tikz-pgf
It would be convenient sometimes to be able to draw an arc in tikz by specifying
i.e., the "natural" way an arc is defined, instead of the "first point of the arc".
Is there a way to do it?
Here is an answer, using the technique described here, with more detail,
\pgfmathsetmacro{\ex}{0}
\pgfmathsetmacro{\ey}{1}
\draw (\ex,\ey) -- ++(-15:1)
(\ex,\ey) -- ++(15:1);
\draw (\ex,\ey) ++(45:.8) arc (45:-45:.8);
The secret to this is in the last line,
\draw (\ex,\ey) ++(45:.8)
Jumps the draw cursor to "the 45 degree position on a circle of radius 0.8", without drawing anything (achieved by using only ++ and not any -- in the command)
Then, from there, we draw an arc
arc (45:-45:.8);
from 45 degrees to -45 degrees, of what would be a circle of radius .8.
Pretty roundabout way to do this, but still it works fine.
My finished eye:
\begin{tikzpicture}
%eye
\pgfmathsetmacro{\eyeSize}{1}
\pgfmathsetmacro{\ex}{0}
\pgfmathsetmacro{\ey}{1}
\pgfmathsetmacro{\eRot}{-10}
\pgfmathsetmacro{\eAp}{-55}
\draw[rotate around={\eRot:(\ex,\ey)}] (\ex,\ey) -- ++(-.5*\eAp:\eyeSize)
(\ex,\ey) -- ++(.5*\eAp:\eyeSize);
\draw (\ex,\ey) ++(\eRot+\eAp:.75*\eyeSize) arc (\eRot+\eAp:\eRot-\eAp:.75*\eyeSize);
% IRIS
\draw[fill=gray] (\ex,\ey) ++(\eRot+\eAp/3:.75*\eyeSize) % start point
arc (\eRot+180-\eAp:\eRot+180+\eAp:.28*\eyeSize);
%PUPIL, a filled arc
\draw[fill=black] (\ex,\ey) ++(\eRot+\eAp/3:.75*\eyeSize) % start point
arc (\eRot+\eAp/3:\eRot-\eAp/3:.75*\eyeSize);
\end{tikzpicture}
Only part of what was required. But it also shows how a key with the required syntax could be programmed:
\documentclass[tikz,border=5]{standalone}
\tikzset{%
insert arc/.style args={#1:#2:#3and#4 with center #5}{
insert path={
\pgfextra{%
\pgfpointxy{#3}{#4}%
\pgfgetlastxy\arcrx\arcry%
\pgfcoordinate{#5}{%
\pgfpoint{\csname tikz@lastx\endcsname+\arcrx*cos(#1+180)}%
{\csname tikz@lasty\endcsname+\arcry*sin(#1+180)}}
}
arc (#1:#2:#3 and #4)
}
}
}
\begin{document}
\begin{tikzpicture}[rotate=40]
\draw [very thick] (0,0) [insert arc={-25:45:3 and 2 with center X}];
\draw [dashed, red] (X) ellipse [x radius=3, y radius=2];
\end{tikzpicture}
\end{document}
And a slightly more comprehensive example:
\begin{tikzpicture}
\draw [very thick] (0,0) -- (0,1)
[insert arc={270:135:3 and 2 with center X}] -- (-4,4)
[insert arc={ 10:-90:1 and 3 with center Y}]
[insert arc={180:360:4 and 2 with center Z}] -- cycle;
\draw [dashed, red] (X) ellipse [x radius=3, y radius=2];
\draw [dashed, green] (Y) ellipse [x radius=1, y radius=3];
\draw [dashed, blue] (Z) ellipse [x radius=4, y radius=2];
\end{tikzpicture}
Best Answer
You can use the parametrization
where
ris the radius of the circle and(a,b)are the coordinates of its center. In Tikz this can be implemented as follows:which produces
Depending on your application, you might like to do this using the
pgfplotspackage.Also, in case this is used frequently, consider defining a custom command
\centerarcas suggested in a comment by Tom Bombadil (this requires\usetikzlibrary{calc}):Then use it by invoking