Interpretation of ARD Descriptions

After removal of all statement fields, and the insertion of any implicit .OR. operators, the fields in an ARD description are treated as a Fortran-like logical expression. Each keyword field forms a logical operand, acted upon by the adjoining operator fields. operators have their usual Fortran precedence (see Section ). The order of evaluation can be changed by enclosing sub-expressions within parentheses as usual. The interpretation of operands depends on the type of keyword:

• Operands for regions which (in general) have non-zero volume (e.g. POLYGON, CIRCLE, BOX, etc.) are .TRUE. if the centre of the current pixel lies on or within the boundary of the region, and are .FALSE. otherwise.

• Operands for regions which have zero volume (e.g. POINT, LINE, ROW, COLUMN, etc.) are .TRUE. if the boundary of the region passes through the current pixel. The pixel with index $I$ along some axis is assumed to cover a range of pixel coordinates $P$ given by $(I-1) < P \leq I$.