[Tex/LaTex] Colour gradient filling in PSTricks figure

colorgradientpstricks-add

Consider the following drawing:

\documentclass[
  a4paper,
  12pt
]{article}

\usepackage[utf8]{inputenc}
\usepackage[
  hmargin=2.4cm,
  vmargin=3cm
]{geometry}
\usepackage{rotating}
\usepackage{pstricks-add}

\newcommand*\forskel{350 140 div }
\newcommand*\udtag[2]{\rput(!10   249 140 div #1 \forskel mul add){\small #2}}
\newcommand*\pil[1]{  \rput(!11.5 249 140 div #1 \forskel mul add){\small $\rightarrow$}}
\newcommand*\destillering[1]{%
  % Ventiler og udtag.
  \psline(!4    2.8  #1 \forskel mul add)%
         (!4    1.8  #1 \forskel mul add)%
         (!4    2.3  #1 \forskel mul add)%
         (!5.3  2.3  #1 \forskel mul add)%
         (!5.3  2.65 #1 \forskel mul add)
  \psline(!5.15 2.5  #1 \forskel mul add)%
         (!5.15 2.8  #1 \forskel mul add)%
         (!5.95 2.8  #1 \forskel mul add)%
         (!5.95 2.5  #1 \forskel mul add)
  \psline(!5.8  2.65 #1 \forskel mul add)%
         (!5.8  2.3  #1 \forskel mul add)%
         (!6.3  2.3  #1 \forskel mul add)%
         (!6.3  2.65 #1 \forskel mul add)
  \psline(!6.15 2.5  #1 \forskel mul add)%
         (!6.15 2.8  #1 \forskel mul add)%
         (!6.95 2.8  #1 \forskel mul add)%
         (!6.95 2.5  #1 \forskel mul add)
  \psline(!6.8  2.65 #1 \forskel mul add)%
         (!6.8  2.3  #1 \forskel mul add)%
         (!7.4  2.3  #1 \forskel mul add)
  \psline(!   8.5       284 140 div #1       \forskel mul add)%
         (!   8.5       214 140 div #1 1 add \forskel mul add)
  \psarc(! 1144 140 div   2.3       #1       \forskel mul add){0.27142857}{180}{270}
  \psarc(! 1144 140 div   2.3       #1       \forskel mul add){0.77142857}{180}{270}
  \psline(!   7.9         2.3       #1       \forskel mul add)%
         (!   8.5         2.3       #1       \forskel mul add)
  \psline(!1144 140 div 284 140 div #1       \forskel mul add)%
         (!  12         284 140 div #1       \forskel mul add)
  \psline(!  12         214 140 div #1       \forskel mul add)%
         (!2861 350 div 214 140 div #1       \forskel mul add)
  % `Mellemventiler'.
  \psline(!3.5  470 140 div #1 \forskel mul add)%
         (!5.3  470 140 div #1 \forskel mul add)%
         (!5.3  519 140 div #1 \forskel mul add)
  \psline(!5.15 498 140 div #1 \forskel mul add)%
         (!5.15 540 140 div #1 \forskel mul add)%
         (!5.95 540 140 div #1 \forskel mul add)%
         (!5.95 498 140 div #1 \forskel mul add)
  \psline(!5.8  519 140 div #1 \forskel mul add)%
         (!5.8  470 140 div #1 \forskel mul add)%
         (!6.3  470 140 div #1 \forskel mul add)%
         (!6.3  519 140 div #1 \forskel mul add)
  \psline(!6.15 498 140 div #1 \forskel mul add)%
         (!6.15 540 140 div #1 \forskel mul add)%
         (!6.95 540 140 div #1 \forskel mul add)%
         (!6.95 498 140 div #1 \forskel mul add)
  \psline(!6.8  519 140 div #1 \forskel mul add)%
         (!6.8  470 140 div #1 \forskel mul add)%
         (!8.1  470 140 div #1 \forskel mul add)%
         (!8.1  392 140 div #1 \forskel mul add)%
}

