at 8:45

In principle, you can just establish a parametric equation of the arc, where the parameter is the arc length. Especially in the case of circles, their parametric equation is fairly simple.

Say you have a point $O$, and two orthogonal unit vectors $\mathbf u$ and $\mathbf v$. Then the parametric equation of the circle of center $O$, radius $r>0$, and in the plane through $O$ with directions $\mathbf u$ and $\mathbf v$, is given by $$M(\theta) = O+r\left(\cos\theta\mathbf u+\sin\theta\mathbf v\right)$$ for $\theta\in\mathbb R$. Restricting the value of $\theta$ to a range of length less than $2\pi$ will yield circular arcs supported by the circle.

To apply this to your problem, you just have to figure out what choice of $\mathbf u$ and $\mathbf v$ will make your life simple. One possibility is to compute the center $O$ and radius $r$ of your circle, then set $\hat{\mathbf u} = \vec{Op_1}$ and $\hat{\mathbf v} = \vec{Op_2}$. Let $\mathbf u$ and $\mathbf v$ be the Gram–Schmidt orthonormalisation of $\hat{\mathbf u}$ and $\hat{\mathbf v}$. Let also $\varphi$ be the unique real number in $(O,2\pi)$ such that $p_3=O+r(\cos\varphi\mathbf u+\sin\varphi\mathbf v)$.

Then, your arc $A$ is the collection of points $M(\theta)$ with $0\le\theta\le\varphi$. If you want to use a fraction of the total arc length, you can use $\theta = f\times\varphi$ with $0\le f\le 1$ your fraction.

Forget about small circles on a sphere. Consider the generalization that we have $3$ points $p_1, p_2, p_3$ lying on some circle centered at $c$ with radius $r$ and $p_2$ is the mid-point of the circular arc $p_1p_3$. Let $\theta_0$ be the half-angle of the arc $p_1p_3$ subtended at $c$.

We can parametrize the circular arc by an angle $\theta \in [-\theta_0,\theta_0]$ using following formula: $$p(\theta) = c + (p_2 - c)\cos\theta + \frac{p_3-p_1}{2}\frac{\sin\theta}{\sin\theta_0} \tag{*1}$$

Independent of value of $c$, we have $p(0) = p_2$. In order for $p(-\theta_0) = p_1$ and $p(\theta_0) = p_3$, we need

$$c + (p_2 - c)\cos\theta_0 = \frac{p_1+p_3}{2} \quad\implies\quad c = \frac{1}{1-\cos\theta_0} \left(\frac{p_1+p_3}{2} - p_2 \cos\theta_0\right)$$ Plug this back in $(*1)$, we obtain

$$\bbox[padding: 1em;border: 1px solid blue]{ p(\theta) = \frac{1-\cos\theta}{1-\cos\theta_0}\frac{p_3+p_2}{2} + \frac{\sin\theta}{\sin\theta_0} \frac{p_3-p_1}{2} + \frac{\cos\theta-\cos\theta_0}{1-\cos\theta_0} p_2}\tag{*2}$$

What remains is to compute $\theta_0$.

Notice $|p_3 - p_1| = 2r\sin\theta_0$ and $|p_2 - p_1| = 2r\sin\frac{\theta_0}{2}$, we have $$\cos\frac{\theta_0}{2} = \frac{\sin\theta_0}{2\sin\frac{\theta_0}{2}} = \frac{|p_3-p_1|}{2|p_2-p_1|} \quad\implies\quad \theta_0 = 2\cos^{-1}\left(\frac{|p_3-p_1|}{2|p_2-p_1|}\right)$$

To parametrize the arc using a fraction $t$, one just need to associate $t \in [0,1]$ with the point $p((2t-1)\theta_0)$ given in formula $(*2)$.

• What is the advantage of this generalization with regards to the problem that I have? – S.A. Jun 4 '18 at 8:46
• @S.A. The advantage is you don't need to tie the formula to a specific sphere. If one day you are forced to use a coordinate system where the sphere is not centered at origin, the same formula continue to work. – achille hui Jun 4 '18 at 10:56