Subpixel analysis is a concept in the IDL-based product, ENVI, which is an elaborate GUI program frequently used for analysis of multi- and hyper-spectral images. In the "language" of ENVI, "subpixel analysis" would refer to looking at the spectral signature of a single pixel and finding in this signature the likely combination of the spectral signatures of multiple known materials. That is, the spectral signature of a single pixel might indicate that there is evidence of the presence of both gold and ... I don't know ... lead in that one pixel location.

Is this what you are referring to? If yes, you should take a close look at https://www.l3harrisgeospatial.com/Software-Technology/ENVI . This kind of analysis is very specialized, very application-specific, and is not based on a single generic IDL procedure or function.

James Jones