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Next: Other Specializations Up: NOTE 216 - Lattice Expression Language Implementation Previous: Optimizations

Subsections

Relational and Logical Expressions

Relational Expressions

So far in the discussion, it has been assumed that the result of all expressions was either a numeric array or scalar. However, we also want to be able to handle operations which result in Booleans. For example, consider Lattices

   Lattice<Float> a;
   Lattice<Float> b;
   Lattice<Bool> c;

and the expression

   c.copyData(a>b);

so the Bool Lattice c is True or False depending upon whether the data values of a were greater than those of b or not.

What has to be handled here is that the output of the > operation is Boolean, whereas the type of the data which went into the operation was Float.

This relational operator, and like ones (<, > =, < =, = =, !=) are handled in the class LELBinaryCmp. It is templated on class T but inherits from LELInterface<Bool> rather than LELInterface<T>.

The LELInterface class eval function is declared as

   // Evaluate the expression and fill the result array
      virtual void eval (Array<T>& result,
                         const PixelRegion& region) const = 0;

This indicates that the array, result, which results from evaluating the expression is of type T. Since LELBinaryCmp inherits from LELInterface<Bool>, the type of its evaluation array, result, is Bool. This is just what we want. The result of b>c is a Bool array.

The LELBinaryCmp class itself is still templated in class T because that is the type of the Lattices that go into it.

Logical Expressions

Take as an example,

   Lattice<Bool> a;
   Lattice<Bool> b;
   Lattice<Bool> c;
   c.copyData(a&&b);

so the Bool Lattice c is True if Lattice a and b are True. This kind of operator can only be defined for Boolean Lattices. Therefore the class LELBinaryBool is not templated and inherits from LELInterface<Bool>. If the data types of the Lattices are not Bool it will throw an exception.

Similarly, the class LELUnaryBool exists to handle unary logical operations such as

   c.copyData(!a)

Next: Other Specializations Up: NOTE 216 - Lattice Expression Language Implementation Previous: Optimizations