\documentclass[a4paper]{article}

\usepackage{pgfplots}
\usepgfplotslibrary{patchplots}

\pgfplotsset{compat=1.8}


\begin{document}

%\tracingcommands=2\tracingmacros=2
\fbox{%
	\begin{tikzpicture}
	\begin{axis}[
		hide axis,
	]
	\addplot3[patch,patch type=biquadratic,shader=interp]
	coordinates {
		(0,0,1) (6,1,0) (5,5,0) (-1,5,0)
		(3,1,0) (6,3,0) (2,6,0) (0,3,0)
		(3,3.75,0)
	};
	\end{axis}
	\end{tikzpicture}%
}



\end{document}

