Transparent shadows under boxes.

Hi everyone.

I just wanted to ask if anybody tried to do what I've been just trying to
achieve. When you use certain styles in beamer, the boxes have a "shadow". This
shadow is a gradient between gray and the background color (if I recall right),
so it does not display correctly when placed over, say, an image.

I've been thinking if it's at all possible to have a fully semi-transparent
shadows that would actually work over images etc. This seems to be a problem
reaching all the way to PDF specification since, according to what I've read,
the gradients cannot be combined with transparency (there are no gradients on
the 'alpha' layer... don't know if I'm clear). Anyway, I came up with some sort
of a solution -- attached. If anybody did (or can) come up with a nicer
implementation, it'd be great to share this knowledge. Thanks!

Dawid




\documentclass[a4paper]{article}

\usepackage[utf8]{inputenc}
\usepackage[OT1]{fontenc}
\usepackage[absolute,overlay]{textpos}
\usepackage{graphicx}
\usepackage[bookmarks=false,pdffitwindow]{hyperref}
\usepackage{tikz}
\usepackage{xcolor}
\usepackage{calc}



%
% Boxed environment with semi-transparent shadow.
%
\newlength{\boxw}
\newlength{\boxh}
\newlength{\shadowsize}
\newlength{\boxroundness}
\newlength{\tmpa}
\newsavebox{\shadowblockbox}

\setlength{\shadowsize}{6pt}
\setlength{\boxroundness}{3pt}

\newenvironment{shadowblock}[1]%
{\begin{lrbox}{\shadowblockbox}\begin{minipage}{#1}}%
{\end{minipage}\end{lrbox}%
\settowidth{\boxw}{\usebox{\shadowblockbox}}%
\settodepth{\tmpa}{\usebox{\shadowblockbox}}%
\settoheight{\boxh}{\usebox{\shadowblockbox}}%
\addtolength{\boxh}{\tmpa}%
\begin{tikzpicture}
    \addtolength{\boxw}{\boxroundness * 2}
    \addtolength{\boxh}{\boxroundness * 2}

    \foreach \x in {0,.05,...,1}
    {
        \setlength{\tmpa}{\shadowsize * \real{\x}}
        \fill[xshift=\shadowsize - 1pt,yshift=-\shadowsize + 1pt,black,opacity=.04,rounded corners=\boxroundness]
            (\tmpa, \tmpa) rectangle +(\boxw - \tmpa - \tmpa, \boxh - \tmpa - \tmpa);
    }

    \filldraw[fill=yellow!50, draw=black!50, rounded corners=\boxroundness] (0, 0) rectangle (\boxw, \boxh);
    \draw node[xshift=\boxroundness,yshift=\boxroundness,inner sep=0pt,outer sep=0pt,anchor=south west] (0,0) {\usebox{\shadowblockbox}};
\end{tikzpicture}}



\begin{document}

%
% We prepare a transparent box first.
%
\newsavebox{\mybox}
\begin{lrbox}{\mybox}
\begin{shadowblock}{5cm}
This is a block of text with a transparent
shadow!
\end{shadowblock}
\end{lrbox}

%
% Over existing text
%
Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Phasellus in nunc
tincidunt sapien sodales tincidunt. Nulla posuere semper nunc. Duis lacinia
libero non arcu. Nam quis eros id tortor ultricies fermentum. Aenean vulputate
adipiscing nulla. Etiam fermentum elit ut est. Duis at est at enim imperdiet
vestibulum. In risus dui, malesuada eu, fermentum vel, adipiscing sodales, eros.
Praesent egestas. Aliquam auctor. Maecenas vulputate mollis est. Nam varius. Nam
auctor fermentum tellus. Curabitur lacus sapien, scelerisque vulputate,
consectetuer laoreet, fringilla laoreet, tellus. Duis dignissim est non mauris.
Nam magna velit, pulvinar vel.

\raisebox{2cm}[0pt][0pt]{\makebox[0pt]{\usebox{\mybox}}}


%
% Over a picture.
%
\noindent\includegraphics[width=\linewidth]{tetons-snake-river}\\
\hspace*{4cm}\raisebox{1cm}[0pt][0pt]{\makebox[0pt]{\usebox{\mybox}}}


%
% Absolute positioning with textpos
%

\begin{textblock}{4}(2,5.2)
\begin{shadowblock}{4cm}
And a small one on top of it.
\end{shadowblock}
\end{textblock}

\begin{textblock}{4}(0,4.5)
\begin{shadowblock}{4cm}
Absolutely positioned box with a lot of text.
Absolutely positioned box with a lot of text.
Absolutely positioned box with a lot of text.
Absolutely positioned box with a lot of text.
\end{shadowblock}
\end{textblock}

\end{document}


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