\pagestyle{empty}

\begin{document}

\vfill
\begin{figure}[htbp]
 \centering
 \psset{unit=1cm}
  \begin{pspicture}(-1.02,0.3)(12,17)
    % Opvarmningsovn.
    \psline(0.7,4.7)(0.7,5)(3.3,5)(3.3,2.9)
    \psline(3.3,2.4)(3.3,2)(0.7,2)(0.7,4.2)
    \psline(-0.8,4.7)(2.3,4.7)
    \psline(-0.8,4.2)(2.3,4.2)
    \psarc(2.3,4){0.2}{270}{90}
    \psarc(2.3,4){0.7}{270}{90}
    \psline(2.3,3.8)(1.7,3.8)
    \psline(2.3,3.3)(1.7,3.3)
    \psarc(1.7,3.1){0.2}{90}{270}
    \psarc(1.7,3.1){0.7}{90}{270}
    \psline(1.7,2.9)(3.5,2.9)
    \psline(1.7,2.4)(3.5,2.4)
    \rput(2,1.75){\small Ovn}
    \rput(1.15,4.45){\small `Kold' råolie}
    \rput(-0.4,4.45){\small $\rightarrow$}
    % Tårnets indre samt udtag.
    \multido{\i=0+1}{5}{%
      \destillering{\i}%
      \pil{\i}%
    }
    \pil{6}
    \udtag{0}{Brændselsolie}
    \udtag{1}{Smøreolie}
    \udtag{2}{Dieselolie}
    \udtag{3}{Petroleum}
    \udtag{4}{Nafta}
    \udtag{6}{Gasser}
    % Tårnets ydre.
    \psline(  3.5,   2.9)%
           (! 3.5  179 140 div 6 \forskel mul add)%
           (! 5.75 179 140 div 6 \forskel mul add)
    \psarc(!  6.5  179 140 div 6 \forskel mul add){0.25}{90}{180}
    \psline(! 6.5  214 140 div 6 \forskel mul add)%
           (!12    214 140 div 6 \forskel mul add)
    \psarc(!  6.5  179 140 div 6 \forskel mul add){0.75}{90}{180}
    \psline(! 6.5  284 140 div 6 \forskel mul add)%
           (!12    284 140 div 6 \forskel mul add)
    \psline(! 6.25 179 140 div 6 \forskel mul add)%
           (! 8.5  179 140 div 6 \forskel mul add)%
           (! 8.5  284 140 div 5 \forskel mul add)%
%           (!12    284 140 div 5 \forskel mul add)
%    \psline(! 8.5  214 140 div 5 \forskel mul add)%
%           (!12    214 140 div 5 \forskel mul add)
    \psline(!8.5 284 140 div 5 \forskel mul add)%
           (!8.5 214 140 div 5 \forskel mul add)
    \psarc(6.5,1){0.25}{180}{270}
    \psline(6.5,0.75)(12,0.75)
    \psarc(6.5,1){0.75}{180}{270}
    \psline(6.5,0.25)(12,0.25)
    \psline(!8.5 214 140 div)(8.5,1)(6.25,1)
    \rput(9,0.5){\small Destillationsrester}
    \rput(11.5,0.5){\small $\rightarrow$}
    \psline(5.75,1)(3.5,1)(3.5,2.4)
  \end{pspicture}
\end{figure}
\vfill

\end{document}

output

How do I gradually colour the tower and the pipes, starting with a dark red at the bottom and ending with light yellow at the top?

