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jemaloQiu

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# Learn Julia (13): Linear Algebra and Rotations

The LinearAlgebra module of Julia brings many matrix-related functionalities to us. Thus I would like to revise some robotic knowledge while learning this module.

A presentation about 3 axes in space and the according rotations:

Now let's code the roll, pitch and yaw matrices:

using LinearAlgebra
function getRotx(theta::Float64, angleInDeg = false)
if (angleInDeg)
theta = theta*pi/180.0
end
return [1 0 0; 0 cos(theta) -sin(theta); 0 sin(theta) cos(theta)]
end

function getRoty(theta::Float64, angleInDeg = false)
if (angleInDeg)
theta = theta*pi/180.0
end
return [cos(theta) 0 sin(theta); 0 1 0;-sin(theta) 0 cos(theta)]
end

function getRotz(theta::Float64, angleInDeg = false)
if (angleInDeg)
theta = theta*pi/180.0
end
return [cos(theta) -sin(theta) 0; sin(theta) cos(theta) 0;0 0 1]
end

Several important groups definition in mathematics:

The orthogonal group in dimension 3 is named SO(3) which is very largely used in Robotics. Any rotation matrix in 3D space belongs to this SO(3) group.

## check if a matrix belongs to group SO(3)
function isInSO3(R)
res = ( size(R) == (3, 3) )
if res
res = (transpose(R) * R == I)
end
res
end

Now define Rotation, Vec3, quaternion as well as their related functions.

# Identity matrix for 3D space
I33 = [1 0 0; 0 1 0; 0 0 1]

##  definition of a 3D vector Datatype
##  for representing a vector in space
struct Vec3
v1::Float64
v2::Float64
v3::Float64
end

# converting a Vec3 to a 3x1 array
function Vec3toArray(v::Vec3)
return [v.v1, v.v2, v.v3]
end

## A rotation in 3d space consists of an axis and an angle about the axis
struct Rot3
unitAxis::Vec3
end

# Quaternion datatype
mutable struct Quaternion
q0::Float64
q1::Float64
q2::Float64
q3::Float64
end

# converting a 3d rotation to a Quaternion respresentation
function getQuaternion(R::Rot3)
q = Quaternion(1.0, 0.0, 0.0, 0.0)
axis = Vec3toArray(R.unitAxis)
if norm(axis) - 1.0 > 1e-10
println("warning: not unit axis")
axis = axis./norm(axis)
end
q.q1 = axis[1]*sin_theta
q.q2 = axis[2]*sin_theta
q.q3 = axis[3]*sin_theta
return q
end

## convert a 3d vector to its skew-matrix form which belongs to group so(3)
function getSkew3(v::Vec3)
return [  0      -v.v3   v.v2;
v.v3    0       -v.v1;
-v.v2    v.v1    0]

end

## converting a 3d rotation to a matrix respresentation
function getRotMatrix(r::Rot3)
q = getQuaternion(r)
# omega = [q.q1, q.q1, q.q1]

if r.thetaInRad == 0.0    #TODO: also include 2n*pi
omega = Vec3(0.0, 0.0, 0.0)
else
alpha = sin( 0.5* r.thetaInRad )
omega = Vec3(q.q1/alpha, q.q2/alpha, q.q3/alpha)
end
sk = getSkew3(omega)
println("omega:   \$omega")
println("skew: ", sk)
return I33 .+ sin( r.thetaInRad )* sk .+ (1 - cos( r.thetaInRad ))* sk* sk
end

Now test the functions defined above:

## rotate about z-axis for 0.3*pi
r1 = Rot3( Vec3(0.0,0.0,1.0), 0.3*pi)
Mrot1 = getRotMatrix(r1)

## rotate about x-axis for 0.2pi
r2 = Rot3( Vec3(1.0,0.0,0.0), 0.2*pi)
Mrot2 = getRotMatrix(r2)

## rotate about y-axis for 0.15pi
r3 = Rot3( Vec3(0.0,1.0,0.0), 0.15*pi)
Mrot3 = getRotMatrix(r3)

println("Mrot1: \$Mrot1 ")

println("det(Mrot1): ", det(Mrot1))
println("trace(Mrot1): ", tr(Mrot1))
println("inv(Mrot1): ", inv(Mrot1))

## check if the results are correct
println(Mrot1 .- getRotz(0.3*pi))
println(Mrot2 .- getRotx(0.2*pi))
println(Mrot3 .- getRoty(0.15*pi))

Now let's do a little visualization test. In the code below, points shall be rotated by 45 deg about z-axis:

points = [0 0 0.5; 1 0 0.5 ]

x = points[:,1]
y = points[:,2]
z = points[:,3]

r4 = Rot3( Vec3(0.0,0.0,1.0), 0.25*pi) # 45 degree about z-axis
Mrot4 = getRotMatrix(r4)

points_new = transpose(Mrot4* transpose(points))
xn = points_new[:,1]
yn = points_new[:,2]
zn = points_new[:,3]

plotting = true
if plotting
using Plots
plotlyjs()

display(plot(x, y, z, xlabel = "x", ylabel = "y", zlabel ="z", w = 10))
display(plot!(xn, yn, zn, w = 10))
display(plot!(zeros(11), zeros(11), 0:0.1:1, w = 3))
display(plot!(zeros(11), 0:0.1:1, zeros(11), w = 3) )
display(plot!(0:0.1:1, zeros(11), zeros(11), w = 3) )
end

I cut two snapshots from the displayed 3D chart, it can be seen that point [1 0 0.5] has been rotated by 45 degrees about z-axis :