tv_diff family contains few plugins which behave bit differently. All of them show somehow changes to reference picture. Most of the plugins use reference picture taken at lebiniou start time.
tv_diff is oldest of plugins. By default it works in overlay mode. It takes reference picture in plugin start, and shows all pixels that differ. Due to webcam inaccuracy, it tends to show all pixels, and overlay tends to cover everything. Plugin uses difference as color value, and therefore tends to show pretty dim values.
tv_diff2, tv_diff3, tv_diff4 all work as default in overlay + lens mode. They all have some tolerance, so that pixels which have small difference are not shown. All behave similar way in sense what pixels are shown and what are used from underlay.
tv_diff2 Uses pixel color value as is.
tv_diff3 Uses absolute value of difference, and therefore has lower color values, just like original tv_diff.
tv_diff4 Calculates pixel values by calculating difference of underlay color and webcam pixel. Therefore it creates kind of translucent pixels, so that picture is showing underlay and overlay same time.
tv_diffbeat is like tv_diff2 but it reacts to beat. On beat, difference required for showing pixel is smaller, and more pixels are show.
To start experimenting with tv_diff plugins we suggest to start plugin over the fadeout plugin so that you see what kind of picture it creates. Note that reference picture for tv_diff2-4 is taken at lebiniou start time, so one pretty obvious use for plugin is start lebiniou with empty venue, and after lebiniou start, ideally plugins will show only artist(s).
All blur type plugins are useless with most of the tv_* plugins.