[Tex/LaTex] Draw double line with empty interior in TikZ

tikz-pgf

I would like to draw a line around given subsets of nodes in Tikz. I have five nodes in total, and two overlapping subsets (red and black) of three nodes sharing one node (cf the attached picture). The issue is that the double lines have a white interior, therefore the red line hides the black one at the overlapping node.

\documentclass[12pt]{article}
\usepackage{tikz}

\begin{document}
\begin{figure}[!htbp]
\centering
\begin{tikzpicture}[scale=4,
    vertex/.style={draw,circle,minimum size=0.75cm,inner sep=0pt},
    arc/.style={draw=blue!50,thick,->},
    arc label/.style={fill=white,font=\tiny,inner sep=1pt},
    loop arc/.style={min distance=2mm}
    ]
% Subset 1
\draw[double=none, double distance=60pt, line join=round, line cap=round, draw=black] (1,0)--(0.3090,0.9511)--(-0.8090,0.5878);
% Subset 2
\draw[double=none, double distance=60pt, line join=round, line cap=round, draw=red] (-0.8090,0.5878)--(-0.8090,-0.5878)--(0.3090,-0.9511);
\end{tikzpicture}
\end{figure}

\end{document}

I thought that the option double=none would solve the issue, but it seems that I am missing something, any ideas?

Thank you in advance for your help.

enter image description here

Best Answer

By playing around with transparency group one can create the desired effect:

\documentclass[12pt]{article}
\usepackage{tikz}

\begin{document}
\begin{figure}[!htbp]
\centering
\begin{tikzpicture}[scale=4,
    vertex/.style={draw,circle,minimum size=0.75cm,inner sep=0pt},
    arc/.style={draw=blue!50,thick,->},
    arc label/.style={fill=white,font=\tiny,inner sep=1pt},
    loop arc/.style={min distance=2mm}
    ]

\pgfsetblendmode{multiply}    

\begin{scope}[transparency group]    
% Subset 1
\draw[double=none, double distance=60pt, line join=round, line cap=round, draw=black] (1,0)--(0.3090,0.9511)--(-0.8090,0.5878);
\end{scope}

\begin{scope}[transparency group]
% Subset 2
\draw[double=none, double distance=60pt, line join=round, line cap=round, draw=red] (-0.8090,0.5878)--(-0.8090,-0.5878)--(0.3090,-0.9511);
\end{scope}
\end{tikzpicture}
\end{figure}

\end{document}

enter image description here

\documentclass[12pt]{article}
\usepackage{tikz}

\begin{document}
\begin{figure}[!htbp]
\centering
\begin{tikzpicture}[scale=4,
    vertex/.style={draw,circle,minimum size=0.75cm,inner sep=0pt},
    arc/.style={draw=blue!50,thick,->},
    arc label/.style={fill=white,font=\tiny,inner sep=1pt},
    loop arc/.style={min distance=2mm}
    ]

\pgfsetblendmode{multiply}

\begin{scope}[transparency group]
\draw[line width=60pt, line join=round, line cap=round, draw=red] (-0.8090,0.5878)--(-0.8090,-0.5878)--(0.3090,-0.9511);
\draw[line width=40pt, line join=round, line cap=round, draw=white] (-0.8090,0.5878)--(-0.8090,-0.5878)--(0.3090,-0.9511);
\draw[line width=60pt, line join=round, line cap=round, draw=black] (1,0)--(0.3090,0.9511)--(-0.8090,0.5878);
\draw[line width=40pt, line join=round, line cap=round, draw=white] (1,0)--(0.3090,0.9511)--(-0.8090,0.5878);
\end{scope}

\begin{scope}[transparency group]
\draw[line width=60pt, line join=round, line cap=round, draw=black] (1,0)--(0.3090,0.9511)--(-0.8090,0.5878);
\draw[line width=40pt, line join=round, line cap=round, draw=white] (1,0)--(0.3090,0.9511)--(-0.8090,0.5878);
\draw[line width=60pt, line join=round, line cap=round, draw=red] (-0.8090,0.5878)--(-0.8090,-0.5878)--(0.3090,-0.9511);
\draw[line width=40pt, line join=round, line cap=round, draw=white] (-0.8090,0.5878)--(-0.8090,-0.5878)--(0.3090,-0.9511);
\end{scope}

\end{tikzpicture}
\end{figure}

\end{document}

enter image description here

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