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Red edge position index (REPO)

//
// Red Edge Position - 4PLI, based on Gholizadeh et al. 2016 (abbrv. REP)
//
// General formula: 700+40*((670nm+780nm/2)-700nm/(740nm-700nm)
//
// URL https://www.indexdatabase.de/db/i-single.php?id=196
// Article: https://www.mdpi.com/1999-4907/7/10/226

let index = 700 + 40 * ((((B04 + B07) / 2) - B05) / (B06 - B05));
let min = 690;
let max = 725;
let zero = 707.5;

// colorBlend will return a color when the index is between min and max and white when it is less than min.
// To see black when it is more than max, uncomment the last line of colorBlend.
// The min/max values were computed automatically based on paper visualization, feel free to change them to tweak the displayed range.
// An arbitrary diverging color map is used. To tweak the value of the break in the color map, change the variable 'zero'.

let underflow_color = [1, 1, 1];
let low_color = [255 / 255, 0 / 255, 0 / 255];
let high_color = [0 / 255, 255 / 255, 0 / 255];
let zero_color = [255 / 255, 255 / 255, 0 / 255];
let overflow_color = [0, 0, 0];

return colorBlend(index, [min, min, zero, max],
	[
		underflow_color,
		low_color,
		zero_color, // divergent step at zero
		high_color,
		//overflow_color // uncomment to see overflows
	]);

Evaluate and Visualize

General description of the script

The red edge position index is sensitive to changes in chlorophyll concentrations, as higher chlorophyll concentrations absorb longer wavelengths. It is calculated as: 700+40*((670nm+780nm/2)-700nm/(740nm-700nm)

Description of representative images

Visualization of the REPO index above Italy. REPO

Credits

The script is based on Gholizadeh et al., 2016. It is described in the Index database as well.