shadow effect with coons patch shading

shadow.tex

\documentclass{article}
\usepackage{luamplib}
\pagestyle{empty}
\begin{document}
\mpfig*
  numeric sh, wd, ht;
  sh = 6; wd = 200; ht = 100;
  path p[];
  p1 = unitsquare xscaled wd yscaled ht;
  p2 = unitsquare xscaled (wd-sh) yscaled sh shifted(sh,-sh);
  p3 = unitsquare xscaled sh yscaled (ht-sh) shifted(wd,0);
  p4 = (origin -- subpath (4,6) of fullcircle -- cycle) scaled 2sh shifted(sh,0);
  p5 = (origin -- subpath (6,8) of fullcircle -- cycle) scaled 2sh shifted(wd,0);
  p6 = (origin -- subpath (0,2) of fullcircle -- cycle) scaled 2sh shifted(wd,ht-sh);
\endmpfig
\mplibgroup{mymask}[asgroup="masking"]
\mpfig
  fill boundingbox(p4--p6)
    withshadingmethod "coons"
    withcoonspatchinit(p2, 0, 0, 1/2, 1/2)
    withcoonspatchinit(p3, 1/2, 0, 0, 1/2)
    withcoonspatchinit(p4, 1/2, 0, 0, 0)
    withcoonspatchinit(p5, 1/2, 0, 0, 0)
    withcoonspatchinit(p6, 1/2, 0, 0, 0)
    ;
\endmpfig
\endmplibgroup
\mpfig
  fill p1 withcolor 1/2[red,white];
  fill boundingbox(p4--p6) withmaskinggroup "mymask";
\endmpfig
\end{document}