Operators

Operators are fields within an ARD description which specify an operation to perform on one or two keyword fields. They cannot be abbreviated, and the leading and trailing dots must be included. The following operators are supported (they are listed in order of decreasing precedence):

.NOT.

- Invert the region specified by the following keyword or expression (i.e. included pixels become excluded, and excluded pixels become included). A minus sign is recognised as a synonym for .NOT..

.AND.

- Take the intersection of the two regions. A pixel is included only if it is within both of the regions given on either side of the .AND. field.

.OR.

- Take the union of the two regions. A pixel is included if it is within either of the regions given on either side of the .OR. field.

.XOR.

- Take the exclusive OR of the two regions. A pixel is included if it is within one, but not both, of the regions given on either side of the .XOR. field.

.EQV.

- Take the equivalence of the two regions. A pixel is included if it is within both of the regions given on either side of the .EQV. field, or if it is within neither.

.XOR. and .EQV. have equal precedence. Opening and closing parentheses (``('' and ``)'') can be used to bracket sub-expressions within an ARD description.