Perpendicular Vegetation Index (PVI)
"The Perpendicular Vegetation Index (PVI) is similar to a difference vegetation index; however, it is sensitive to atmospheric variations. When using this method to compare different images, it should only be used on images that have been atmospherically corrected.
NIR = pixel values from the near-infrared band
Red = pixel values from the red band
a = slope of the soil line
b = gradient of the soil line
This index outputs values between -1.0 and 1.0." (ESRI, 2018)