The WebKit Plugin provides an API for incorporating Quartz Composer presentations into Dashboard Widgets
, it is also supported in Safari
The simplest and most straightfoward of all the tutorials is Apple's own about enhancing Dashboard Widgets with Quartz Composer
The WebKit API for Quartz Composer plug in
also allows for some simple playback controls to be run upon .qtz files run in the browser.
See the Apple Developer links below for further information.
From dedicated presentations, browser presentations, interface designs, application specific facilities, or system wide services, Quartz Composer's graphical ubiquity seems to never end.
Currently the lack of graphical processing power and necessary Quartz Extreme frameworks limits the capabilities of the iPhone, iTouch and iPad to properly render complex, shader dependent Quartz Composer compostitions, although with each succesive iPhone OS update and improvements upon the chipset employed within the iPhone this will, no doubt, change.