tikz - gradient along a bendy path

Question

I'm drawing pipes (like the Pipeline game), and I want them to be shaded to give the effect that the pipes are cylindrical.

A bit like the pipes below: the outer edges are black, the middle is white (I tried bottom color=black,middle color=white,bottom color=black but the pipes just showed up as black. Perhaps because I'm getting my pipe width from the line width instead of as a fill? I'm amenable to changing the way the pipes are defined, so long as I can still draw them easily, with the details abstracted away in the pipe style).

(image credit: "Plumber Game")

There are a number of questions (example) that deal with applying a gradient along a path (as in say green on one end of the path, red on the other); but I want the gradient to be perpendicular to that.

\documentclass{standalone}
\usepackage{tikz}
\begin{document}

\tikzset{
  pipe/.style = {
    draw,
    rounded corners,
    line width=3mm
  }
}
\begin{tikzpicture}
  % a pipe.
  \draw [pipe] (0, 0) -- (0, 3) -- (1, 3);
\end{tikzpicture}
\end{document}

Update

@Symbol-1's suggestion of pgf-blur leads me to the following: I define the blur colour to be white (the centre colour of the pipe). Since the blur fades to transparent, I draw the solid colour of the pipe underneath, first, with preaction.

However in order for the blur to render appropriately I need to draw my right-angle pipe as two separate line segments, which then don't join smoothly. I can try separately drawing the underlying pipe (smoothly) and then the blur (nonsmoothly) but then you can see the blur doesn't quite meet in the corners (plus it is a small pain to redefine the pipe and shadow twice for each pipe, but I can live with this)::

\documentclass{standalone}
\usepackage{tikz}
  \usetikzlibrary{shadows.blur}
\makeatletter
\tikzset{
    /tikz/render blur shadow/.code={
        \pgfbs@savebb
        \pgfsyssoftpath@getcurrentpath{\pgfbs@input@path}%
        \pgfbs@compute@shadow@bbox
        \pgfbs@process@rounding{\pgfbs@input@path}{\pgfbs@fadepath}%
        \pgfbs@apply@canvas@transform
        \colorlet{pstb@shadow@color}{white!\pgfbs@opacity!black}%
        \pgfdeclarefading{shadowfading}{\pgfbs@paint@fading}%
        \pgfsetfillcolor{\blurcolor}%
        \pgfsetfading{shadowfading}%
            {\pgftransformshift{\pgfpoint{\pgfbs@midx}{\pgfbs@midy}}}%
        \pgfbs@usebbox{fill}%
        \pgfbs@restorebb
    }
}
\tikzset{
  basepipe/.style = {
    rounded corners,
    line width=3mm
  },
  pipe/.style = {
    basepipe,
    % shadow stuff
    blur shadow, shadow xshift=0, shadow yshift=0,
    shadow blur radius=1.5mm, shadow opacity=100, shadow blur steps=100,
  }
}
\def\blurcolor{white}
\begin{document}
\begin{tikzpicture}
  % a pipe.
  \path [preaction={draw,basepipe}][pipe] (0, 0) -- (0, 3) -- cycle (0,3) -- (1, 3);
  % another pipe, without cycle
  \begin{scope}[xshift=1.5cm]
  \path [preaction={draw,basepipe}][pipe] (0, 0) -- (0, 3) -- (1, 3);
  \end{scope}
  % another pipe
  \begin{scope}[xshift=3cm]
  \draw [basepipe] (0,0) -- (0,3) -- (1,3);
  \path [pipe] (0, 0) -- (0, 3) -- cycle (0,3) -- (1, 3);
  \end{scope}
\end{tikzpicture}
\end{document}

Can anyone help me get the last step?

Answer 1

The reason why the blur doesn't get the rounded Corners is because, as stated in pfg-blur manual, it only works for closed paths, so if you want the joins to be smooth, you have to close the path with a cycle where the joins are. If you read carefully @Symbol1's answer you'll see that the drawing goes like

(a) -- (b) -- cycle (b) -- (c) -- cycle (c) ...

And wherever there's a cycle, there's an equivalent to a blurred rounded line cap, that's the catch. You're also using rounded corners when you actually want line join=round (the blurred path won't follow the rounded corners). So, I created a few styles to help with the drawing: pipe=<dim> (default is 15mm) which is for the pipe's background, pipe shading=<dim> (default is also 15mm) which sets the blurred foreground and pipe color=<color> sets the blur color. Let's got the MWE then.

I've set two examples, one with line cap=butt and one with line cap=round. There's only one trick to use the line cap=butt, your pipe has to have more than 2 segments, that is: \draw (0,0)--(0,3)--cycle (0,3)--(1,3); (which is the example in you MWE) won't work, instead you need to have an extra segment, even if it won't change the outcome, like this: \draw (0,0)--(0,1) (0,1)--(0,3)--cycle (0,3)--(1,3);.

MWE

\documentclass[tikz]{standalone}
\usetikzlibrary{shadows.blur}
\makeatletter
\tikzset{
    blur shadow color/.store in=\blurcolor,
    blur shadow color=black,
    render blur shadow/.code={
        \pgfbs@savebb
        \pgfsyssoftpath@getcurrentpath{\pgfbs@input@path}%
        \pgfbs@compute@shadow@bbox
        \pgfbs@process@rounding{\pgfbs@input@path}{\pgfbs@fadepath}%
        \pgfbs@apply@canvas@transform
        \colorlet{pstb@shadow@color}{white!\pgfbs@opacity!black}%
        \pgfdeclarefading{shadowfading}{\pgfbs@paint@fading}%
        \pgfsetfillcolor{\blurcolor}%
        \pgfsetfading{shadowfading}%
            {\pgftransformshift{\pgfpoint{\pgfbs@midx}{\pgfbs@midy}}}%
        \pgfbs@usebbox{fill}%
        \pgfbs@restorebb
    },
    pipe color/.style={blur shadow color=#1},
    pipe/.style={line cap=round, line join=round, line width=#1},pipe/.default={15mm},
    pipe shading/.style={blur shadow,shadow xshift=0,shadow yshift=0,shadow opacity=100,shadow blur steps=100, shadow blur radius=#1/2},pipe shading/.default={15mm}
}
\begin{document}
    \begin{tikzpicture}
        \draw[pipe, line cap=butt] (1,0)--(1,6)--(6,6)--(7,4)--(9,4);
        \path[pipe color=white, pipe shading]
        (1,0)--(1,6)
        (1,6)--(6,6)--cycle
        (6,6)--(7,4)--cycle
        (7,4)--(9,4);
        \begin{scope}[yshift=-7.5cm]
        \draw[pipe=10mm] (1,0)--(1,6)--(6,6)--(7,4)--(9,4);
        \path[pipe color=gray, pipe shading=10mm]
        (1,0)--(1,6)--cycle
        (1,6)--(6,6)--cycle
        (6,6)--(7,4)--cycle
        (7,4)--(9,4)--cycle;
        \end{scope}
    \end{tikzpicture}
\end{document}

Note how the drawing with line cap=butt has the blurred path start and end segments without cycle, because the use of cycle introduces the equivalent of a rounded blurred line cap, which is also responsible for the joining visual effect.