# How to draw circles on a line and rotate them

Two questions about the following MWE:

\documentclass[12pt,a4paper]{scrartcl}  %%KOMA class
\setkomafont{sectioning}{\rmfamily\bfseries\boldmath}  %%

\usepackage{tikz}
\usetikzlibrary{rulercompass}
\usetikzlibrary{intersections,quotes,angles}
\usetikzlibrary{calc}

\begin{document}
\begin{tikzpicture}
\draw [color=black!5] (0,0) grid (14,10);
\draw (14,0) coordinate (a) node[right, below] {$x$}
-- (0,0) coordinate (b) node[left] {(0,0)}
-- (0,10) coordinate (c) node[left] {$y$};
\draw [->, ultra thick] (0,2) coordinate (ad) node[left] {(0,2)}  -- (30:15cm) coordinate (dd)  node[above] {$l$};
\draw (ad) -- (14,2)  coordinate (l);

\path (ad) -- (dd) coordinate[pos=0.355](c1) coordinate[pos=0.692](c2);
%circle A
\draw [fill=red!15] (c1) circle [radius=2.365];
%circle B
\draw [fill=green!15] (c2) circle [radius=2.365];
% centre  circles
\draw (ad) -- (c1) node{$\bullet$} -- (c2) node {$\bullet$}--(dd)
pic["$\alpha$", draw=red, <<-, angle eccentricity=1.1, angle radius=5cm]{angle=l--ad--dd}
pic["$\alpha$", draw=red, <<-, angle eccentricity=1.1, angle radius=9.8cm]{angle=l--ad--dd};
\end{tikzpicture}
\end{document}

1. How do I calculate the second coordinate[pos=0.692] in terms of the circle radius [radius=2.365]?
2. How do I draw the rotation of the oblique line l as to center the two circles on the y=2 line at the correct position?

The following almost fixes it (code to be cleaned):

 \coordinate
let
in
node (c1n) at (\n1,2) node (c2n) at (\n2,2);
\draw [fill=red!25] (c1n) circle [radius=2.365];
\draw [fill=green!25] (c2n) circle [radius=2.365];

\draw
let
in
(ad) -- (c1n) node{$\bullet$} -- (c2n) node{$\bullet$} {}--(l)
pic["$\alpha$", draw=red, <<-, angle eccentricity=1.05, angle radius=\n1]{angle=l--ad--dd}
pic["$\alpha$", draw=red, <<-, angle eccentricity=1.02, angle radius=\n2]{angle=l--ad--dd};
\draw (ad) -- (14,2)  coordinate (l);


• Probably I'm slow/ignorant, but I don't understand what you want to do in point 2. Can you try to explain more thoroughly? May 23, 2017 at 10:37
• @Torbjørn T. pls. see new picture (which I draw by guessing the radius values of the two pics, which is not good). I wish to know if I may instruct tikz to calculate how to draw the two circles after rotation of line l from its present position to y=2. Am i clear? May 23, 2017 at 10:49

You can use the features of the calc library for both problems. For 1., define the second coordinate as \coordinate (c2) at ($(c1)!2*2.365 cm!(dd)$);, i.e. the point that is twice the radius away from c1, towards dd.

For the second you can use the let syntax to calculate the distance from dd to each of the circle centers, and use that as the angle radius, and to define the center points of the two circles on the horizontal line.

I also added a second possible method for making the circles, by using nodes with an appropriate anchor set.

Small note: I wouldn't use \node {$\bullet$} in the circle centers, as that is positioned a bit wrong. I used a filled, circular node instead, another option would be e.g. \fill (c1) circle[radius=2pt];

\documentclass[12pt,a4paper]{scrartcl}  %%KOMA class
\setkomafont{sectioning}{\rmfamily\bfseries\boldmath}  %%

\usepackage{tikz}
\usetikzlibrary{rulercompass}
\usetikzlibrary{intersections,quotes,angles}
\usetikzlibrary{calc}
\tikzset{bullet/.style={circle,inner sep=0pt,minimum size=4pt,fill,draw}}
\begin{document}
\begin{tikzpicture}
\draw [color=black!5] (0,0) grid (14,10);
\draw (14,0) coordinate (a) node[right, below] {$x$}
-- (0,0) coordinate (b) node[left] {(0,0)}
-- (0,10) coordinate (c) node[left] {$y$};
\draw [->, ultra thick] (0,2) coordinate (ad) node[left] {(0,2)}  -- (30:15cm) coordinate (dd)  node[above] {$l$};

\path (ad) -- (dd) coordinate[pos=0.355](c1);
\coordinate (c2) at ($(c1)!2*2.365 cm!(dd)$);
%circle A
\draw [fill=red!15] (c1) circle [radius=2.365];
%circle B
\draw [fill=green!15] (c2) circle [radius=2.365];

\path
let
in
(ad) ++(\n1,0) coordinate (c1n)
(ad) ++(\n2,0) coordinate (c2n);

\draw [fill=red!15] (c1n) circle [radius=2.365];
%circle B
\draw [fill=green!15] (c2n) circle [radius=2.365];

\draw (ad) -- (14,2)  coordinate (l);
% draw angles
\draw
let
in
(ad) -- (c1) node[bullet]{} -- (c2) node[bullet] {}--(dd)
pic["$\alpha$", draw=red, <<-, angle eccentricity=1.05, angle radius=\n1]{angle=l--ad--dd}
pic["$\alpha$", draw=red, <<-, angle eccentricity=1.02, angle radius=\n2]{angle=l--ad--dd};
\end{tikzpicture}

\begin{tikzpicture}[
declare function={
alpha=25; % angle
L=14cm; % length of ray
circpos=0.5; % position of circle tangent along ray
},
mycirc/.style={
circle,
draw=black,
thin,
fill=#1,
minimum size=R*2,
outer sep=0pt,
label={[bullet]center:}
}
]

\draw [color=black!5] (0,0) grid (L,10);
\draw (L,0) coordinate (a) node[right, below] {$x$}
-- (0,0) coordinate (b) node[left] {(0,0)}
-- (0,10) coordinate (c) node[left] {$y$};

\draw [->, ultra thick] (0,2) coordinate (ad) node[left] {(0,2)}  -- ++(alpha:L) coordinate (dd)
node[above] {$l$}
node[mycirc=red!15,anchor=alpha,pos=circpos] (c1) {}
node[mycirc=green!15,anchor=alpha+180,pos=circpos] (c2) {};

% draw second set of circles
\path (ad) -- ++(0:L)
node[mycirc=red!15,anchor=0,pos=circpos] {}
node[mycirc=green!15,anchor=180,pos=circpos]  {};

\draw (ad) -- (L,2)  coordinate (l);

\draw (c1.alpha+180) -- (c2.alpha);

\draw
let
pic["$\alpha$", draw=red, <<-, angle eccentricity=1.05, angle radius=\n1]{angle=l--ad--dd}
pic["$\alpha$", draw=red, <<-, angle eccentricity=1.02, angle radius=\n2]{angle=l--ad--dd};\end{tikzpicture}

• Op only asks how to set the center of the rightmost green circle, relatively to the left pink one, but it seems they actually want both circles to be tangent in the middle of the (ad)--(dd) ray. In that case, both (c1) and (c2) should be computed in the same way relatively to the middle. May 23, 2017 at 11:01
• @marsupilam Maybe you're right. mario: is that correct, should the circles meet at the exact midpoint of the line l? May 23, 2017 at 11:06
• I am checking. Also, how do convert to .png: I use convert -density 300 0041.pdf -quality 100 0041a.png, but your picture looks better, why? May 23, 2017 at 11:06