Semitransparent outside "clipped" region?

Question

To highlight part of a graph for a presentation that I'm writing, I would like to "circle" an area and make semitransparent everything outside that area. Anything inside that area should remain opaque. Is there a way to accomplish this inside TikZ?

As an example, in the following picture, I would like to make nodes a and c semitransparent, as well as the fraction of the edge from a to b that lies outside the circle.

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}
\begin{tikzpicture}[vertex/.style={draw, fill={#1}}]
  \node (a) [vertex=blue] {};
  \node (b) [vertex=blue, right=of a] {};
  \node (c) [vertex=green, right=of b] {};

  \draw (a) edge (b);

  \draw [red] (b) circle [radius=0.75];
\end{tikzpicture}
\end{document}

Of course, I could always accomplish this by manually making the desired pieces transparent, but for a complex picture it would be better to use some kind of clipping. I've read through the manual and vaguely understand clipping and transparency, but I don't see how to put those keys together to get what I want.

Answer 1

Two attempts here, the first is not ideal, but "fakes" transparency by overlaying a semi-transparent white rectangle and uses the spy library:

\documentclass[tikz,border=5]{standalone}
\usetikzlibrary{positioning,spy}
\tikzset{%
  white out/.style={
    preaction={%
      even odd rule, 
      fill=white, 
      fill opacity=0.75,
      insert path={
        % This covers the current picture.
        (current bounding box.south west) 
        rectangle
        (current bounding box.north east) 
      }
    }
  }
}

\begin{document}
\begin{tikzpicture}[vertex/.style={draw, fill={#1}}, spy using outlines={circle, magnification=1}]

  \node (a) [vertex=blue] {};
  \node (b) [vertex=blue, right=of a] {};
  \node (c) [vertex=green, right=of b] {};

  \draw (a) edge (b);

  \spy [size=0.75cm] on (b) in node [thin, red, white out] at (b);

\end{tikzpicture}
\end{document}

The second manages the semi-transparency but must capture the original picture inside a scope:

\documentclass[tikz,border=5]{standalone}
\usetikzlibrary{positioning}

\newbox\tikzcapturebox
\tikzset{capture scope/.style={
   execute at begin scope={\global\setbox\tikzcapturebox=\hbox\bgroup}, 
   execute at end scope={\egroup\copy\tikzcapturebox}
},
captured scope as path picture/.style={
  preaction={path picture={\pgfextra{\copy\tikzcapturebox}}}
}}


\begin{document}
\begin{tikzpicture}[vertex/.style={draw, fill={#1}}]

\node [text=gray!50, font=\footnotesize] at (1.25,0) {some text behind the figure};

\begin{scope}[transparency group, opacity=0.5, capture scope]
  \node (a) [vertex=blue] {};
  \node (b) [vertex=blue, right=of a] {};
  \node (c) [vertex=green, right=of b] {};
  \draw (a) edge (b);
\end{scope}

\draw [red, captured scope as path picture] (a) circle [radius=0.5];
\draw [blue, captured scope as path picture] ([xshift=-0.25cm]c) circle [radius=0.25];

\end{tikzpicture}
\end{document}

Answer 2

Here is a very simple solution (trick: a white background).

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}[vertex/.style={draw, fill={#1}}]
  \node (a) [vertex=blue] {};
  \node (b) [vertex=blue, right=of a] {};
  \node (c) [vertex=green, right=of b] {};
  \draw (a) edge (b);
  \draw [red] (b) circle [radius=0.75];

  \fill[white,fill opacity=.75]
  (current bounding box.south west) rectangle (current bounding box.north east)
  (b) circle[radius=0.75cm+.5\pgflinewidth];
\end{tikzpicture}
\end{document}

Answer 3

A layman's solution (whose doesn't like spies ;)):

\documentclass[tikz,border=5pt]{standalone}
\usetikzlibrary{positioning}

\begin{document}
\begin{tikzpicture}[vertex/.style={draw, fill={#1}}]
  \node (a) [vertex=blue] {};
  \node (b) [vertex=blue, right=of a] {};
  \node (c) [vertex=green, right=of b] {};

  \draw (a) edge (b);

  \draw [red] (b) circle [radius=0.75];
  \fill[white,fill opacity=0.75](current bounding box.south west) rectangle 
                                (current bounding box.north east);
  \begin{scope}
    \clip (b) circle [radius=0.75];
    \node [vertex=blue, right=of a] {};
    \draw (a) edge (b);
    \draw [red] (b) circle [radius=0.75];
  \end{scope}
\end{tikzpicture}
\end{document}

Answer 4

This looks like the same problem with inverse clipping and plus filling with some color with opacity less than 1. I stole Paul's nice solution from How can I invert a 'clip' selection within TikZ?. And to make it do more than clipping you need to reuse the path and hence the preaction.

EDIT: I confused myself. No clipping is necessary obviously (thanks Henry). Just filling is enough. Renamed it to invselect. Result stays the same.

\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{positioning}

\tikzset{
    vertex/.style={draw, fill={#1}},
    invselect/.style={insert path={{[reset cm]
      (-16383.99999pt,-16383.99999pt) rectangle (16383.99999pt,16383.99999pt)
    }}
  },
  hilite/.style={fill=#1,opacity=0.9,overlay,invselect}
}

\begin{document}
\begin{frame}

\begin{tikzpicture}
  \fill[yellow] (-3,-2) rectangle (3,2);
  \node (a) [vertex=blue] {};
  \node (b) [vertex=blue, right=of a] {};
  \node (c) [vertex=green, right=of b] {};

  \draw (a) edge (b);

  \draw [red] (b) circle [radius=0.75];

  \path<2>[hilite=white] (b) circle (0.75) ;
  \path<3>[hilite=brown] (a) circle (1);
\end{tikzpicture}

\end{frame}
\end{document}