Update

Do to Werner's help, here is what I ended up with:

\documentclass[
  a4paper,
  12pt
]{article}

\usepackage[utf8]{inputenc}
\usepackage[
  hmargin=2.4cm,
  vmargin=3cm
]{geometry}
\usepackage{rotating}
\usepackage{
  pst-slpe,
  pstricks-add
}

\newcommand*\forskel{350 140 div }
\newcommand*\udtag[2]{\rput(!10   249 140 div #1 \forskel mul add){\small #2}}
\newcommand*\pil[1]{  \rput(!11.5 249 140 div #1 \forskel mul add){\small $\rightarrow$}}
\newcommand*\destillering[1]{%
  % Ventiler.
  \psline(!4    2.8  #1 \forskel mul add)%
         (!4    1.8  #1 \forskel mul add)%
         (!4    2.3  #1 \forskel mul add)%
         (!5.3  2.3  #1 \forskel mul add)%
         (!5.3  2.65 #1 \forskel mul add)
  \psline(!5.15 2.5  #1 \forskel mul add)%
         (!5.15 2.8  #1 \forskel mul add)%
         (!5.95 2.8  #1 \forskel mul add)%
         (!5.95 2.5  #1 \forskel mul add)
  \psline(!5.8  2.65 #1 \forskel mul add)%
         (!5.8  2.3  #1 \forskel mul add)%
         (!6.3  2.3  #1 \forskel mul add)%
         (!6.3  2.65 #1 \forskel mul add)
  \psline(!6.15 2.5  #1 \forskel mul add)%
         (!6.15 2.8  #1 \forskel mul add)%
         (!6.95 2.8  #1 \forskel mul add)%
         (!6.95 2.5  #1 \forskel mul add)
  \psline(!6.8  2.65 #1 \forskel mul add)%
         (!6.8  2.3  #1 \forskel mul add)%
         (!7.4  2.3  #1 \forskel mul add)
  \psline(!   8.5       284 140 div #1       \forskel mul add)%
         (!   8.5       214 140 div #1 1 add \forskel mul add)
  % `Mellemventiler'.
  \psline(!3.5  470 140 div #1 \forskel mul add)%
         (!5.3  470 140 div #1 \forskel mul add)%
         (!5.3  519 140 div #1 \forskel mul add)
  \psline(!5.15 498 140 div #1 \forskel mul add)%
         (!5.15 540 140 div #1 \forskel mul add)%
         (!5.95 540 140 div #1 \forskel mul add)%
         (!5.95 498 140 div #1 \forskel mul add)
  \psline(!5.8  519 140 div #1 \forskel mul add)%
         (!5.8  470 140 div #1 \forskel mul add)%
         (!6.3  470 140 div #1 \forskel mul add)%
         (!6.3  519 140 div #1 \forskel mul add)
  \psline(!6.15 498 140 div #1 \forskel mul add)%
         (!6.15 540 140 div #1 \forskel mul add)%
         (!6.95 540 140 div #1 \forskel mul add)%
         (!6.95 498 140 div #1 \forskel mul add)
  \psline(!6.8  519 140 div #1 \forskel mul add)%
         (!6.8  470 140 div #1 \forskel mul add)%
         (!8.1  470 140 div #1 \forskel mul add)%
         (!8.1  392 140 div #1 \forskel mul add)%
  % Udtag.
  \psarc(! 1144 140 div   2.3       #1       \forskel mul add){0.27142857}{180}{270}
  \psarc(! 1144 140 div   2.3       #1       \forskel mul add){0.77142857}{180}{270}
  \psline(!   7.9         2.3       #1       \forskel mul add)%
         (!   8.5         2.3       #1       \forskel mul add)
  \psline(!1144 140 div 284 140 div #1       \forskel mul add)%
         (!  12         284 140 div #1       \forskel mul add)
  \psline(!  12         214 140 div #1       \forskel mul add)%
         (!2861 350 div 214 140 div #1       \forskel mul add)
}

