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In a recent tutorial style answer for How to draw this diagram with TikZ?, I encountered some strange behavior with tikz. The code below is a slightly reduced version of the code from that answer. I attempted to reduce it even further, but that made the issue harder to see.

Here is what I get if I end the line below in black with -- cycle:

\draw (A1) -- (intersection-1) -- (A4) -- cycle;

enter image description here

Notice how the black lines go past the points at A1 and A4. But, by not using -- cycle and instead re-specifying the starting point as the ending point, I get better results for the A1:

\draw (A1) -- (intersection-1) -- (A4) -- (A1);

enter image description here

A way to fix this is below, but why is the behavior different if using --cycle or re-specifying the starting point as the end point?

References:

enter image description here

Code:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{intersections}

\begin{document}

\begin{tikzpicture}

% Define all the points:
    \coordinate (A1) at (0,4);
    \coordinate (A2) at (3,6);
    \coordinate (A3) at (5,5);
    \coordinate (A4) at (6,3);
    \coordinate (A5) at (4.5,1.5);
    \coordinate (An) at (0,0);

% Draw the lines
    \draw [ultra thick, blue, solid ] (An) -- (A1) -- (A2) -- (A3) -- (A4) -- (A5);

% Label the nodes:
    \foreach \Index/\Position in {1/left, 2/above, 3/above right, 4/right, 5/below right, n/left} {
        \node [\Position] at (A\Index) {$A_{\Index}$};
    }%

    \draw [gray, ultra thick] 
            [name path=A1A3] (A1) -- (A3)
            [name path=A2A4] (A2) -- (A4)
            [name path=A1A4] (A1) -- (A4);


    \fill [fill=red!20, draw=black, ultra thick, name intersections={of=A1A3 and A2A4}]
        (A1) -- (intersection-1) -- (A4) -- cycle;% produces wacky vertices
        %(A1) -- (intersection-1) -- (A4) -- (A1);% this fixes problem at (A)

%    % Following fix works
%    \fill [fill=red!20, draw=black, line join=round, ultra thick, name intersections={of=A1A3 and A2A4}]
%       (A1) -- (intersection-1) -- (A4) -- cycle;

\end{tikzpicture}
\end{document}
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1 Answer

up vote 7 down vote accepted

From PGF manual, I found the following statement.

enter image description here

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Well that was easy. I guess for the angles/line widths I have the left triangle from the manual works better. –  Peter Grill Oct 19 '12 at 4:54
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