Color gradient for \psline in PSTricks

Question

I'd like to produce a \psline (or, even better, \pscurve) in PSTricks that has a color gradient. I can't find a package that allows me to do so. Is there one? (pst-slpe produces sophisticated gradients for areas, but not, as far as I can see, for lines.)

(On the page https://comp.text.tex.narkive.com/tjLnOyJ4/applying-a-gradient-color-to-a-line Werner reports that he had asked the same question on the PSTricks mailing list in 2003 and received a response from Manuel Luque with the definition of a (complicated) function to create a parametric plot with a color gradient. It doesn't seem easy to adapt that code to \psline.)

Answer 1

Run with lualatex

\documentclass{standalone}
\usepackage{pst-hsb}
\begin{document}
\begin{pspicture}(-0.75,-0.75)(11,5)
% \psgrid(0,-4)(10,4)
\psaxes{->}(0,0)(11,5)

\pslineHSBlinewidth=1mm,HueBegin=0,HueEnd=0.5(10,1) 
\pslineHSBlinewidth=1mm,HueBegin=0.5,HueEnd=0.7(10,3) 
\pslineHSBlinewidth=1mm,HueBegin=0.8,HueEnd=1(10,5) 
\end{pspicture}
\end{document}

Or only for two given colors. Maybe that it will be part of an update of PSTricks ...

\documentclass{article}
\usepackage{pstricks-add}
\makeatletter
\define@key[psset]{pstricks}{startColor}[black]{%
  \colorlet{tempcolor}[rgb]{#1}%   we need rgb
  \pst@getcolor{tempcolor}\ps@startColor}
\define@key[psset]{pstricks}{endColor}[white]{%
  \colorlet{tempcolor}[rgb]{#1}%
  \pst@getcolor{tempcolor}\ps@endColor}
\psset[pstricks]{startColor=black,endColor=white}
\def\pscolorLine{\def\pst@par{}\pst@object{pscolorLine}}
\def\pscolorLine@i{%
  \pst@getarrows{%
    \begin@OpenObj
    \pscolorLine@ii}}
\def\pscolorLine@ii(#1){@ifnextchar({\pscolorLine@iii(#1)}{\pscolorLine@iii(0,0)(#1)}}%
\def\pscolorLine@iii(#1,#2)(#3,#4){%
  \use@par
  \ifx\psk@arrowA@empty\else\psline[linecolor=\ps@startColor]{->}(!#3 0.5 mul #4 0.5 mul)(#1,#2)\fi
  \ifx\psk@arrowB@empty\else\psline[linecolor=\ps@endColor]{->}(!#1 0.5 mul #2 0.5 mul)(#3,#4)\fi
  \pst@getcoor{#1,#2}\pst@tempA
  \pst@getcoor{#3,#4}\pst@tempB
  \addto@pscode{
    10 dict begin
    [ \psgetRGBColorValues{\ps@startColor} ] dup == aload length 1 eq { dup dup } if 
      3 copy 3 array astore /startColor ED
    [ \psgetRGBColorValues{\ps@endColor} ] dup == aload length 1 eq { dup dup } if 
      3 copy 3 array astore /endColor ED
%   on stack R1 G1 B1 R2 G2 B2 
    4 -1 roll               % R1 G1 R2 G2 B2 B1
    sub 256 div /dB ED      % R1 G1 R2 G2 
    3 -1 roll               % R1 R2 G2 G1 
    sub 256 div /dG ED      % R1 R2 
    exch sub 256 div /dR ED
    \pst@tempB /Y2 ED /X2 ED 
    \pst@tempA /Y1 ED /X1 ED 
    Y2 Y1 sub X2 X1 sub atan /Angle ED
    Y2 Y1 sub Angle sin dup 0 eq { pop }{ div } ifelse /L_line ED     % line length
    \ifx\psk@arrowA@empty\else 
      X2 X1 sub L_line div \psk@arrowlength\space mul \pst@number\psxunit mul X1 add /X1 ED
      Y2 Y1 sub L_line div \psk@arrowlength\space mul \pst@number\psyunit mul Y1 add /Y1 ED
     \fi
    \ifx\psk@arrowB@empty\else 
      X2 X1 sub L_line div \psk@arrowlength\space mul \pst@number\psxunit mul X2 exch sub /X2 ED
      Y2 Y1 sub L_line div \psk@arrowlength\space mul \pst@number\psyunit mul Y2 exch sub /Y2 ED
     \fi
    Y2 Y1 sub 256 div /dY ED
    X2 X1 sub 256 div /dX ED
%    dR dG dB == == ==
%startColor == endColor ==
    \ifPst@noCurrentPoint\else\pst@cp\fi      % current point?
    0 1 255 {

      /dIndex ED
%startColor == 
      X1 Y1 moveto
      X1 dX add  Y1 dY add % start for coordinate array
      gsave
      lineto
      CP /Y1 ED /X1 ED
      \pst@number\pslinewidth SLW
      startColor aload pop 3 copy setrgbcolor
      % on stack r g b
      dB add 3 1 roll    % b+dB r g 
      dG add 3 1 roll    % b+dB g+dG r
      dR add 3 1 roll    % b+dB g+dG r+dR
      3 array astore /startColor ED
      stroke
      grestore
    } for 
    end
  }%
  \end@OpenObj
  \ignorespaces
}
\makeatother
\begin{document}
\begin{pspicture}linewidth=12pt,showgrid(5,5)
\pscolorLine(0,-5)(0,5)
\pscolorLinestartColor=cyan,endColor=magenta(5,0)
\pscolorLine[startColor=blue,endColor=green]{->}(-5,5)(5,-5)
\pscolorLine[startColor=red,endColor=blue]{<->}(-5,-5)(5,5)
\end{pspicture}
\end{document}

Answer 2

While you are waiting for PStricks help, here is my attempt at a Metapost solution, for comparison.

\documentclass[border=5mm]{standalone}
\usepackage{luamplib}
\begin{document}
\mplibtextextlabel{enable}
\begin{mplibcode}
vardef cgdraw(expr P, a, b) = 
    save s, n; 
    numeric s, n;
    4s = xpart urcorner currentpen - xpart llcorner currentpen;
    n = arclength P / s;
    for i=0 upto n:
        draw point arctime s * i of P of P withcolor (i/n)[a, b];
    endfor
enddef;
beginfig(1);
    path p; p = origin .. (89, 34) .. (144, -13) .. (200, 0);
    cgdraw(p, red, blue);
endfig;
\end{mplibcode}
\end{document}

This is wrapped in luamplib, so please compile it with lualatex.

It works simply by drawing dots along the path. You can use the same approach to taper the width of the line, for example by changing the line in the loop to

draw point arctime s * i of P of P withpen currentpen scaled (1-i/n);

which would give you this

• Note that draw with a single point in MP just draws a dot at the point using the current pen. (Same as drawdot).