The BLFEI operator algorithm computes the Built-Up Land Features Extraction Index from a given product.

The BLFEI results from the following equation:

BLFEI = (((G + R + S2) / 3.0) - S1) / (((G + R + S2) / 3.0) + S1)

, where:

The bands are:

- G is the Green band
- R is the Red band
- S1 is the SWIR 1 band
- S2 is the SWIR 2 band

Also the processor computes an additional flags band called 'blfei_flags' with the following bit coding:

Bit Position	Description
Bit 0	The computed value for BLFEI is NAN or is Infinite
Bit 1	The computed value for BLFEI is less than -1 (minus one)
Bit 2	The computed value for BLFEI is greater than 1 (one)