I'm using graphicsmagick to trim some PNG images with transparent backgrounds. I need to know what the offset of the new image is compared with the old one so I can reconstruct it later.

For example, take this image:
Code:
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==*==*==
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=*====*=
==*==*==
===**===
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Then, graphicsmagick trims it to this:
Code:
=*==*=
======
*====*
=*==*=
==**==
How can I get graphicsmagick to tell me that there's now an offset of 3 pixels for y and 1 pixel for x?