# How to draw parametric curve in TikZ deltoid and astroid?

Without using MATLAB, Python or any vector graphic language, is it possible to draw the parametric curves deltoid (a/b = 3) and astroid (a/b = 4) using TikZ?

\begin{tikzpicture}

\draw (0,0) circle [radius=4cm];
\draw[dashed] (0,0) circle [radius=3cm];
\draw[red] (3,0) circle [radius=1cm];
\draw[smooth] (4,0) .. controls(3,0) ..(1.4,1.4).. controls(0,3).. (0,4);
\end{tikzpicture} ## 2 Answers

Just for comparison here is a version in Metapost and the luamplib package. Compile with lualatex. \documentclass[border=5mm]{standalone}
\usepackage{luamplib}
\begin{document}
\mplibtextextlabel{enable}
\begin{mplibcode}
beginfig(1);
numeric r, n;
r = 1cm;
n + 1 = 4;

path xx, yy, C[];
xx = (left--right) scaled ((n+2)*r);
yy = xx rotated 90;

numeric theta; theta = 120/n;
C1 = fullcircle scaled 2r rotated (-n * theta) shifted ((n*r, 0) rotated theta);
C2 = fullcircle scaled (2r * n);
C3 = fullcircle scaled (2r * (n+1));

path delta; numeric s; s = 1/2;
delta = for t=s step s until 360:
(r, 0) rotated (-n*t) shifted ((n*r, 0) rotated t) --
endfor cycle;

draw xx withcolor 2/3 blue;
draw yy withcolor 2/3 blue;

draw C1 withcolor 2/3 green;
draw C2 dashed evenly withcolor 2/3 green;
draw C3 withcolor 2/3 green;

draw origin -- center C1 -- point 0 of C1 withcolor 3/4;

draw delta withpen pencircle scaled 1 withcolor 2/3 red;

dotlabel.lrt("$O$", origin);
dotlabel.lrt("$" & decimal (n+1) & "r$", point 0 of C3);
dotlabel.llft("$A$", point 0 of C1);

label("$r$", 1/2[point 0 of C1, center C1] shifted (5 * unitvector(direction 0 of C1)));
label("$" & decimal n & "r$", 1/2 center C1 shifted (5 * unitvector(center C1 rotated 90)));

endfig;
\end{mplibcode}
\end{document}


Changing the value of n will draw other hypocycloids. So with n+1 = 5; you get: See also this answer for more detail.

# THE RAW SOLUTION: THE DELTOID

Here's a quick solution in plain TikZ, using parametric definition of a deltoid (with b=3a):

\documentclass[tikz,border=3.14159mm]{standalone}

\begin{document}

\begin{tikzpicture}
\draw[cyan,very thin] (-4,-4) grid (4,4);
\draw[->] (-4,0) -- (4,0);
\draw[->] (0,-4) -- (0,4);
\draw (0,0) circle (3);
\def\a{1} \def\b{3}

\draw[line width=1pt,blue] plot[samples=100,domain=0:360,smooth,variable=\t] ({(\b-\a)*cos(\t)+\a*cos((\b-\a)*\t/\a},{(\b-\a)*sin(\t)-\a*sin((\b-\a)*\t/\a});

\end{tikzpicture}

\end{document} # A QUICK EDIT: HYPOCYCLOIDS

If you want to be able to change the value of b to obtain other hypocycloids and the grid to be edited automatically, here's a better solution (this time with b=5a):

\documentclass[tikz,border=3.14159mm]{standalone}

\begin{document}

\begin{tikzpicture}
\def\a{1} \def\b{5}
\draw[cyan,very thin] (-\b,-\b) grid (\b,\b);
\draw[->] (-\b,0) -- (\b,0);
\draw[->] (0,-\b) -- (0,\b);
\draw (0,0) circle (\b);

\draw[line width=2pt,red] plot[samples=100,domain=0:\a*360,smooth,variable=\t] ({(\b-\a)*cos(\t)+\a*cos((\b-\a)*\t/\a},{(\b-\a)*sin(\t)-\a*sin((\b-\a)*\t/\a}); <-- edited (to add \a*360)

\end{tikzpicture}

\end{document} EDIT: I forgot to write \a*360 in the plot, which occured a, incomplete curve in case of a being different of 1. It's now ok.

# THE COMPLETE VERSION

Now with all construction lines and possibility to chose the position of the rolling circle centre.

\documentclass[tikz,border=3.14159mm]{standalone}

\begin{document}

\begin{tikzpicture}
\def\clr{olive}

% Define parameters a and b of the hypocycloid
\def\a{2} \def\b{7}

% Define the parameters for plotting the function
\newcommand{\xt}{(\b-\a)*cos(#1)+\a*cos((\b-\a)*#1/\a}
\newcommand{\yt}{(\b-\a)*sin(#1)-\a*sin((\b-\a)*#1/\a}

% Coordinate system and grid
\draw[cyan,very thin] (-\b-1,-\b-1) grid (\b+1,\b+1);
\draw[->] (-\b-1,0) -- (\b+1,0);
\draw[->] (0,-\b-1) -- (0,\b+1);

% The circle into which the rolling circle rolls
\draw[\clr,thick] (0,0) circle (\b);

% The circle on which moves the center of the rolling circle
\draw[\clr,dashed,very thin] (0,0) circle (\b-\a);

% Plot the hypocycloid
\draw[line width=2pt,orange!80!red] plot[samples=100,domain=0:\a*360,smooth,variable=\t] ({\xt{\t}},{\yt{\t}});

% Define the value of t0 (current point)
% (i.e. the angular abscissa for the center of the rolling circle
% and draw the construction lines

\def\t0{40}
\draw[\clr!50!black] (\t0:\b-\a) circle (\a);
\draw[purple,fill] ({\xt{\t0}},{\yt{\t0}}) circle (2pt) -- (\t0:\b-\a) circle (1pt) node [midway, sloped, above] {\scriptsize $r=a$} -- (0,0) circle (1pt) node [midway, sloped, above] {\scriptsize $R=b-a$} node [below right] {$O$};

% Printing the values of a and b in the upper left corner
\node[\clr,rounded corners,fill=white,draw,text width=1cm,align=center] at (-\b,\b) {$a=\a$ $b=\b$};
\end{tikzpicture}

\end{document} • I had to edit my post to add this \a*360 to draw the complete curve in case of a not equal to 1. – SebGlav Feb 28 at 12:49