\pagestyle{empty}

\begin{document}

\vfill
\begin{center}
 \psset{
   unit=1cm
 }
  \begin{pspicture}(-0.8,0.25)(12,17)
    % Farvegraduering.
   {\psset{
      linestyle = none,
      slopeangle = 90,
      fillstyle = slope,
      slopebegin = red!90!black,
      slopeend = yellow
    }
      \psframe[
        slopesteps = 50
      ](0.7,2)(3.3,5)
      \pscustom[
        slopesteps = 200
      ]{
        \multido{\iA = 0+1}{5}{%
          \psline(! 8.5 284 140 div \iA\space \forskel mul add)%
                 (!12   284 140 div \iA\space \forskel mul add)%
                 (!12   214 140 div \iA\space \forskel mul add)%
                 (! 8.5 214 140 div \iA\space \forskel mul add)%
        }
        \psline(8.5,1)(6.3,1)
        \psarc(6.5,1){0.25}{180}{270}
        \psline(6.5,0.75)(12,0.75)(12,0.25)(6.5,0.25)
        \psarcn(6.5,1){0.75}{270}{180}
        \psline(5.75,1)(3.5,1)(3.5,2.4)(3.5,16.3)(5.75,16.3)
        \psarc(6.5,16.3){0.75}{90}{180}
        \psline(6.5,17.05)(12,17.05)(12,16.55)(6.5,16.55)
        \psarc(6.5,16.3){0.25}{90}{180}
        \psline(6.5,16.3)(8.5,16.3)
      }
   }
   {\psset{
      linestyle = none,
      slopeangle = 90,
      fillstyle = slope,
      slopebegin = red!80!yellow,
      slopeend = blue!50!black
    }
      \pscustom[
        slopesteps = 50
      ]{
        \psline(-0.8,4.2)(2.3,4.2)
        \psarc(2.3,4){0.2}{-90}{90}
        \psline(2.3,3.8)(1.7,3.8)
        \psarcn(1.7,3.1){0.7}{-90}{90}
        \psline(1.7,2.4)(3.5,2.4)(3.5,2.9)(1.7,2.9)
        \psarc(1.7,3.1){0.2}{90}{-90}
        \psline(1.7,3.3)(2.3,3.3)
        \psarcn(2.3,4){0.7}{90}{-90}
        \psline(2.3,4.7)(-0.8,4.7)
      }
   }
    % Opvarmningsovn.
    \psline(0.7,4.7)(0.7,5)(3.3,5)(3.3,2.9)
    \psline(3.3,2.4)(3.3,2)(0.7,2)(0.7,4.2)
    \psline(-0.8,4.7)(2.3,4.7)
    \psline(-0.8,4.2)(2.3,4.2)
    \psarc(2.3,4){0.2}{270}{90}
    \psarc(2.3,4){0.7}{270}{90}
    \psline(2.3,3.8)(1.7,3.8)
    \psline(2.3,3.3)(1.7,3.3)
    \psarc(1.7,3.1){0.2}{90}{270}
    \psarc(1.7,3.1){0.7}{90}{270}
    \psline(1.7,2.9)(3.5,2.9)
    \psline(1.7,2.4)(3.5,2.4)
    \rput(2,1.75){\small Ovn}
    \rput(1.15,4.45){\small `Kold' råolie}
    \rput(-0.4,4.45){\small $\rightarrow$}
    % Tårnets indre samt udtag.
    \multido{\iB = 0+1}{5}{%
      \destillering{\iB}%
      \pil{\iB}%
    }
    \pil{6}
    \udtag{0}{Brændselsolie}
    \udtag{1}{Smøreolie}
    \udtag{2}{Dieselolie}
    \udtag{3}{Petroleum}
    \udtag{4}{Nafta}
    \udtag{6}{Gasser}
    % Tårnets ydre.
    \psline(  3.5,   2.9)%
           (! 3.5  179 140 div 6 \forskel mul add)%
           (! 5.75 179 140 div 6 \forskel mul add)
    \psarc(!  6.5  179 140 div 6 \forskel mul add){0.25}{90}{180}
    \psline(! 6.5  214 140 div 6 \forskel mul add)%
           (!12    214 140 div 6 \forskel mul add)
    \psarc(!  6.5  179 140 div 6 \forskel mul add){0.75}{90}{180}
    \psline(! 6.5  284 140 div 6 \forskel mul add)%
           (!12    284 140 div 6 \forskel mul add)
    \psline(! 6.25 179 140 div 6 \forskel mul add)%
           (! 8.5  179 140 div 6 \forskel mul add)%
           (! 8.5  284 140 div 5 \forskel mul add)%
%           (!12    284 140 div 5 \forskel mul add)
%    \psline(! 8.5  214 140 div 5 \forskel mul add)%
%           (!12    214 140 div 5 \forskel mul add)
    \psline(!8.5 284 140 div 5 \forskel mul add)%
           (!8.5 214 140 div 5 \forskel mul add)
    % Nederste udtag.
    \psarc(6.5,1){0.25}{180}{270}
    \psline(6.5,0.75)(12,0.75)
    \psline(12,0.25)(6.5,0.25)
    \psarc(6.5,1){0.75}{180}{270}
    \psline(!8.5 214 140 div)(8.5,1)(6.25,1)
    \rput(9,0.5){\small Destillationsrester}
    \rput(11.5,0.5){\small $\rightarrow$}
    \psline(5.75,1)(3.5,1)(3.5,2.4)
  \end{pspicture}
\end{center}
\vfill

