\documentclass{standalone}
\usepackage{graphicx}
\usepackage{tikz}
\usetikzlibrary{calc,fadings}
\tikzfading[name=fade left,
left color=transparent!100,
right color=transparent!0]
\tikzfading[name=fade right,
right color=transparent!100,
left color=transparent!0]
\tikzfading[name=fade down,
bottom color=transparent!100,
top color=transparent!0]
\tikzfading[name=fade up,
top color=transparent!100,
bottom color=transparent!0]