# Coordinate transformation from three known points

I have 3 coordinates, moving_points. I know where each of these is supposed to really be, fixed_points. I want to transform them (and many other coordinates) to these known locations (involving translation, rotation, but also a bit of stretching).

How can I create a transformation so that when it’s applied to these 3 coordinates they become a set of known other coordinates?

MWE:

fixed_points = [[0,0], [0,50], [130,50]]
moving_points = [rand(2) for _ in 1:3]
trans = # Probably something involving CoordinateTransformations, the fixed_points, and moving_points
@assert fixed_points ≈ trans(moving_points)


If your “fixed points” are y_1, y_2, y_3, and your “moving points” are x_1, x_2, x_3, you are probably looking for A,b such that

Ax_i + b = y_i \quad i=1,2,3

which is a linear system, so just formulate it as one and use \.

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Do you mean this?

fixed_points = [[0,0], [0,50], [130,50]]
X = hcat(vcat(fixed_points...), ones(2*3))

moving_points = [rand(2) for _ in 1:3]
y = vcat(moving_points...)

p = X\y


which wouldn’t work for any of the points. Or this:

Ab = [hcat(m, [1,1])\f for (m, f) in zip(moving_points, fixed_points)]


but then, how do I apply that…? As you can see, I’m a wee bit lost.

I was under the impression that trans would be a 3×3 transformation matrix…

Trans would be 3x3 and solve

    M*[xs ys ones(3)] = [xsnew ysnew ones(3)] ?


Edit: Yes

julia> M = [xsnew ysnew ones(3)]/[xs ys ones(3)]
3×3 Array{Float64,2}:
1.02264   -0.615986   0.593346
0.140419   0.425081   0.434501
2.92372   -1.79821   -0.125504

julia> [xsnew ysnew ones(3)]
3×3 Array{Float64,2}:
0.485407  0.575846  1.0
0.232837  0.587207  1.0
0.905755  0.955727  1.0

julia> M*[xs ys ones(3)]
3×3 Array{Float64,2}:
0.485407  0.575846  1.0
0.232837  0.587207  1.0
0.905755  0.955727  1.0


This one works (of course, it can be written in more efficient way, it’s only proof of concept)

X = permutedims(hcat(map(x -> [x..., 1.0], moving_points)...))
Y = hcat(map(x -> x, fixed_points), map(x -> x, fixed_points))

c = X \ Y
c = c'

A = c[1:2, 1:2]
b = c[:, 3]


And transformation can be written like this

mov_points = hcat(moving_points...)
A * mov_points .+ b

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That works! Awesome! I can’t help imagine that there is (or should be) a mechanism for this somewhere in https://github.com/JuliaGeometry/CoordinateTransformations.jl

Here is something:

using CoordinateTransformations
f = Translation(b)∘LinearMap(A)
@assert f.(moving_points) ≈ fixed_points


You can use directly f = AffineMap(A, b), there is no need to compose two transformations.

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Here’s an OK implementation that works:

using CoordinateTransformations
function CoordinateTransformations.AffineMap(ps::Vector{Pair{T,R}}) where {T, R}
X = vcat(hcat(last.(ps)...), ones(1, length(ps)))'
Y = hcat(first.(ps)...)'
c = (X \ Y)'
A = c[:, 1:2]
b = c[:, 3]
AffineMap(A, b)
end
fixed_points = [[0,0], [0,50], [130,50]]
moving_points = [rand(2) for _ in 1:3]
f = AffineMap(Pair.(fixed_points, moving_points))
@assert fixed_points ≈ f.(moving_points)

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OK… I’m stumped again.

So while this works great for three coordinates, I’m trying to make it work for only two coordinates. But the problem is that this implementation “flattens” all the coordinates I apply it to:

ps = [[0,0] => [1,1], [10,0] => [9,4]] # using just two coordinates
f = AffineMap(ps)
@assert first.(ps) ≈ f.(last.(ps)) # it "works"

n = 1000
for (x,y) in zip(rand(-20:20, n), rand(-20:20, n))
_, y2 = f([x,y]) # but no matter what the moving point is
@assert y2 == 0  # its y value is always zero
end


I’m sure this makes sense because there is no information about the y axis in those two coordinates (both have zero y-values). But I’m now wondering how to make this work. Now the transformation is a translation followed by a rotation and finally stretching just one of the axis (in this case, the x axis).

With two coordinates, you need an additional constraint, for example that the map is only rotation and scaling, but no shearing. Scaling can be computed from the change in the distance of the points, rotation from change in the angle of the line connecting the two points.

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Wow, I have never worked with CoordinateTransformations before, but it is really very powerful.

Solution to this problem is in the sequence of linear transformations. We should start from moving points to the origin, than rotate them to align along x axis, than scale, than rotate backward in such a way so points will be collinear and finally shift them to fixed_point (it’s not very clear from this description, try to read the code and draw necessary transformations). And great thing: these are all linear transformations and CoordinateTransformations internally concatenate them together into single AffineMap so no Linear Algebra is needed, it’s pure geometry!

using CoordinateTransformations
using Rotations

fixed_points = [[1, 1], [9, 4]]
moving_points = [rand(2) for _ in 1:2]

z1 = moving_points .- moving_points
z2 = fixed_points .- fixed_points

α = norm(z2)/norm(z1)
θ1 = PolarFromCartesian()(z1).θ
θ2 = PolarFromCartesian()(z2).θ

t1 = Translation(-moving_points...)
lm1 = LinearMap(RotZ(-θ1)[1:2, 1:2])
lm2 = LinearMap([α 0; 0 1])
lm3 = LinearMap(RotZ(θ2)[1:2, 1:2])
t2 = Translation(fixed_points...)

am = t2 ∘ lm3 ∘ lm2 ∘ lm1 ∘ t1  # This is a single affine map!

@assert fixed_points ≈ am.(moving_points)

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Perhaps lm2 = LinearMap([α 0; 0 α])?

It can be, but OOP explicitly said “finally stretching just one of the axis (in this case, the x axis).”

As a side note, one can remove Rotations dependence, and do instead

nz1 = z1 ./norm(z1)
lm1 = LinearMap([nz1 -nz; nz nz)


and the same for lm3

Thanks a lot!!! I’ve managed to avoid this kind of transformation altogether.

But! the bigger problem of creating a transformation from a set of fixed and moving coordinates is still open (see issues 30 and 33 in CoordinateTransformations.jl). Matlab solves it nicely, see “More About” and then “Transformation Types” here. Perhaps an additional package that does exactly that would be beneficial. It would be extra nice if said package would solve it for when there are more than the minimum number of coordinate pairs and the resulting transformation object would be an optimum considering all of the input pairs.

I think you were suggested a perfectly fine solution in that issue:

For exactly determined linear problems (like the original one here), just use the linear solver. For overdetermined linear problems you can use eg least squares (again \), and for nonlinear transformations just minimize a cost function.

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Yea, for sure. I need to get on that.

I’m surprised no one has mentioned https://en.wikipedia.org/wiki/Orthogonal_Procrustes_problem
with a julia blog post here https://simonensemble.github.io/2018-10-27-orthogonal-procrustes/
Another related post on the topic, although quite old at this point
Calculating rigid transformation between two point set

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Does this orthogonal Procrustes problem include all the possible transformations, namely translation, scale, shear/stretching or “just” rotation? Cause otherwise that blog post would basically solve camera calibration for me…

It’s no longer an orthogonal transformation if it includes shearing?

In general, if the correspondence between the points in the two sets to be aligned is known, the problem is relatively easy for linear operators. If the correspondence is not known it becomes a bit messier.

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