\end{document}

output

Best Answer

The "easiest" way is to create a \pscustom path containing the entire tower+pipes structure and fill it using pst-slpe's slope fill.

Below I only show a portion of the custom path, leaving out some of the pipes. However, the procedure would be the same. Also, the fill is actually solid red, with a fading applied at a 90 degree angle:

enter image description here

\documentclass[
  a4paper,
  12pt
]{article}

\usepackage[utf8]{inputenc}
\usepackage[
  hmargin=2.4cm,
  vmargin=3cm
]{geometry}
\usepackage{rotating}
\usepackage{pstricks-add,pst-slpe}

\newcommand*\forskel{350 140 div }
\newcommand*\udtag[2]{\rput(!10   249 140 div #1 \forskel mul add){\small #2}}
\newcommand*\pil[1]{  \rput(!11.5 249 140 div #1 \forskel mul add){\small $\rightarrow$}}
\newcommand*\destillering[1]{%
  % Ventiler og udtag.
  \psline(!4    2.8  #1 \forskel mul add)%
         (!4    1.8  #1 \forskel mul add)%
         (!4    2.3  #1 \forskel mul add)%
         (!5.3  2.3  #1 \forskel mul add)%
         (!5.3  2.65 #1 \forskel mul add)
  \psline(!5.15 2.5  #1 \forskel mul add)%
         (!5.15 2.8  #1 \forskel mul add)%
         (!5.95 2.8  #1 \forskel mul add)%
         (!5.95 2.5  #1 \forskel mul add)
  \psline(!5.8  2.65 #1 \forskel mul add)%
         (!5.8  2.3  #1 \forskel mul add)%
         (!6.3  2.3  #1 \forskel mul add)%
         (!6.3  2.65 #1 \forskel mul add)
  \psline(!6.15 2.5  #1 \forskel mul add)%
         (!6.15 2.8  #1 \forskel mul add)%
         (!6.95 2.8  #1 \forskel mul add)%
         (!6.95 2.5  #1 \forskel mul add)
  \psline(!6.8  2.65 #1 \forskel mul add)%
         (!6.8  2.3  #1 \forskel mul add)%
         (!7.4  2.3  #1 \forskel mul add)
  \psline(!   8.5       284 140 div #1       \forskel mul add)%
         (!   8.5       214 140 div #1 1 add \forskel mul add)
  \psarc(! 1144 140 div   2.3       #1       \forskel mul add){0.27142857}{180}{270}
  \psarc(! 1144 140 div   2.3       #1       \forskel mul add){0.77142857}{180}{270}
  \psline(!   7.9         2.3       #1       \forskel mul add)%
         (!   8.5         2.3       #1       \forskel mul add)
  \psline(!1144 140 div 284 140 div #1       \forskel mul add)%
         (!  12         284 140 div #1       \forskel mul add)
  \psline(!  12         214 140 div #1       \forskel mul add)%
         (!2861 350 div 214 140 div #1       \forskel mul add)
  % `Mellemventiler'.
  \psline(!3.5  470 140 div #1 \forskel mul add)%
         (!5.3  470 140 div #1 \forskel mul add)%
         (!5.3  519 140 div #1 \forskel mul add)
  \psline(!5.15 498 140 div #1 \forskel mul add)%
         (!5.15 540 140 div #1 \forskel mul add)%
         (!5.95 540 140 div #1 \forskel mul add)%
         (!5.95 498 140 div #1 \forskel mul add)
  \psline(!5.8  519 140 div #1 \forskel mul add)%
         (!5.8  470 140 div #1 \forskel mul add)%
         (!6.3  470 140 div #1 \forskel mul add)%
         (!6.3  519 140 div #1 \forskel mul add)
  \psline(!6.15 498 140 div #1 \forskel mul add)%
         (!6.15 540 140 div #1 \forskel mul add)%
         (!6.95 540 140 div #1 \forskel mul add)%
         (!6.95 498 140 div #1 \forskel mul add)
  \psline(!6.8  519 140 div #1 \forskel mul add)%
         (!6.8  470 140 div #1 \forskel mul add)%
         (!8.1  470 140 div #1 \forskel mul add)%
         (!8.1  392 140 div #1 \forskel mul add)%
}

