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Expressions

A LEL expression can be as simple or as complex as one likes using the standard arithmetic, comparison, and logical operators. Parentheses can be used to group subexpressions.
The operands in an expression can be lattices, constants, functions, and condition masks. lattice regions and masks. E.g.

  lat1 + 10
  lat1 + 2 * max(lat2,1)
  amp(lat1, lat2)
  lat1 + mean(img[region1])
  lat1 + mean(lat2[lat2>5 && lat2<10])

The last example shows how a boolean expression can be used to form a mask on a lattice. Only the pixels fulfilling the boolean condition will be used when calculating the mean.

In general the result of a LEL expression is a lattice, but it can be a scalar too. If is is a scalar, it will be handled correctly by C++ and glish functions using it as the source in, say, an assignment to another lattice.

LEL fully supports masks. In most cases the mask of a subexpression is formed by and-ing the masks of its operands. It is fully explained in a later section.

LEL supports the following data types:

Bool

Float

single precision real (which includes integers)

Double

double precision real

Complex

single precision complex

DComplex

double precision complex

All these data types can be used for scalars and lattices.
LEL will do automatic data type promotion when needed. E.g. when a Double and a Complex are used in an operation, they will be promoted to DComplex. It is also possible to promote explicitly using the conversion functions (FLOAT, DOUBLE, COMPLEX and DCOMPLEX). These functions can also be used to demote a data type (e.g. convert from Double to Float), which can sometimes be useful for better performance.

Region is a specific data type. It indicates a region of any type (in pixel or world coordinates, relative, fractional). A region can only be applied to a lattice subexpression using operator [].