The convertvp tool is used internally by Squish to handle image verifications. However, some customers have found one of its facilities useful: its ability to take two images and produce a third image which is a diff (shows the difference) of the other two. When used with two identical images, it simply produces a plain white image the same size as the two images it was given.

Here is an example of it in use:

convertvp --diff screenshot1.jpg screenshot2.jpg diff.png

The tool takes four arguments: --diff, followed by the names of the two images files that are to be compared, followed by the name of the resultant image file. The filenames may include paths. The program supports all the image formats that the underlying Qt library supports, which at a minimum includes, BMP, JPG, PNG, PPM, TIFF, XBM, and XPM.

The tool has several other options, but the only one likely to be of use is --topng, which converts an image from one format to another. Use convertvp -h to get a list of the commands and their arguments.