[Dvipng] one pixel added to top of image?

[jw]

Hello,
I have this simple .tex file:

% begin file

\nopagenumbers
\long\def\leftrule#1{\line{\vrule width 5pt\kern-5pt\vbox to\vsize{#1}}}

\leftrule{hello, how are you?

I am doing well, thanks.}
\bye

% end file

When I run this through dvipng, I notice the vrule at the left starts
one pixel down from the top.
It is always one pixel, regardless of resolution.
dvips does not seem to have the same behavior - is this a bug, or do I
miss something?

-John