% worm.m4
%
define(`setfillcolour',`ifdef(`pstricks_',
  `m4v = max(max(`$1',`$2'),`$3')
  command sprintf("\definecolor{fcolor}{rgb}{%0.4f, %0.4f, %0.4f}",\
    (`$1')/m4v, (`$2')/m4v, (`$3')/m4v)
  \pscustom[fillcolor=fcolor,fillstyle=solid]{',
  `ifdef(`gpic_',`m4v = sqrt((`$1')^2+(`$2')^2+(`$3')^2)/sqrt(3)
  command sprintf("\special{sh %g}",1-m4v)')')
  ')
%
.PS
  linethick_(0)
  dimen = 1.25       # scale parameter; try dimen=2 for a larger picture
  r = 0.15 * dimen   # worm radius
  S: dimen,0
  for i = 1 to 360 do {
    R: dimen*cosd(3*i), dimen*sind(2*i)
    D: R-(S.x,S.y)
    h = sqrt(max(0,r^2/(D.x^2+D.y^2)-1/4))
    Ra: 0.5<S,R> + (h*D.y,-h*D.x)
    Rb: 0.5<S,R> - (h*D.y,-h*D.x)
    setfillcolour( (1+sind(2*i))/2, (1+sind(i))/2, (1+sind(3*i))/2 )
    arc  cw from Ra to Rb rad r with .c at S
    arc ccw from Rb to Ra rad r with .c at R
    ifdef(`pstricks_',`\relax}')
    S:R
    }
.PE