How does the Piwik image tracker know full page URL?

This is probably a very basic question, but I thought it might help me solve this.

I could be wrong, but from what I’ve read it does appear that the Piwik image tracker is able to figure out the full URL of the page that is being visited (i.e. not just that a page from has been loaded but that was loaded).

Where does Piwik get that information from? Is it in a header that’s sent along when the image is loaded - and if so, how reliable is this (do some browsers not include that information?).