Here you can find very simple explanation of classical NDVI 


https://www.youtube.com/watch?v=rxOMhQwApMc

Classical formula for Normalized Difference Vegetation Index (NDVI) is:


Thus - classical NDVI index varies:

  • from -1 (maximum reflection of RED =  absence of vegetation)
  • to 1 (no red reflection = highest level of vegetation)


In the skyglyph we have a normalized scale (0-1) with the same sense:

  • 0 - the absence of vegetation,
  • 1 - the highest level of vegetation


And also as (in general cases) we have cameras with NIR-GREEN-BLUE  (no red channel) - we cannot calculate real NDVI.

Instead, we use Green Normalized Difference Vegetation Index (GNDVI)

It has almost the same sense and minimizes the influence of green reflection in the picture (when we have two same green leaves, and one of them is with a higher level of vegetation - it will have more value of GNDVI).


For visualization we use a classical scheme:

  • the minimal value of index - red ,
  • maximum - green




Also, for RGB photos we have VARI index (Visible Atmospherically Resistant Index).

VARI was designed and tested to work with RGB data rather than near-infrared (NIR) data. It is a measure of "how green" an image is. 

VARI is not intended as a substitute for a NIR camera, but it is meaningful when working with non-NDVI imagery. The VARI index was developed on a measurement of corn and soybean crops in the Midwestern United States.

VARI is used to estimate the fraction of vegetation in a scene with low sensitivity to atmospheric effects (exactly what we have with low-altitude drone monitoring)