SQUID, or Sam's Qt Undocumented Image Differ, is a simple program that allows you to quickly compare two image files. With three toggle-able panes, SQUID supports a variety of comparison methods, ranging from simple side-by-side to automatically identifying different areas and zooming in on them.
(If you like SQUID, check out CALAMARI, the newer version!)
SQUID supports all image formats directly supported by Qt - there are no plans at present to try to support other formats. Since SQUID is a Qt application, it can in theory be ported to other platforms by anyone with a Qt license and the inclination to do so.
SQUID was designed for use with Perforce, but can also be used standalone. SQUID lives in the Perforce Public Depot - source can be found there along with executables.
As implied by its name, SQUID is simple enough not to require full-fledged documentation, but brief installation and usage notes follow.
Windows installation:
Usage:
Contact me: spamwise at gmail dot com. Questions, suggestions, and feedback welcomed. Especially positive feedback.
Disclaimer: this tool is completely unsupported by Perforce Software, Inc.