How to set a background image on a custom background color in beamer?

Question

I haven't found this question anywhere and therefore no answer to it.

I have a beamer presentation where I use \usebackgroundtemplate in order to put a background image. Now on some slides I would like the part left out by the image to be on a backround color other than white. I tried several positions of \setbeamercolor (inside/outside the frame, in a group with the frame, included in curly brackets, etc.) but nothing seems to work. It looks to me that \usebackgroundtemplate overrides the command \setbeamercolor.

Here is a minimal example:

\documentclass[12pt,a4paper,ucs,t,german]{beamer}
\usetheme{Ophaniel}
\usepackage[utf8x]{inputenc}
\usepackage{graphicx}
\usepackage{eurosym}
\usepackage[export]{adjustbox}

\title{\textcolor{black}{Playa Turquesa - Dominikanische Republik}}

\setbeamercolor{background canvas}{bg=}
\setbeamertemplate{frametitle}[default][center]

\begin{document}

\usebackgroundtemplate{%
  \parbox[c][\paperheight][c]{\paperwidth}{\centering{\includegraphics[scale=1.0]{../../../immobili/Repubblica-Dominicana/playaturquesa/Playa-Turquesa-Long-Logo-2.png}}}%
}

\begin{frame}
%\setbeamercolor{background}{bg=violet} % just a try, doesn't work
%\setbeamercolor{background canvas}{bg=violet} % this doesn't work either
\frametitle{\fcolorbox{black}{white}{Beispielpreise}}
Frame text here
\end{frame}

end{document}

The image is a .png with transparent background, so I would assume it should let see what's behind it.

Is there an easy way to do this?

Edited the question, thank you.

Ophaniel is my own theme that I did by just modifying another one. So I guess the code is not very compilable, if you don't have it. I'm not sure if there's anything there that could disturb the functioning of \setbeamercolor{background}. Should I publish the theme as well for you to be able to compile it?

Answer 1

\documentclass[t]{beamer}

\setbeamerfont{frametitle}{size=\fontsize{24}{28},series=\bfseries}
\setbeamercolor{frametitle}{fg=black}

\setbeamercolor{bgcolor}{fg=black,bg=blue!20}
\pgfdeclareimage[width=\paperwidth]{mybackground}{back.pdf}

\setbeamertemplate{background canvas}{%
    \begin{picture}(30,273)
     \begin{beamercolorbox}[wd=1.1\paperwidth,ht=\paperheight]{bgcolor}
     \end{beamercolorbox}
        \put(-310,0){%
            \pgfuseimage{mybackground}
        }
    \end{picture}
}

\begin{document}

    \begin{frame}
        test
    \end{frame}

    \setbeamercolor{bgcolor}{fg=black,bg=red!20}
    \begin{frame}
        test
    \end{frame}

\end{document}

Answer 2

Using a macro (e.g. \opacity) works:

\documentclass{beamer}
\usepackage{tikz}
\usepackage{mwe}

\usebackgroundtemplate{%
\tikz\node[opacity=\opacity,inner sep=0pt]%
{\includegraphics[height=\paperheight,width=\paperwidth]{example-image}};}

\begin{document}
\def\opacity{0.3}
\begin{frame}
first frame
\end{frame}

\def\opacity{1}
\begin{frame}
second frame
\end{frame}

\end{document}