Modeling expressions are similar to numeric expressions but have some special extensions and limitations. The special extensions include the ability to use channels in the expressions and a number of special modeling variables.
To understand the limitations of modeling expressions it can be helpful to realize that modeling expressions are evaluated in "chunks" over the whole requested region. Each operation in the expression is done over a chunk, taking zero or more blocks of image data as input and creating an output block of image data. This means that modeling expressions cannot generally be used in place of numeric expressions unless the function treats modeling values specially.
Nevertheless, use of modeling expressions is fairly intuitive when used in simple assignment (LET) statements.
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