Suppressing the frametitle after a specific command within a frame (e.g. titlepage)

Question

I'm writing on a .sty - file for a theme, which others will use too.

My aim is to have as less as possible additional (visible) code in the document environment of a corresponding .tex-file using the particular theme.

I run across a obstacle concerning the frametitle - titlepage interaction. I want to avoid that people give the titlepage an additional frame title.

MWE

\documentclass{beamer}

\pgfdeclareimage[width=.5\paperwidth]{mybackground}{beamericononline}

\defbeamertemplate*{title page}{mytitlepage}
{
    \begin{picture}(0,0)
    \put(-20,-60){%
        \pgfuseimage{mybackground}
    }
    \put(165,0){%
        %\colorbox{teal}{
        \begin{minipage}[b][10mm][t]{0.5\textwidth}
        \usebeamerfont{title}{\inserttitle\par}
        \end{minipage}%
        %}
    }
    \end{picture}
}

\title{A proper test title for a test presentation?}

\begin{document}
%%----------------------------------------------------------------------
%% This is how it should look like, even if you uncomment the frametitle
%%----------------------------------------------------------------------

\begin{frame}%{An extra frame title\\[2ex] is not allowed}
\titlepage
\end{frame}

%%----------------------------------------------------------------------
%% This should not be allowed, an error shall stop the compiling    
%%----------------------------------------------------------------------

\begin{frame}{An extra frame title\\[2ex] is not allowed}
    \titlepage
\end{frame}

%%----------------------------------------------------------------------
%% Frametitle suppressed, but I prefer to handle the added code in the 
%% .sty file, i.e. before the document environment
%%----------------------------------------------------------------------

{
\setbeamertemplate{frametitle}{}
\begin{frame}{An extra frame title\\[2ex] is not allowed}
    \titlepage
\end{frame}
}

I already tried several stuff shifting the

{
\setbeamertemplate{frametitle}{}
...
}

before the document, e.g. inside the title page template. Other ideas using additional \newif,\if,\ifxor defining a key reacting to the input \titlepage (as part of the frame body), similar to tex.stackexchange.com/questions/173201 did not work.

Is there a way (next to the given one) to suppress the output of the frame title using (only) a style file?

Answer 1

If there is always a title page, you could test for the page number within the definition of the frametitle and only show it starting from the second page:

\documentclass{beamer}

\pgfdeclareimage[width=.5\paperwidth]{mybackground}{beamericononline}

\defbeamertemplate*{title page}{mytitlepage}
{
    \begin{picture}(0,0)
    \put(-20,-60){%
        \pgfuseimage{mybackground}
    }
    \put(165,0){%
        %\colorbox{teal}{
        \begin{minipage}[b][10mm][t]{0.5\textwidth}
        \usebeamerfont{title}{\inserttitle\par}
        \end{minipage}%
        %}
    }
    \end{picture}
}

\title{A proper test title for a test presentation?}


\setbeamertemplate{frametitle}{%
    \ifnum\thepage>1
        \insertframetitle
    \fi
}

\begin{document}

\begin{frame}{An extra frame title\\[2ex] is not allowed}
    \titlepage
\end{frame}

\begin{frame}
\frametitle{title}
normal frame
\end{frame}

\end{document}