How to define different head/footlines for title/regular/plain pages in Beamer

Question

I am developing a very simple beamer theme from scratch. Just a few additions to the default. I am putting all color/font/inner/outer stuff into one file beamerthemeBlabla.sty:

\RequirePackage{tikz}
\definecolor{mygray}{gray}{0.5}

\mode
<presentation>

\defbeamertemplate*{footline}{}
{
  \leavevmode%
  \begin{tikzpicture}
    \useasboundingbox (0,0) rectangle(\the\paperwidth,.8);
    \draw[color=blue,thick,rounded corners=8pt]
      (0,.6) -- (12.45,.6) -- (12.45,0);
    \node[color=mygray,text width=1cm] at (1.3,.3)
      {\insertshortpart};
    \node[color=mygray,align=right] at (12,.3)
      {\insertframenumber};
  \end{tikzpicture}
}

\defbeamertemplate*{title page}{}[1][]
{ 
   \vskip3cm%
      {\Huge\inserttitle}\par%
    \vskip1.75cm%
      {\scriptsize\insertauthor}%
     \vskip0.2cm%
      {\scriptsize\insertdate}%
  \vfill
}

\mode
<all>

Which is then used in a presentation starting with

\documentclass[]{beamer}
\usetheme{Blabla}

I get exactly the footline I want for regular slides. However, I need a different footline for the title page. Since I fail to understand the setbeamertemplate/defbeamertemplate mechanism, I have no clue.

Furthermore, I would also like to have a minimalistic footline on "plain" slides. Is that possible, or does it categorically conflict with the definition of "plain" style?

Answer 1

Different footline:

\defbeamertemplate*{footline}{}[1]{%
  \ifnum\insertframenumber>1\relax%
  <code for standard slides>
  \else%
  <code for title page>
  \fi%
}

From what I know (which is likely not too much) plain frames can't have footline, but you can \defbeamertemplate* a "my plain frame" and \usebeamertemplate***{my plain frame} in a frame which has the option [plain].