Abstract: A desktop personal computer 410 is provided with a system for building custom applications. A set of filters 3 is assembled by the user in an operating environment 4 by selection from a library 5 of available filters, each filter performing a respective processing task on data objects which may be input from a data memory 6. An output of the system is processed by a user interface controller 10 for display on a display screen 20 in which data is displayed in a data display area 21, the sequence of filters being represented graphically by a stack of filter boxes 24 in a filter display window 23, and the selection of filters from the library 5 being facilitated by a filter selector window 25. Each of the filters is controllable by the user interface provided within a respective filter box 24, data and user interface description objects being communicated by respective channels 811 and 88 connecting the filters in sequence.