In imaging applications, input is usually image channels containing raw imagery. In GIS applications, input usually is attribute data and image channels containing data `layers'. Layers may include raw imagery, elevation, classification themes, digitized maps, and proximity maps.
A single PCIDSK database (FILE) is assumed to contain all the image channels/layers. Attribute data may be held in text files or as segments in the PCIDSK file. The text describing the model (SOURCE) is read in and the equations are applied to each pixel in the PCIDSK database. If the model specifically generates a report, then this is sent to the current report file (REPORT). Since imagery is often of unpredictable quality, it is possible to specify the results of undefined operations (UNDEFVAL) rather than have the MODEL program terminate immediately upon the detection of such an operation.