Semitransparent graphics?

[Denis Girou]

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The following message is a courtesy copy of an article
that has been posted to comp.text.tex as well.


>>>>> "Nigel.Stanger" == nigel.stanger at stonebow.otago.ac.nz (Nigel Stanger) writes:

    Nigel.Stanger > I have a bit of an interesting problem with a diagram I'm working on. It's
    Nigel.Stanger > effectively a graph (in the mathematical sense) consisting of a collection
    Nigel.Stanger > of ovalnodes (I'm using pstricks btw), some nested within others, with
    Nigel.Stanger > arcs connecting the nodes. Some of the connectors connect to nested nodes,
    Nigel.Stanger > which means I have to do a \ncarc[border=2pt] to make a nice little gap.
    Nigel.Stanger > This is all fine.
    Nigel.Stanger > 
    Nigel.Stanger > The problem I've got is that I want to highlight a subset of the graph by
    Nigel.Stanger > surrounding the appropriate nodes with a filled gray blob shape. I can
    Nigel.Stanger > produce the blob shape fine, but (a) I have to draw the blob first so it
    Nigel.Stanger > doesn't wipe out the stuff it's highlighting, and (b) the [border=2pt]
    Nigel.Stanger > options on the \ncarc commands means that they leave a big white streak
    Nigel.Stanger > through the middle of the blob, which looks bad. Setting the bordercolor
    Nigel.Stanger > is no use, beacuse some of the connectors pass outside the blob area, so
    Nigel.Stanger > I'd get gray streaks instead of white ones, which would look even worse.
    Nigel.Stanger > (If you can't visualise what I'm trying to do here, I can sent you an
    Nigel.Stanger > example.)
    Nigel.Stanger > 
    Nigel.Stanger > So, what I'm wondering is if it's possible to draw the blob over the top
    Nigel.Stanger > of the graph in a semitransparent manner; effectively some sort of
    Nigel.Stanger > additive drawing mode or something similar. At the moment the blob is an
    Nigel.Stanger > EPS, but that's only because I constructed it using a drawing tool ---
    Nigel.Stanger > building it manually in pstricks/whatever is OK if I have to.
    Nigel.Stanger > 
    Nigel.Stanger > I couldn't find anything obvious in pstricks and I don't really have the
    Nigel.Stanger > time to go on a massive search for something to do this --- I've got a
    Nigel.Stanger > deadline approaching, so if I can't find anything soon, I'll just `unfill'
    Nigel.Stanger > the blob and use just the frame, which will work but not look quite as
    Nigel.Stanger > good.

  I did not (yet) receive the example but I believe to understand the
question. The desired effect can be obtained by usage of the clipping
mechanism of PSTricks.

  I think it is not too difficult to understand and to adapt but it can be
considered as rather "advanced" usage. For description of the clipping
mechanism see pages 54-55 of the documentation. Another problem to solve is
that the highlighted area must be drawn exactly at the same place, but the
psmatrix entries change the horizontal and vertical positions. I solve here
this problem using a node defined at the beginning of the psmatrix environment.

  It also require that the highlighted area is drawn with PSTricks, but I do
not see why you could not been able to do that.

\documentclass{article}

\usepackage{pstcol} % To use the standard "color" package with PSTricks
\usepackage{pst-node}

\definecolor{LightGray}{gray}{.93}

% The area to highlight
\newcommand{\HighlightedArea}[1][linestyle=solid]{%
\psccurve[#1](-1.5,-5)(3,-2.5)(10,-11)(3,-9)}

\pagestyle{empty}

\begin{document}

\begin{center}
  \Huge
  \SpecialCoor
  \begin{psmatrix}[rowsep=4,colsep=4,mnode=oval,
                   fillstyle=solid,fillcolor=white]
    \pnode{Origin} % To be able to redraw later the highlighted area
                   % at the same place
    % The area to highlight (starting point at (0,0))
    \rput(Origin){%
      \pscustom[linewidth=2pt,fillstyle=solid,fillcolor=LightGray]{%
        \HighlightedArea}}
    One   & Two \\
    Three & [colsep=6] Four \\
    Five  & Six
  \end{psmatrix}
  % Connections
  \psset{fillstyle=none,arrows=->,arrowscale=2,border=2mm,nodesep=3pt}
  \ncarc{1,1}{2,2}
  \ncarc{1,1}{3,2}
  \ncarc{3,1}{1,2}
  \ncarc[arcangle=-60]{3,2}{1,2}
  % We redraw the highlighted area as clipping region
  \begin{psclip}{\rput(Origin){\HighlightedArea[linewidth=2pt,border=0]}}
    % We redraw the line connections, but only the parts inside
    % the highlighted area, with a bordercolor equal to the background one
%    \psset{bordercolor=red} % To clearly see the effect...
    \psset{bordercolor=LightGray}
    \ncarc{1,1}{3,2}
    \ncarc{3,1}{1,2}
    \ncarc[arcangle=-60]{3,2}{1,2}
    % We redraw the highlighted area one time more, here to have everywhere
    % for it the correct width of the line...
    \rput(Origin){\HighlightedArea[linewidth=2pt,border=0]}
 \end{psclip}
\end{center}

\end{document}

Denis Girou
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Centre National de la Recherche Scientifique                              |
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