---
author: Stéphane Laurent
date: '2018-05-01'
highlighter: kate
linenums: True
output:
html_document:
keep_md: False
md_document:
preserve_yaml: True
variant: markdown
prettify: True
prettifycss: 'twitter-bootstrap'
tags: 'R, graphics, rgl'
title: 'Hopf Torus (3/3): the sinusoidal case'
---
In this first and last part of our articles about Hopf tori, we will
take a sinusoidal curve on $S^2$.
``` {.r}
hopfinverse <- function(q, t){
1/sqrt(2*(1+q[3])) * c(q[1]*cos(t)+q[2]*sin(t),
sin(t)*(1+q[3]),
cos(t)*(1+q[3]),
q[1]*sin(t)-q[2]*cos(t))
}
stereog <- function(x){
c(x[1], x[2], x[3])/(1-x[4])
}
```
In order to draw a sinusoidal curve on the sphere, I used this equation,
that I found on
[mathcurve.com](https://www.mathcurve.com/courbes3d/sinusoidespherique/sinusoidespherique.shtml):
$$
\begin{equation}
x = \frac{\cos u}{\sqrt{1+k^2\cos^2(nu)}} \\
y = \frac{\sin u}{\sqrt{1+k^2\cos^2(nu)}} \\
z = \frac{k \cos(nu)}{\sqrt{1+k^2\cos^2(nu)}}
\end{equation}
$$
``` {.r}
plotSphereWithSinusCurve <- function(){
plotSphereEquator()
view3d(0,90)
u_ <- seq(-pi, pi, len=1000)
k <- 3
for(i in 1:length(theta_)){
u <- u_[i]
den <- sqrt(1+k^2*cos(3*u)^2)
x <- cos(u)/den
y <- sin(u)/den
z <- k*cos(3*u)/den
points3d(x,y,z, color="blue")
}
}
```
``` {.r}
open3d(windowRect=c(50,50,500,500))
view3d(0,90)
t_ <- seq(0, 2*pi, len=200)
k <- 3
u <- seq(0, 2*pi , len=300)
for(i in 1:length(u_)){
u <- u_[i]
den <- sqrt(1+k^2*cos(3*u)^2)
x <- cos(u)/den
y <- sin(u)/den
z <- k*cos(3*u)/den
circle4d <- sapply(t_, function(t){
hopfinverse(c(x,y,z),t)
})
circle3d <- t(apply(circle4d, 2, stereog))
shade3d(cylinder3d(circle3d, radius=0.1), color="purple")
}
```
This time, we obtain a Hopf torus with three lobes (because we took
$n=3$ in the formula above).
Below is an interactive rendering with `three.js`. Go to [this
post](https://laustep.github.io/stlahblog/posts/threejsTorus.html) if
you want to know how I've drawn the circles.
