Sstrhsrtj

thanks for taking the time.
I have the problem, that I want to get normal vectors on a smooth plot, furthermore I have lines intersecting said plot, but they also need to end in the graph. My approach so far is using the decorations for the normal vectors. I need to create the next normal vector at the intersection of $A$ with the boundary, and I'd like to cut $A$ short at exactly the intersection.

  begin{tikzpicture}[

decoration={

markings,

mark=at position 0.4 with {draw[->] (0,0)--(0,1);},

mark=at position 0.4 with {draw[->] (0,0)--(2,-2) node[below]{A};},

mark=at position 0.4 with {draw[<-] (0,0)--(-.8,-.8);}

}

]

draw[postaction={decorate}] plot [smooth cycle] coordinates {(0,0) (1,1) (3,1) (3,0) (2,-1)};

end{tikzpicture}

I hope my problem has a solution.
Thanks in advance.
Fabian

Welcome to TeX.SE! You can add arbitrarily many commands to one draw command.

documentclass[tikz,border=3.14mm]{standalone}

usetikzlibrary{decorations.markings}

begin{document}

begin{tikzpicture}[

decoration={

markings,

mark=at position 0.4 with {draw[->] (0,0)--(0,1);

draw[->] (0,0)--(2,-2) node[below]{A};

draw[<-] (0,0)--(-.8,-.8);}

}

]

draw[postaction={decorate}] plot [smooth cycle] coordinates {(0,0) (1,1) (3,1) (3,0) (2,-1)};

end{tikzpicture}

end{document}

What is more more, you can name the coordinates used in markings, and access it from outside.

documentclass[tikz,border=3.14mm]{standalone}

usetikzlibrary{decorations.markings}

begin{document}

begin{tikzpicture}[

decoration={

markings,

mark=at position 0.4 with {draw[->] (0,0)coordinate(X) --(0,1);},

}]

draw[postaction={decorate}] plot [smooth cycle] coordinates {(0,0) (1,1) (3,1) (3,0) (2,-1)};

draw[-latex] (1.5,-1.5) -- (X);

draw[-latex] (X) -- (2,2);

end{tikzpicture}

end{document}