Highlighting intersections

Question

I would like to highlight the region C in the Euler diagram below. I tried to play around with fill opacity command, but I was not able to highlight C alone. Should I use the intersections libraries to figure out the coordinates of intersection of C and B and proceed? Can someone help me with this?

enter image description here

\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\pagestyle{empty}
\begin{tikzpicture}
        \draw[fill=green, draw = black,fill opacity=0.5] (-1.5,0) circle (3);
        \draw[fill=blue!30!white, draw = black,fill opacity=0.5] (1.5,0) circle (3);
        \node at (-2,1) (B) {\large\textbf{B}};
        \node at (2,1) (C) {\large\textbf{C}};
        \node at (0,0) (D) {\large\textbf{D}};
\end{tikzpicture}
\end{document}

What exactly do you mean by highlight the region C? How exactly do you want to highlight this region? — Gonzalo Medina, Jun 03 '13 at 17:57
have you seen Tikz: joining points on a circle it looks pretty close/relevant :) — cmhughes, Jun 03 '13 at 17:58
@GonzaloMedina I want the opacity of region $B$ and $D$ to be say $0.1$ and the opacity of region $C$ to close to $1$. — , Jun 03 '13 at 18:00
Andrew Stacey posted a very nice solution for drawing three-circle Venn diagrams at How to generate all possible Venn diagrams (with the case below) efficiently?. It's quite simple to reduce it to two circles. — Jake, Jun 03 '13 at 18:07

score 8 · Answer 1 · edited Apr 13 '17 at 12:34

I used Paul Gaborit's invclip style from his answer to How can I invert a 'clip' selection within TikZ?:

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

\tikzset{invclip/.style={clip,insert path={
      (-16383.99999pt,-16383.99999pt) rectangle (16383.99999pt,16383.99999pt)
    }}}


\begin{document}

\begin{tikzpicture}
% The intersection region D
\begin{scope}
\clip (1.5,0) circle (3cm);
\clip (-1.5,0) circle (3cm);
\fill[blue,fill opacity=0.2] (-1.5,0) circle [radius=3cm];
\end{scope}
% The B+D region
\begin{scope}
\clip (-1.5,0) circle (3cm);
\fill[green,fill opacity=0.2] (-1.5,0) circle [radius=3cm];
\end{scope}
\node at (-2,1) (B) {\large\textbf{B}};
\node at (0,0) (D) {\large\textbf{D}};
% Region C
\begin{pgfinterruptboundingbox} % useful to avoid the rectangle in the bounding box
    \path[invclip]
    (-1.5,0) circle (3cm);
  \end{pgfinterruptboundingbox} 
\fill[blue,fill opacity=0.9] (1.5,0) circle [radius=3cm];
\node at (2,1) (C) {\large\textbf{C}};
\end{tikzpicture}

\end{document}

enter image description here

score 6 · Answer 2 · answered Jun 03 '13 at 18:35

You can do it by using two clip commands:

Code

\documentclass[tikz,border=2mm]{standalone}
% declare a new layer
\pgfdeclarelayer{background} 
% give the order of layers from back to front
\pgfsetlayers{background,main}

\begin{document}
\begin{tikzpicture}
    \draw[fill=green, draw = black,fill opacity=0.2] (-1.5,0) circle (3);
    \draw[fill=blue!50, draw = black,fill opacity=0.2] (1.5,0) circle (3);
    \node at (-2,1) (B) {\large\textbf{B}};
    \node at (2,1) (C) {\large\textbf{C}};
    \node at (0,0) (D) {\large\textbf{D}};
    % begin a background layer to avoid painting over stuff
    \begin{pgfonlayer}{background}
        % scope to keep clipping local
        \begin{scope}
            % clip a large enough rectangle, then cut out the left circle
            \clip (-4.5,-3) rectangle (4.5,3) (-1.5,0) circle (3);
            % clips do stack; so clip only the left circle from the remaining region --> region C remains
            \clip (1.5,0) circle (3);
            % fill right circle, due to clipping only region C
            \fill[blue!50,opacity=1] (-5,-4) rectangle (5,4);
        \end{scope}
    \end{pgfonlayer}    
\end{tikzpicture}
\end{document}

Output

enter image description here

Highlighting intersections

2 Answers2

Code

Output