\pagestyle{empty}

\begin{document}

\vfill
\begin{figure}[htbp]
 \centering
 \psset{unit=1cm}
  \begin{pspicture}(-1.02,0.3)(12,17)
    % Opvarmningsovn.
    %\psgrid
    \psframe[linestyle=none,slopeangle=90,slopesteps=50,
      fillstyle=slope,slopebegin=red,slopeend=yellow
      ](0.7,2)(3.3,5)% Ovn
    \pscustom[linestyle=none,slopeangle=90,slopesteps=200,
      fillstyle=slope,slopebegin=red,slopeend=yellow
      ]{
      \psline(12,0.25)(6.5,0.25)
      \psarcn(6.5,1){0.75}{270}{180}
      \psline(5.8,1)(3.5,1)(3.5,2.4)(1.7,2.4)
      \psarcn(1.7,3.1){0.7}{-90}{90}
      \psline(1.7,3.8)(2.3,3.8)
      \psarc(2.3,4){0.2}{-90}{90}
      \psline(2.3,4.2)(-0.8,4.2)(-0.8,4.7)(2.3,4.7)
      \psarcn(2.3,4){0.7}{90}{-90}
      \psline(2.3,3.3)(1.7,3.3)
      \psarc(1.7,3.1){0.2}{90}{-90}
      \psline(1.7,2.9)(3.5,2.9)(3.5,16.3)(8.5,16.3)
        (8.5,12)(12,12)(12,11.5)(8.5,11.5)% Nafta
        (8.5,9.5)(12,9.5)(12,9)(8.5,9)% Petroleum
        (8.5,7)(12,7)(12,6.5)(8.5,6.5)% Dieselolie
        (8.5,4.5)(12,4.5)(12,4)(8.5,4)% Smoreolie
        (8.5,2)(12,2)(12,1.5)(8.5,1.5)% Braendselolie
        (8.5,1)(6.3,1)
      \psarc(6.5,1){0.25}{180}{270}
      \psline(6.5,0.75)(12,0.75)(12,0.25)
    }
    \psline(0.7,4.7)(0.7,5)(3.3,5)(3.3,2.9)
    \psline(3.3,2.4)(3.3,2)(0.7,2)(0.7,4.2)
    \psline(-0.8,4.7)(2.3,4.7)
    \psline(-0.8,4.2)(2.3,4.2)
    \psarc(2.3,4){0.2}{270}{90}
    \psarc(2.3,4){0.7}{270}{90}
    \psline(2.3,3.8)(1.7,3.8)
    \psline(2.3,3.3)(1.7,3.3)
    \psarc(1.7,3.1){0.2}{90}{270}
    \psarc(1.7,3.1){0.7}{90}{270}
    \psline(1.7,2.9)(3.5,2.9)
    \psline(1.7,2.4)(3.5,2.4)
    \rput(2,1.75){\small Ovn}
    \rput(1.15,4.45){\small `Kold' råolie}
    \rput(-0.4,4.45){\small $\rightarrow$}
    % Tårnets indre samt udtag.
    \multido{\i=0+1}{5}{%
      \destillering{\i}%
      \pil{\i}%
    }
    \pil{6}
    \udtag{0}{Brændselsolie}
    \udtag{1}{Smøreolie}
    \udtag{2}{Dieselolie}
    \udtag{3}{Petroleum}
    \udtag{4}{Nafta}
    \udtag{6}{Gasser}
    % Tårnets ydre.
    \psline(  3.5,   2.9)%
           (! 3.5  179 140 div 6 \forskel mul add)%
           (! 5.75 179 140 div 6 \forskel mul add)
