Defining a Filter

A filter is constructed from one or more clauses: instructions to compare a particular attribute of a file with the provided values. When Opus applies a filter to an operation, every potential file that the operation could affect is compared against the clauses in the filter, and the file is only processed if it matches the filter.

Filters are effectively Boolean operations expressed in human-readable form.

• Individual clauses can be set to Match (true) or No Match (false), which give the effect of a not operator.
• Clauses are linked with the Boolean operators and or or.
• A filter can also have sub-clauses, which give the effect of parentheses in Boolean operations.

For example, consider the following Boolean expression. The effect of this is to match all .jpg files that are larger than 100KB or all .gif files that are smaller than 50KB (quite why you'd want to do that is another question):

    ( Name matches *.jpg and size > 100KB ) or

    ( Name matches *.gif and size < 50KB )

This would be repesented in a filter control by the following clauses:

The two expressions enclosed in parentheses are represented by sub-clauses. Each sub-clause contains two clauses linked by and operators; a Name match which compares the filename against a wildcard pattern, and a Size match which compares the size of the file against a given value. Finally the two sub-clauses are linked by an or operator. You can see that in the filter control the contents of sub-clauses are indented from their parent clause.

In the above example, all the clauses are set to Match which means the condition in the clause must be true for it to match. If a clause is set to No Match then it must be false for the clause to match. Imagine if we wanted to match all .jpg files that are larger than 100KB, but not those that are larger than 1MB or have the word "horse" in their name. The human-readable form of this would be:

    Name matches *.jpg and size > 100KB and
    not ( size > 1MB or name contains "horse" )

In the filter control you could represent this in the following way:

The sub-clause in the above example is set to No Match meaning the filter won't match a file unless sub-clause fails to match. The sub-clause contains two child clauses, both of which are set to Match and linked by the or operator. So for the file in question, if its Size is greater than 1 MB or its Name contains "horse", one or other (or both) of those clauses will match, which means its parent sub-clause will match, which means (because the sub-clause was set to No Match) the filter will fail. Confused yet? :)