Rotate a node but not its content: the case of the ellipse decoration

Question

What I want to do: Draw a node decorated with bumps, but with the decoration starting from the right (ie, opposite to the usual) of the node.

Why I want to do it: I will have a node hiding the part of the bumps that connects weirdly, but I want to be able to chose where this connection is made. Luckily, I only need "left of the node" and "right of the node".

What I thought was the solution: I thought I could use shape border rotate, so that only the border would be rotated. Define:

  \newcommand{\drawprost}[2][Prost!]{
    \node [draw,
           decorate,decoration={bumps,mirror},
           #2,
           postaction={decorate,decoration={markings,mark=at position 1 with
                        {\arrow[line width=5pt,blue]{>}}}}]
          {#1};}

Now the following two commands work as expected:

  \drawprost{shape=trapezium,shape border rotate=180}
  \drawprost{shape=trapezium,shape border rotate=0}

with the ugly blue arrow being rotated 180 degrees in the second drawing. But without shape=trapezium or with shape=ellipse, this fails miserably.

An ugly solution: The only solution I came up with is (using ellipse, otherwise I don't even have a solution):

  \drawprost[\phantom{Prost!}]{rotate=180,shape=ellipse,label=center:Prost!}

that is, phantoming the label, and redrawing it.

Question: Is there a better way to do it? Bonus point for a method that would shift the start of the decoration (rotate) at any given point :-)

---

Full code

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations,decorations.pathmorphing,shapes,decorations.markings}

\begin{document}
\begin{tikzpicture}
  \newcommand{\drawprost}[2][Prost!]{
    \node [draw,
           decorate,decoration={bumps,mirror},
           #2,
           postaction={decorate,decoration={markings,mark=at position 1 with
                        {\arrow[line width=5pt,blue]{>}}}}]
          {#1};}

  \newlength\yshift
  % I get the expected result only with trapezium.
  \foreach \shape in {,shape=trapezium,shape=ellipse}
    \foreach \rotate in {0, 180}
    {
      \expandafter\drawprost\expandafter{%
        \shape,yshift=\yshift,shape border rotate=\rotate};
      \global\advance\yshift -2cm
    }

  % What I do want:
  \drawprost[\phantom{Prost!}]{yshift=\yshift,rotate=180,shape=ellipse,label=center:Prost!}
\end{tikzpicture}
\end{document}

Answer 1

The source of the difficulty is that ellipses are constructed in a particular way in TikZ. They are paths that start from the x-axis and proceed counter-clockwise around their centre. The vast majority of the time, the exact parametrisation doesn't matter. You appear to have found the one situation where it does!

In the actual question, you only want to be able to mirror the ellipse, and so draw it starting from the negative x-axis (the title of the question suggests a more flexible approach). That's actually not too hard since we can exploit the symmetry of the ellipse. The key is to provide it with a negative x-radius, since then it will start from the negative x-axis (and proceed clockwise, but we could correct for that by negating the y-radius as well). To do this, we interrupt the call from the node shape to the drawing command and flip the sign of the x-radius. The simplest way to do this is to redefine the \pgfpathellipse macro to do the negation and then call the original macro. The following code does this.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations,shapes,decorations.markings}
\makeatletter
\let\origpgfpathellipse=\pgfpathellipse
\def\revpgfpathellipse#1#2#3{%
  #2%
  \pgf@xa=-\pgf@x
  \origpgfpathellipse{#1}{\pgfqpoint{\pgf@xa}{0pt}}{#3}}
\makeatother

\tikzset{
  reversed ellipse/.style={
    ellipse,
    reverse the ellipse%
  },
  reverse the ellipse/.code={
    \let\pgfpathellipse=\revpgfpathellipse
  }
}
\begin{document}
\begin{tikzpicture}
\node[ellipse,
  draw,
  postaction={
    decorate,
    decoration={
      markings,
      mark=at position 1 with {
        \arrow[line width=5pt,blue]{>}
      }
    }
  }
] at (0,0) {hello world};

\node[reversed ellipse,
  draw,
  postaction={
    decorate,
    decoration={
      markings,
      mark=at position 1 with {
        \arrow[line width=5pt,blue]{>}
      }
    }
  }
] at (0,-2) {hello world};
\end{tikzpicture}

\end{document}

Here's the result:

(the arrow got clipped, but you can see where it lies)

Answer 2

You could move the label to be part of the macro and then specifying the \phantom within the macro allows the width of the node to be computed:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations,decorations.pathmorphing,shapes,decorations.markings}

\newcommand{\drawprost}[2][Prost!]{
    \node [draw,
           decorate,decoration={bumps,mirror},
           #2, label=center:#1,
           postaction={decorate,decoration={markings,mark=at position 1 with
                        {\arrow[line width=5pt,blue]{>}}}}]
          {\phantom{#1}};}

\begin{document}
\begin{tikzpicture}
  \drawprost[Prost!]{rotate=180,shape=ellipse}
\end{tikzpicture}
\end{document}