Color gradient filling along a path

Question

I want to make the gradient color to follow the pipe curve below rather than to span from the left to the right.

The following MWE shows the default behavior which is not what I'm looking for.

\documentclass[pstricks,border=12pt]{standalone}

\usepackage{pst-slpe} 

\begin{document}

\begin{pspicture}(8,-7)
\pscustom[fillstyle=slope]{
  \psline(6,0)
  \psarcn(6,-2){2}{90}{-90}
  \psline(2,-4)
  \psarc(2,-5){1}{90}{-90}
  \psline(8,-6)
  \psline(8,-7)(2,-7)
  \psarcn(2,-5){2}{-90}{90}
  \psline(6,-3)
  \psarc(6,-2){1}{-90}{90}
  \psline(0,-1)
  \closepath}
\end{pspicture}

\end{document}

Could you help me to accomplish my objective?

Update

Using Pierre's solution, here is what I ended up with:

\documentclass{article}  

\usepackage{pst-plot}

\makeatletter
 \pst@addfams{pst-HSB}
 \define@key[psset]{pst-HSB}{HueBegin}{%
   \def\PstParametricplotHSB@HueBegin{#1}}
 \define@key[psset]{pst-HSB}{HueEnd}{%
   \def\PstParametricplotHSB@HueEnd{#1}}
 \define@boolkey[psset]{pst-HSB}[Pst@]{HSB}[true]{}
 % Default values
 \psset[pst-HSB]{HueBegin = 0, HueEnd = 1, HSB = true}
 \psset{dimen = outer}

 \def\parametricplotHSB{\pst@object{parametricplotHSB}}
 \def\parametricplotHSB@i#1#2#3{{%
  \begin@ClosedObj
  \addto@pscode{%
    /t #1 def
    /dt #2 t sub \psk@plotpoints\space div def
    /t t dt sub def
    /Counter 0 def
    1 setlinejoin
    \psk@plotpoints {
      /t t dt add def
      /Counter Counter 1 add def
      #3
      \pst@number\psyunit mul exch
      \pst@number\psxunit mul exch
      1 Counter eq { moveto currentpoint /OldY ED /OldX ED }
        {\ifPst@HSB
          /PointY exch def
          /PointX exch def
          Counter \psk@plotpoints\space div
          \PstParametricplotHSB@HueEnd\space
          \PstParametricplotHSB@HueBegin\space sub mul
          \PstParametricplotHSB@HueBegin\space add
          1 1 sethsbcolor
          OldX OldY PointX PointY lineto lineto 
          stroke
          PointX PointY moveto
      /OldX PointX def /OldY PointY def
        \else lineto \fi } ifelse
     } repeat }%
   \end@ClosedObj}
  \ignorespaces}
\makeatother

\begin{document}

\begin{pspicture}(-0.5,-0.5)(4.5,4.5)
 \psset{
   linewidth = 1cm,
   plotpoints = 300
 }
  \parametricplotHSB[
    HueBegin = 0.70,
    HueEnd   = 0.66
  ]{3.05}{0}{t 4}
  \parametricplotHSB[
    HueBegin = 0.70,
    HueEnd   = 0.78
  ]{0}{180}{t sin 3 add t cos 3 add}
  \parametricplotHSB[
    HueBegin = 0.78,
    HueEnd   = 0.86
  ]{3.05}{0.95}{t 2}
  \parametricplotHSB[
    HueBegin = 0.86,
    HueEnd   = 0.92
  ]{0}{-180}{t sin 1 add t cos 1 add}
  \parametricplotHSB[
    HueBegin = 0.92,
    HueEnd   = 1
  ]{0.95}{4}{t 0}
\end{pspicture}

\end{document}

Update 2

I've not managed to get vertical ends on the curve and a black boundry; can anyone help with this?

Update 3

I've now found http://pstricks.blogspot.dk/2015/10/helice-tournant-autour-de-son-axe-et.html; can a similar coloring as alone the helix be obtained?

