Image registration, also known as image alignment, is the process of transforming a given image to achieve a best match with another image. The need for such a tool is given in situations where you want to combine two or more images which are similar, but geometrically do not have a perfect match.
A typical use is the combination of 2 or 3 photos in HDR methods. Unless you use a stable tripod, the source images will always be slightly out of alignment, so the very first step will be to re-align the images. The alignment step is normally built into dedicated HDR software, but for doing simple HDR (using for example Gimp's layer masks), you'll need to align the source images yourself, and this is where this plug-in can be helpful.
For installing the plug-in, download the latest pre-built binaries for your system from
unpack it to some folder on your computer and add that folder to the list of directories where Gimp looks for installed plug-ins.Visit the installation page for more detils. This page also contains detailed information on building the plug-in from sources.
