Tikz randomly placed groups of objects

I would like to place several groups of three objects (two circle paths and one node) in a triangular area. I came up with the following code, which is based on a subdivision of the triangle in horizontal slices and a random positioning within the slice. (used inside a tikzpicture environment in beamer, and with all the related tikz libs [calc, decorations, decorations.text, decorations.pathmorphing, shapes.callouts, shapes.symbols] loaded and working):

\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{calc, decorations,decorations.text,decorations.pathmorphing,shapes.callouts,shapes.symbols}

\begin{document}
\begin{frame}{MWE}
\centering
\begin{tikzpicture}
\foreach \l in {1,2,...,10}
{
\coordinate (mycenterpoint) at (${(.5*\l+.1)*rand}*(1,0)+{rnd-\l-1}*(0,1)$);
\coordinate (myangle) at (rand*180:1.4pt);
%\node[starburst,fill=cyan,fill opacity=0.3] at (mycenterpoint) {};
\fill[red] (mycenterpoint) ++(myangle) circle (2pt);
\fill[green] (mycenterpoint) ++($-1*(myangle)$) circle (2pt);
}
\end{tikzpicture}
\end{frame}
\end{document}


which places as I expected the red and green circles, namely in a random position in the respective triangle slice. Unfortunately, if i uncomment the starburst line, something in the rand is jeopardized, and all the circles and nodes are placed on a "line" just as the rand and rnd in the first coordinate definition were set to -1 and 0 respectively.

does anyone have hints on how and where the rand function gets spoiled?

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Welcome to TeX.sx! –  lockstep Dec 11 '12 at 12:19
Could you turn the code snippet into a full minimal example document, so starting from \documentclass? That makes it much easier for others to try out the code to come up with a solution. –  Jake Dec 11 '12 at 12:51
done. there are still "locally" useless tikzlibraries which i need in other slides. –  davide Dec 11 '12 at 12:57
I believe the problem comes with the choice of the shape: if you use cloud, you won't have problems. Causes of such an issue are mysterious to me... –  Claudio Fiandrino Dec 11 '12 at 15:11
@ClaudioFiandrino : it looks like the starburst shape resets the seed internally somehow. I found out a way to overcom this, but it's not realy "clean" –  davide Dec 11 '12 at 16:05

I somehow found out that setting a loop-variable dependent random seed does the trick. Something like:

\pgfmathsetseed{\l}
\coordinate (mycenterpoint) at (${(.5*\l+.1)*rand}*(1,0)+{rnd-\l-1}*(0,1)$);
\coordinate (myangle) at (rand*180:1.4pt);
\node[starburst,fill=cyan,fill opacity=0.3] at (mycenterpoint) {};


in place of the first three lines in the \foreach loop in the example above.

The problem comes along with the starburst shape, as @ClaudioFiandrino suggested, which apparently resets the seed internally. I think there should be some cleaner way to achieve the same though I have no clue how to obtain it

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Do you want to keep the current seed? I think I know where it is reset but I don't know what you want to keep. –  percusse Dec 11 '12 at 16:11