Graphics, Figures & Tables ⇒ xy coordinates for a point

[topsquark]

I have a point, I0, that I have calculated through the intersection of two curves.
\path[name intersections={of=c0 and c1,by=I0}];

I can use the through library to put a circle through this point:
\node[draw,circle through={(I0)}] at (O) {};

This works fine. However, I find I'm in need of the radius of this circle. So I did something different. I found the xy coordinates of the point I0:
\path[transform canvas] (I0); \pgfgetlastxy{\x}{\y};

The problem is, the point (\x,\y) is not the same as the point I0. (The inner circle should be the same as the outer circle going through the intersection point.) What am I doing wrong?

Thanks!

-Dan

Addendum: Oh! When I did the square root there was some kind of size problem. I had to do a scaling. That's a whole other question.

Code: Select all

Code, edit and compile here:

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\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{intersections,through}
\begin{document}
\newcommand*{\myangle}{10}
\begin{tikzpicture}[scale=0.6]
     \coordinate (O) at (0,0);
          \path[name path=c0] (120:4) arc(-167.2:-219.2:10) arc(39.2:-12.8:10);
          \draw (120:4) arc(-167.2:-219.2:10) arc(39.2:-12.8:10);
          \path[name path=c1] (120+\myangle:4) arc(-167.2+\myangle:-219.2+\myangle:10) arc(39.2+\myangle:-12.8+\myangle:10);
          \draw (120+\myangle:4) arc({-167.2+\myangle}:-219.2+\myangle:10) arc(39.2+\myangle:-12.8+\myangle:10);
     \path[name intersections={of=c0 and c1,by=I0}];
     \path[transform canvas] (I0); \pgfgetlastxy{\x}{\y};
     \pgfmathsetmacro\radiusI{10*sqrt((\x/10)^2+(\y/10)^2)};
     \draw (O) circle(\radiusI pt);
     \node[draw,circle through={(I0)}] at (O) {};
\end{tikzpicture}
\end{document}
 
 
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Run LaTeX here

[rais]

Here, \pgfgetlastxy{\x}{\y} returns x,y for point I0 after scaling, that's why your calculated radius is only 60% of what it should have been:

Code: Select all

Code, edit and compile here:

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\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{intersections,through}
\newlength\myx
\newlength\myy
\begin{document}
\newcommand*{\myangle}{10}
\begin{tikzpicture}[scale=0.6]
  \coordinate (O) at (0,0);
  \draw[name path=c0] (120:4) arc(-167.2:-219.2:10) arc(39.2:-12.8:10);
  \draw[name path=c1] (120+\myangle:4) arc(-167.2+\myangle:-219.2+\myangle:10) arc(39.2+\myangle:-12.8+\myangle:10);
  \path[name intersections={of=c0 and c1,by=I0}];
  \path[transform canvas] (I0); \pgfgetlastxy{\x}{\y}%;
  \pgfmathsetmacro\radiusI{16.666667*sqrt((\x/10)^2+(\y/10)^2)}% 10/scale ...
  \draw (O) circle(\radiusI pt);
  \setlength\myx{\x}%
  \setlength\myy{\y}%
  \setlength\myx{1.666667\myx}% \myx / scale
  \setlength\myy{1.666667\myy}%
  \draw[black!30] (O) ++(-0.5,-0.5) -- ++(1,1) ++(-1,0) -- ++(1,-1);
  \draw[green] (I0) -- ++(-\myx, 0) -- ++(0, -\myy) --cycle;
  \node[draw,circle through={(I0)}] at (O) {};
\end{tikzpicture}
\end{document}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Run LaTeX here

BTW: neither \pgfgetlastxy nor \pgfmathsetmacro require a trailing `;' character; in fact, you should get a message like

Code: Select all

1
Missing character: There is no ; in font nullfont!
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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for both cases.

KR
Rainer

[topsquark]

Here, \pgfgetlastxy{\x}{\y} returns x,y for point I0 after scaling, that's why your calculated radius is only 60% of what it should have been:

Code: Select all

Code, edit and compile here:

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\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{intersections,through}
\newlength\myx
\newlength\myy
\begin{document}
\newcommand*{\myangle}{10}
\begin{tikzpicture}[scale=0.6]
  \coordinate (O) at (0,0);
  \draw[name path=c0] (120:4) arc(-167.2:-219.2:10) arc(39.2:-12.8:10);
  \draw[name path=c1] (120+\myangle:4) arc(-167.2+\myangle:-219.2+\myangle:10) arc(39.2+\myangle:-12.8+\myangle:10);
  \path[name intersections={of=c0 and c1,by=I0}];
  \path[transform canvas] (I0); \pgfgetlastxy{\x}{\y}%;
  \pgfmathsetmacro\radiusI{16.666667*sqrt((\x/10)^2+(\y/10)^2)}% 10/scale ...
  \draw (O) circle(\radiusI pt);
  \setlength\myx{\x}%
  \setlength\myy{\y}%
  \setlength\myx{1.666667\myx}% \myx / scale
  \setlength\myy{1.666667\myy}%
  \draw[black!30] (O) ++(-0.5,-0.5) -- ++(1,1) ++(-1,0) -- ++(1,-1);
  \draw[green] (I0) -- ++(-\myx, 0) -- ++(0, -\myy) --cycle;
  \node[draw,circle through={(I0)}] at (O) {};
\end{tikzpicture}
\end{document}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Run LaTeX here

BTW: neither \pgfgetlastxy nor \pgfmathsetmacro require a trailing `;' character; in fact, you should get a message like

Code: Select all

1
Missing character: There is no ; in font nullfont!
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

for both cases.

KR
Rainer

Actually, I figured out the scaling problem last night and was just coming in to report on it. Thank you!

As to the ; it was in the examples I found. I'll have to look those up again.

Thank you for the comments!

-Dan