I have been trying to use the ppi tool on a coastal image which has the water masked out. I continually get an output which highlights all the masked pixels (which have a value of zero) as spectrally pure when I thought it would ignore these pixels altogether. Someone told me to try and mask out all the NaN values but when I run that  it masks out the entire image as opposed to just the blacked out areas. Does anyone know how to get the ppi tool to ignore these masked zero values? Any info would be greatly appreciated as this dilemma is holding up my thesis research. Thanks