Hyperspectral Data Processing: Algorithm Design and Analysis

18.2 Four Types of Pixels

According to the definition given by Schowengerdt (1997), an endmember is an idealized pure signature for a spectral class. Therefore, an endmember is, in general, not a pixel. It is a spectral signature that is completely specified by the spectrum of a single-material substance. Accordingly, a pixel is pure if its spectral signature is an endmember. In other words, a pixel whose spectral signature has 100% of purity formed of a single-material substance is a pure pixel, which can also be referred to as an endmember pixel. In this chapter, the endmember pixel and pure pixel will be used interchangeably, whichever is more appropriate for explanation. To the contrary, a mixed pixel is a pixel whose spectral signature is not an endmember. Instead, it is composed of more than one material substance. With this interpretation, when an endmember extraction algorithm (EEA) is implemented, it is expected that signatures or pixels extracted are supposed to be pure signatures or pure pixels, that is, endmember pixels. But, is this really the case in practical applications? In addition to the pure pixel and mixed pixel described above, a concept of homogenous pixel is further introduced for pixel information analysis. A pixel is called a homogeneous pixel if its spectral signature is similar to the spectral signatures of the pixels in its surroundings subject to small deviations. In other words, all the pixels within a neighborhood of a homogenous pixel should have ...