A graphics system comprising a series of calculation units. The calculation units comprise a first subset and a second subset of calculation units. A first calculation unit of the series generates a first digital video stream and a second digital video stream. Each calculation unit of the first subset: (a) passes the second digital video stream to a next calculation unit of the series unmodified; and (b) computes first pixel values, injects or mixes the first pixel values into the first digital video stream, and passes the modified first digital video stream to the next calculation unit. Similarly, each calculation unit of the second subset injects or mixes second pixel values into the second digital video stream, and passes the first digital video stream unmodified. A last calculation unit of the series drives one or more display devices in response to the first and second digital video streams.