    \psarc(!  6.5  179 140 div 6 \forskel mul add){0.25}{90}{180}
    \psline(! 6.5  214 140 div 6 \forskel mul add)%
           (!12    214 140 div 6 \forskel mul add)
    \psarc(!  6.5  179 140 div 6 \forskel mul add){0.75}{90}{180}
    \psline(! 6.5  284 140 div 6 \forskel mul add)%
           (!12    284 140 div 6 \forskel mul add)
    \psline(! 6.25 179 140 div 6 \forskel mul add)%
           (! 8.5  179 140 div 6 \forskel mul add)%
           (! 8.5  284 140 div 5 \forskel mul add)%
%           (!12    284 140 div 5 \forskel mul add)
%    \psline(! 8.5  214 140 div 5 \forskel mul add)%
%           (!12    214 140 div 5 \forskel mul add)
    \psline(!8.5 284 140 div 5 \forskel mul add)%
           (!8.5 214 140 div 5 \forskel mul add)
    \psarc(6.5,1){0.25}{180}{270}
    \psline(6.5,0.75)(12,0.75)
    \psarc(6.5,1){0.75}{180}{270}
    \psline(6.5,0.25)(12,0.25)
    \psline(!8.5 214 140 div)(8.5,1)(6.25,1)
    \rput(9,0.5){\small Destillationsrester}
    \rput(11.5,0.5){\small $\rightarrow$}
    \psline(5.75,1)(3.5,1)(3.5,2.4)
  \end{pspicture}
\end{figure}
\vfill

\end{document}

You draw the fill first, and then overlay the rest of the construction as-is. The alignment along the side tubes are not 100% on, but that's because I don't fully understand where they're drawn within the mix of coordinates.

I've increased the number of steps used the define the slope colours.

plotpoints

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

[Tex/LaTex] Add gradient to colorbox

One option using the tcolorbox package to define a color box with a vertical shading; since the class uses titlesec to format the titles, the necessary redefinition for sections is quite simple

\titleformat{\section}
  {\large\sffamily\bfseries\filcenter}
  {}
  {0em}
  {\begin{mybox}[center upper,colupper=color1]\arabic{section}.~#1\end{mybox}}
  []
\titleformat{name=\section,numberless}
  {\large\sffamily\bfseries\filcenter}
  {}
  {0em}
  {\begin{mybox}[center upper,colupper=color1]#1\end{mybox}}
  []

where mybox is the newly defined box; a redefinition of \maketitle is also required to use the shading for the box.

