In a prior recipe, we used the expression-based map-algebra function ST_MapAlgebra() to convert the PRISM pixel values to their true values. The expression-based ST_MapAlgebra() method is easy to use, but limited to operating on at most two raster bands. This restricts the ST_MapAlgebra() function's usefulness for processes that require more than two input raster bands, such as the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI).

There is a variant of ST_MapAlgebra() designed to support an unlimited number of input raster bands. Instead of taking an expression, this ST_MapAlgebra() variant requires a callback function. This callback function is run for each set of input pixel values, and returns either a new pixel value, or NULL for the output pixel. Additionally, this variant of ST_MapAlgebra() permits operations on neighborhoods (sets of pixels around a center pixel).

PostGIS comes with a set of ready-to-use ST_MapAlgebra() callback functions. All of these functions are intended for neighborhood calculations, such as computing the average value of a neighborhood, or interpolating empty pixel values.