Band math doesn't work in GEE

Question

I'm trying to calculate WDVI index but for some reason the computation failes and I don't find my mistake. The function works but all the pixels are 0.

var addWDVI = function(image) {
  var red=image.select('B4');
  var RedEdge=image.select('B8A');
  var WDVI = RedEdge.subtract(red.multiply(RedEdge.divide(red)))
  .rename('WDVI')
  .copyProperties(image,['system:time_start']);
  return image.addBands(WDVI);

};

This is the link to my code- https://code.earthengine.google.com/43e76e0665ceb3dc20dcbc8a8cae5e25

My end goal- to add this index to my code

Answer 1

The way you defined the equation, the outcome is always zero. Let's take a simplified example:

WDVI = 3 - (2 * (3/2)) = 3 - 3 = 0

In "The application of a weighted infrared-red vegetation index for estimating leaf area index by correcting for soil moisture", (Clevers e1989), is stated that you should use the reflectance of the soil for the red and RedEdge band in this part (RedEdge.divide(red)). Thus, you should first derive or estimate soil reflectance values for these bands in your are of interest.