Change the colors for the shading (inside the \fill in the definition of mybox) according to your liking:

\documentclass[fleqn,10pt]{SelfArx}
\usepackage[nopar]{lipsum}
\usepackage[many]{tcolorbox}

\setlength{\columnsep}{0.55cm}
\setlength{\fboxrule}{0.75pt}

\definecolor{color1}{RGB}{0,0,90} % Color of the article title and sections
\definecolor{color2}{RGB}{0,20,20} % Color of the boxes behind the abstract and headings

\usepackage{hyperref}
\hypersetup{hidelinks,colorlinks,breaklinks=true,urlcolor=color2,citecolor=color1,linkcolor=color1,bookmarksopen=false,pdftitle={Title},pdfauthor={Author}}

\JournalInfo{Journal, Vol. XXI, No. 1, 1-5, 2013}
\Archive{Additional note}

\PaperTitle{Article Title}

\Authors{John Smith\textsuperscript{1}*, James Smith\textsuperscript{2}}
\affiliation{\textsuperscript{1}\textit{Department of Biology, University of Examples, London, United Kingdom}}
\affiliation{\textsuperscript{2}\textit{Department of Chemistry, University of Examples, London, United Kingdom}}
\affiliation{*\textbf{Corresponding author}: john@smith.com}

\Keywords{Keyword1 --- Keyword2 --- Keyword3}
\newcommand{\keywordname}{Keywords}

\Abstract{\lipsum[1]}

\newtcolorbox{mybox}[1][]{
  freelance,
  frame code={},
  interior code={
    \fill[top color=color2!45,bottom color=color2!5,middle color=color2!15]
      (interior.north west) rectangle (interior.south east);
  }
  outer arc=0pt,
  arc=0pt,
  boxrule=0pt,
  nobeforeafter,
  left=0pt,
  right=0pt,
  top=0pt,bottom=0pt,
  #1
}

\titleformat{\section}
  {\large\sffamily\bfseries\filcenter}
  {}
  {0em}
  {\begin{mybox}[center upper,colupper=color1]\arabic{section}.~#1\end{mybox}}
  []
\titleformat{name=\section,numberless}
  {\large\sffamily\bfseries\filcenter}
  {}
  {0em}
  {\begin{mybox}[center upper,colupper=color1]#1\end{mybox}}
  []

\makeatletter
\renewcommand{\@maketitle}{%
\twocolumn[{%
\thispagestyle{empty}%
\vskip-36pt%
{\raggedleft\small\sffamily\bfseries\@JournalInfo\\\@Archive\par}%
\vskip20pt%
{\raggedright\color{color1}\sffamily\bfseries\fontsize{20}{25}\selectfont \@PaperTitle\par}%
\vskip10pt%
{\raggedright\color{color1}\sffamily\fontsize{12}{16}\selectfont  \@Authors\par}%
\vskip18pt%
\fcolorbox{color1}{white}{%
\parbox{\textwidth-2\fboxsep-2\fboxrule}{\centering%
\begin{mybox}
\ifx\@Keywords\@empty%
\sffamily\textbf{\abstractname}\\\@Abstract%
\else%
\sffamily\textbf{\abstractname}\\\@Abstract\\[4pt]%
\textbf{\keywordname}\\\@Keywords%
\fi%
\end{mybox}
\vskip4pt%
\begingroup%
\raggedright\sffamily\small%
\footnotesize\@affiliation\par%
\endgroup%%
}%
}%
\vskip25pt%
}]%
}%
\makeatother

\begin{document}

\flushbottom
\maketitle
%\tableofcontents
%\thispagestyle{empty}

\section*{Introduction}

%\addcontentsline{toc}{section}{Introduction}

\lipsum[1-3]
 and some mathematics $\cos\pi=-1$ and $\alpha$ in the text\footnote{And some mathematics $\cos\pi=-1$ and $\alpha$ in the text.}.

\section{Methods}

\lipsum[4]

\end{document}

enter image description here

Best Answer

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[Tex/LaTex] Color gradient filling along a path

plotpoints

[Tex/LaTex] Add gradient to colorbox

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