Tikz Pattern lines are not exactly straight

I do have a short problem with Tikz

\documentclass{article}
\usepackage{pgf,tikz}
\usetikzlibrary{patterns}
\begin{document}
\begin{tikzpicture}
\fill[pattern=north east lines] (0,0) -- (0,1) -- (1,1) -- (1,0) -- cycle;
\end{tikzpicture}
\end{document}


I simply want to fill differently shaped areas with north east lines. Which is done as is it's supposed to be. But the problem i can not figure out is why they are not exactly straight. Certainly nobody would notice in print but in the pdf it just looks distorted. Is there a way to improve this?

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This is yet another side effect of the PDF-viewer, has nothing to do with TikZ or pstricks if that is used. Try zoom in and see if it disappears? It does in my case. – zeroth Apr 7 '12 at 9:43
– JohnReed Jun 7 '12 at 0:46