A secure digital display data compositor apparatus for simultaneous display of different classes of display data (104, 106, 107), each class of display data corresponding to a unique independent computing domain (101, 102, 103), comprising: a plurality of digital display data input interfaces (311/321, 312/322, 313/323 and 208) for receiving digital display data, each interface coupled to a respective single independent computing domain (101, 102, 103) and each digital display data input interface configured to receive a digital data stream comprising the digital display data from the respective independent computing domain; a digital display data compositor, that is configured to: identify predetermined regions of the digital display data for each independent computing domain by identifying an in-band signal in the digital display data from each independent computing domain, wherein each respective in-band signal defines the respective predetermined regions of the digital display data of the respective independent computing domain; define an ordering on the identified predetermined regions; associate a predetermined display characteristic with each identified predetermined region; and composite, according to the defined ordering, the identified predetermined regions and associated display characteristics from each of the independent computing domains into a single digital display data set, and a digital display data output interface (317/307) for outputting the composited digital display data. a plurality of digital display data input interfaces (311/321, 312/322, 313/323 and 208) for receiving digital display data, each interface coupled to a respective single independent computing domain (101, 102, 103) and each digital display data input interface configured to receive a digital data stream comprising the digital display data from the respective independent computing domain; a digital display data compositor, that is configured to: identify predetermined regions of the digital display data for each independent computing domain by identifying an in-band signal in the digital display data from each independent computing domain, wherein each respective in-band signal defines the respective predetermined regions of the digital display data of the respective independent computing domain; define an ordering on the identified predetermined regions; associate a predetermined display characteristic with each identified predetermined region; and composite, according to the defined ordering, the identified predetermined regions and associated display characteristics from each of the independent computing domains into a single digital display data set, and identify predetermined regions of the digital display data for each independent computing domain by identifying an in-band signal in the digital display data from each independent computing domain, wherein each respective in-band signal defines the respective predetermined regions of the digital display data of the respective independent computing domain; define an ordering on the identified predetermined regions; associate a predetermined display characteristic with each identified predetermined region; and composite, according to the defined ordering, the identified predetermined regions and associated display characteristics from each of the independent computing domains into a single digital display data set, and a digital display data output interface (317/307) for outputting the composited digital display data.