Why does viewport produce a strange result

I use pdflatex to compile the following input file:

\documentclass{article}
\usepackage{graphicx}
\begin{document}
\fbox{\includegraphics[viewport=10 10 20 20]{bald.png}}
\end{document}


The image to be imported is:

But the resulting PDF looks very strange as follows:

Note: the gray frame is Adobe Reader background, it is not the part of PDF output.

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you'd forgotten to clip

\fbox{\includegraphics[viewport=10 10 20 20,clip]{bald.png}}

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Looks quite normal. You are setting a small viewport, so for latex the graphic is only 10x10pt large. The rest of the graphics "sticks" out. Use clip if you want to see the part set by viewport.

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