Answer 1

I've tried to follow everyone's advices,

• I have drawn 3 lines and 2 half-circles (instead of a shape)
• I've used the \parametricplotHSB to have the gradient

and here's what I got :

    \documentclass[xcolor]{article}  
    \usepackage[utf8]{inputenc}
    \usepackage{pstricks,pst-plot,pst-xkey}

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    % Essai de Manuel Luque 19 février 2003
    % transformé par Denis Girou le 25 février 2003
    % révisé le 9 mai 2007
    % modifié par Pierre Vivegnis le 2 juin 2014
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    \makeatletter
    \pst@addfams{pst-plothsb}

    \define@key[psset]{pst-plothsb}{HueBegin}{% Between 0 and 1
    \edef\PstParametricplotHSB@HueBegin{#1}}

    \define@key[psset]{pst-plothsb}{HueEnd}{% Between 0 and 1
    \edef\PstParametricplotHSB@HueEnd{#1}}

    \newif\ifPst@HSB
    \define@key[psset]{pst-plothsb}{HSB}[true]{\@nameuse{Pst@HSB#1}}

    % Default values
    \psset{HueBegin=0,HueEnd=1,HSB=true}

    \def\parametricplotHSB{\pst@object{parametricplotHSB}}
    \def\parametricplotHSB@i{\@ifnextchar[{\parametricplotHSB@do}{\parametricplotHSB@do[]}}
    \def\parametricplotHSB@do[#1]#2#3#4{{%
    \psset{#1}%
    \begin@ClosedObj
    \addto@pscode{%
    /t  #2 def
    /dt #3 t sub \psk@plotpoints\space div def
    /t t dt sub def
    /Counter 0 def
    \psk@plotpoints {
      /t t dt add def
      /Counter Counter 1 add def
    /F@pstplot \ifPst@algebraic (#4)
                        tx@AlgToPs begin AlgToPs end cvx
                     \else { #4 } 
              \fi  
       def
    \ifPst@algebraic 
       F@pstplot
        \else  #4  
    \fi 
      \pst@number\psyunit mul exch
      \pst@number\psxunit mul exch
      1 Counter eq
        {moveto}                    % First point
        {\ifPst@HSB                 % Other points than the first one
           /PointY exch def
           /PointX exch def
           Counter \psk@plotpoints\space div
               \PstParametricplotHSB@HueEnd\space
                 \PstParametricplotHSB@HueBegin\space sub mul
               \PstParametricplotHSB@HueBegin\space add
             1 1 sethsbcolor
           PointX PointY lineto
           stroke
           PointX PointY moveto
         \else
           lineto
         \fi} ifelse
        } repeat}% fin du code ps
    \end@ClosedObj}} % fin de la commande PSTricks

    \makeatother

    \begin{document}
        \begin{pspicture}[showgrid](-1, -1)(5,5)
          \psset{plotpoints=360, linewidth=10mm}%, HSB=false}%
            \parametricplotHSB[HueBegin=0,HueEnd=0.2]{-0.5}{3.01}{t 4}
            \parametricplotHSB[HueBegin=0.2,HueEnd=0.4]{0}{181}{t sin 3 add t cos 3 add}
            \parametricplotHSB[HueBegin=1,HueEnd=0.8]{4.5}{0.99}{t 0}
            \parametricplotHSB[HueBegin=0.8,HueEnd=0.6]{-180}{0}{t sin 1 add t cos 1 add}
            \parametricplotHSB[HueBegin=0.4,HueEnd=0.6]{3}{0.95}{t 2}
        \end{pspicture}
    \end{document}

and I gives me that :

But I can't figure out how, when you add, the HSB=false option, manage the colours...

I hope my answer doesn't come too late... After 6 months :P

plotpoints

I tried increasing the number of plotted points, but this was the best... For that I can't help you, sorry...