TikZ: Draw angle with label between lines

Question

How do I draw an angle with a label between two lines when the lines are not necessarily drawn in the same \draw call?

I need to draw an angle with a label, theta, between the y-axis and the pendulum string (see picture below).

Other suggestions/improvements of my code/diagram are welcome.

Current code:

\documentclass[tikz,border=10pt]{standalone}
\usetikzlibrary{calc,patterns}
\begin{document}
\begin{tikzpicture}
\coordinate (origo) at (0,0);
\coordinate (pivot) at (1,5);

% draw axes
\fill[black] (origo) circle (0.05);
\draw[thick,gray,->] (origo) -- ++(4,0) node[black,right] {$x$};
\draw[thick,gray,->] (origo) -- ++(0,-4) node[black,below] {$y$};

% draw roof
\fill[pattern = north east lines] ($ (origo) + (-1,0) $) rectangle ($ (origo) + (1,0.5) $);
\draw[thick] ($ (origo) + (-1,0) $) -- ($ (origo) + (1,0) $);

\draw[thick] (origo) -- ++(300:3) coordinate (bob);
\fill (bob) circle (0.2);
\end{tikzpicture}
\end{document}

Current output:

enter image description here

For illustration, I want something like this in my diagram:

enter image description here

cfr · Accepted Answer · 2014-12-22T03:03:58.357

44

You can use the angles library which defines a pic for this purpose. The quotes library is used for ease of labelling.

\documentclass[tikz,border=10pt]{standalone}
\usetikzlibrary{calc,patterns,angles,quotes}
\begin{document}
  \begin{tikzpicture}
    \coordinate (origo) at (0,0);
    \coordinate (pivot) at (1,5);

    % draw axes
    \fill[black] (origo) circle (0.05);
    \draw[thick,gray,->] (origo) -- ++(4,0) node[black,right] {$x$};
    \draw[thick,gray,->] (origo) -- ++(0,-4) node (mary) [black,below] {$y$};

    % draw roof
    \fill[pattern = north east lines] ($ (origo) + (-1,0) $) rectangle ($ (origo) + (1,0.5) $);
    \draw[thick] ($ (origo) + (-1,0) $) -- ($ (origo) + (1,0) $);

    \draw[thick] (origo) -- ++(300:3) coordinate (bob);
    \fill (bob) circle (0.2);

    \pic [draw, ->, "$\theta$", angle eccentricity=1.5] {angle = mary--origo--bob};
  \end{tikzpicture}
\end{document}

marked angle

If you want the angle in red with two-way arrows, just modify the last line:

\pic [draw=red, <->, "$\theta$", angle eccentricity=1.5] {angle = mary--origo--bob};

modified marked angle

EDIT (Respond to query in comments)

To change the colour of the label as well, just set the text key:

\pic [draw=red, text=blue, <->, "$\theta$", angle eccentricity=1.5] {angle = mary--origo--bob};

modified modified marked angle

edited Dec 22 '14 at 03:03

answered Dec 22 '14 at 02:36

cfr

198,882

2

May I ask why you called the node mary? :-) – Dec 22 '14 at 02:54
@teelf Why did you call the other one bob? I don't really know, is the answer. For some reason 'Bob' -> 'Other name?' got me 'Mary'. Why? – cfr Dec 22 '14 at 03:01
1

@teelf Didn't you ask how to change the colour of the marker? It seems to have disappeared but, just in case, see edit above ;). – cfr Dec 22 '14 at 03:04
Ha, It's a bob in a pendulum. – Dec 22 '14 at 03:06
@teelf That makes sense ;). I wondered about that as I answered your question - but the node was already mary by then, I'm afraid. If you'd called it weight, I'd have picked something less anthropomorphic! – cfr Dec 22 '14 at 03:09
@cfr Is there a way to finely customize the position of the label? Using above, right, etc. works, but e.g. above=5 doesn't increase the distance. – Janosh Jan 18 '17 at 10:22
@Casimir The label of what? Those keys aren't used for labelling the angle, so I'm not sure what you are trying to do. In general, see the positioning library which allows things like above=<dimension> of <location> etc. – cfr Jan 19 '17 at 00:19
@cfr I meant the position of the angle label, i.e. theta in your example. – Janosh Jan 19 '17 at 07:37
1

@Casimir Try changing angle eccentricity. – cfr Jan 20 '17 at 01:21
2

I tried out your solution and it works great. For other users i have two tips: 1. If you use angle=(0,0)--(1,0)--(0,-1) with points and not coordinates your latex code will throw an error. You need to add a coordinate for each point of your angle. 2. If you wish to fill the angle with a certain color you can use the fill option in the same style as in \draw. – michip96 Jun 11 '18 at 13:40
I would like to add one more note: Unlike with notation for lines eg \draw (mary) -- (bob); I must not have spaces between the coordinate names. So \pic [draw] {angle = mary -- origo -- bob}; did not work for me, but only \pic [draw] {angle = mary--origo--bob}; did. – BadAtLaTeX Jan 22 '19 at 11:13

score 4 · Answer 2 · edited Apr 13 '17 at 12:35

Just for comparison, here's a version in plain Metapost with some explanatory comments.

a swinging pendulum

prologues := 3;
outputtemplate := "%j%c.eps";

beginfig(1);

% first define the unit to use
u = 1cm;

% now define the paths and points
% next define the axes, the bob position, and the path of the pendulum
path xx, yy, pendulum; pair bob;
xx = (left -- 3 right) scaled u;
yy = (origin -- 4 down) scaled u;

theta = 24;
bob = 3 down scaled u rotated theta;
pendulum = origin -- bob;

% also define an angle mark, rotated to start on the yy axis and go as far as the pendulum
% this assumes theta is positive by the way
path angle_mark; angle_mark = fullcircle rotated 270 scaled 3/2u cutafter pendulum;

% now we can get on with drawing

% first do the striped fill for the roof area
path roof_area; roof_area = unitsquare shifted 1/2 left xscaled 2u yscaled 1/2u;
picture roof_fill;   
roof_fill = image(for x=-2u step 1/8u until 2u: draw (left--right) scaled 2u rotated 45 shifted (x,0); endfor);
clip roof_fill to roof_area;
draw roof_fill;

% now draw the axes in grey
drawarrow xx withcolor .5 white; label.rt (btex $x$ etex, point 1 of xx);
drawarrow yy withcolor .5 white; label.bot(btex $y$ etex, point 1 of yy);
% and the bottom of the roof area in black
draw subpath(0,1) of roof_area;
% draw the pendulum, the pivot at the origin, and the bob on the end of the pendulum
draw pendulum;
fill fullcircle scaled dotlabeldiam;
fill fullcircle scaled 1/3u shifted bob;

% now the angle mark - note it will lie on top of the axis and the pendulum
% if this bothers you, draw it first
ahlength := 2.5; 
drawarrow angle_mark withcolor .54 red;

% finally do the label, attached to the angle_mark
label(btex $\theta$ etex, point 1/2 of angle_mark + (1,-6)) withcolor .67 blue;

% and some optional go-faster stripes for the bob
for i=2 step 2 until 10:
  draw quartercircle scaled 1/3u rotated (135+theta-i) shifted (bob rotated -i) withcolor (i/10)*white;
endfor
endfig;
end.

TikZ: Draw angle with label between lines

2 Answers2

EDIT (Respond to query in comments)

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