Is it possible to apply two pathmorphing/pathreplacing decorations in succession?

Question

Suppose one wants to achieve the following:

Current Status

Based upon another post, I was able to create a "MWE" of how I would like this to work:

\documentclass[border=5pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations, decorations.pathmorphing, decorations.pathreplacing}
\def\width{5mm}
\pgfdeclaredecoration{make wide}{initial}
{
    \state{initial}[width=1pt]
    {
        \pgfpathmoveto{\pgfpoint{0pt}{-0.5\width}}
        \pgfpathlineto{\pgfpoint{1pt}{-0.5\width}}
        \pgfpathmoveto{\pgfpoint{0pt}{0.5\width}}
        \pgfpathlineto{\pgfpoint{1pt}{0.5\width}}
    }
    \state{final}
    {
        \pgfpathmoveto{\pgfpoint{0pt}{0.5\width}}
        \pgfpathlineto{\pgfpoint{1pt}{0.5\width}}
        \pgfpathmoveto{\pgfpoint{0pt}{-0.5\width}}
        \pgfpathlineto{\pgfpoint{1pt}{-0.5\width}}
    }
}
\tikzstyle{make wide and ticks} = [
    decoration={make wide},
    decorate,
    postaction={
        decoration={ticks},
        decorate
    }
]
\begin{document}
    \begin{tikzpicture}
        \drawmake wide and tickstoout=0, in=180;
    \end{tikzpicture}
\end{document}

However, the output is:

To rule out any weird order of execution issues that I previously encountered with tikzstyle, I also tried it without defining a tikzstyle with the same result (just like one would expect). From my understanding of the implementation of postaction, it is not possible to to use every decoration within the postaction phase.

Questions:

1. Is there a way to achieve the desired output with decorations?
2. Is there another simple way that also works for arbitrary shaped paths?

Answer 1

The solution is realized in two steps:

1. points characterizing a certain tubular neighborhood of the path are constructed through a decoration towards tubular which takes two arguments (step and distance from the path that are taken in pt)
2. the two corresponding paths of the tubular neighborhood are constructed and decorated with the predefined decoration ticks through the commands L ticks and R ticks (left and right tubular borders).

Note, in the first drawing of the above figure, that there is a problem with the last tick of the decoration; it does not "cover" the last part of the initial path (drawn in black). For this reason, some "tail" is added and then the decoration towards tubular is invoked (see the middle drawing). The tail is added through the option e.

I hope that the last drawing approaches what you are looking for.

The code

\documentclass[11pt, margin=10pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{math}
\usetikzlibrary{decorations.markings, decorations.pathreplacing}
\begin{document}
\tikzset{%
  e/.style={% step length -- in points
    % needed for the last tick of open curves
    insert path={-- ([turn]0: #1pt)}
  },
  towards tubular/.style 2 args={% step length, width -- in points
    % create the points describing the tubular neighborhood
    decoration={markings,
      mark=between positions 0 and 1 step #1pt with {
        \tikzmath{%
          integer \pNumber;
          \pNumber = \pgfkeysvalueof{/pgf/decoration/mark
            info/sequence number};
        }
        \pgfextra{\xdef\pNumber{\pNumber}}
        \path (0, #2pt) coordinate (TL-\pNumber);
        \path (0, -#2pt) coordinate (TR-\pNumber);
      }
    },
    postaction=decorate
  },
  L ticks/.style 2 args={% step length, tick length -- in points
    decorate,
    decoration={ticks, segment length=#1pt, amplitude=#2pt},
    insert path={%
      (TL-1) \foreach \i in {2, ..., \pNumber}{-- (TL-\i)}
    }
  },
  R ticks/.style 2 args={% step length, tick length -- in points
    decorate,
    decoration={ticks, segment length=#1pt, amplitude=#2pt},
    insert path={%
      (TR-1) \foreach \i in {2, ..., \pNumber}{-- (TR-\i)}
    }
  }
}
\begin{tikzpicture}
  \draw[help lines, opacity=.3] (-4, -3) grid (4, 4);
% first drawing
  \draw[towards tubular={3}{6}]
  (-3, -2.5) .. controls ++(30:5) and ++(210:3) .. ++(-1, 6);
  \draw[blue, L ticks={4}{2}];
  \draw[red, R ticks={4}{2}];
% second drawing
  \path[towards tubular={6}{6}]
  (0, -2.5) .. controls ++(30:5) and ++(210:3) .. ++(-1, 6) [e=5];
  \draw
  (0, -2.5) .. controls ++(30:5) and ++(210:3) .. ++(-1, 6);
  \draw[blue, L ticks={4}{2}];
  \draw[red, R ticks={4}{2}];
% third drawing
  \path[towards tubular={3}{6}]
  (3, -2.5) .. controls ++(30:5) and ++(210:3) .. ++(-1, 6) [e=3];
  \draw[blue, L ticks={4}{2}];
  \draw[red, R ticks={4}{2}];
\end{tikzpicture}
\end{document}