image masking with luamplib

image-mask.tex

\documentclass[a5paper]{article}
\usepackage{luamplib,graphicx}
\directlua{
  tex.print(-2,{
    token.create"graphicspath",
    token.new(0,1), token.new(0,1),
    kpse.var_value"TEXMFDIST", "/doc/generic/pgf/images/",
    token.new(0,2), token.new(0,2),
  })
}
\pagestyle{empty}
\begin{document}
\begin{mplibgroup}{mymask}[asgroup="masking"]
  %\mpfig
  %  draw btex 
  \includegraphics[decodearray={1 0},width=200bp]{brave-gnu-world-logo-mask}
  %  etex;
  %\endmpfig
\end{mplibgroup}
\mpfig
  picture pic;
  pic = btex \includegraphics[width=200bp]{brave-gnu-world-logo} etex;
  bboxmargin := 10;
  fill bbox pic
    withshadingmethod "linear"
    withshadingvector (3.2, 0)
    withshadingstep(
      withshadingfraction 2/4
      withshadingcolors (red,green)
    )
    withshadingstep(
      withshadingfraction 1
      withshadingcolors (green,blue)
    )
    ;
  draw pic
    withmaskinggroup "mymask"
    ;
\endmpfig
\end{document}