This content is quite old. You might want to check out this modern way of visual diffing, using Puppeteer instead
Wraith uses either PhantomJS, CasperJS or SlimerJS to create screen-shots of webpages on different environments and then creates a diff of the two images, the affected areas are highlighted in blue
By the folks over at the BBC. Video tutorial right here:
We were working on http://backtrac.io tool for visual testing. It has all the features of wraith plus many more (like excluding elements from pages, monitoring, authentication). Welcome to give it a try.