script.js

// CyanoLakes Bloom Index
// Kravitz, J.A., Matthews, M.W., 2020

// Water body detection - Credit : Mohor Gartner
var MNDWI_threshold=0.42; //testing shows recommended 0.42 for Sentinel-2 and Landsat 8. For the scene in article [1] it was 0.8.
var NDWI_threshold=0.4; //testing shows recommended 0.4 for Sentinel-2 and Landsat 8. For the scene in article [1] it was 0.5.
var filter_UABS=true;
function wbi(r,g,b,nir,swir1,swir2) {
    //water surface
    let ws=0;
    //try as it might fail for some pixel
    try {
        //calc indices
        //[4][5][1][8][2][3]
        var ndvi=(nir-r)/(nir+r),mndwi=(g-swir1)/(g+swir1),ndwi=(g-nir)/(g+nir),ndwi_leaves=(nir-swir1)/(nir+swir1),aweish=b+2.5*g-1.5*(nir+swir1)-0.25*swir2,aweinsh=4*(g-swir1)-(0.25*nir+2.75*swir1);
        //[10][11][12]
        var dbsi=((swir1-g)/(swir1+g))-ndvi,wii=Math.pow(nir,2)/r,wri=(g+r)/(nir+swir1),puwi=5.83*g-6.57*r-30.32*nir+2.25,uwi=(g-1.1*r-5.2*nir+0.4)/Math.abs(g-1.1*r-5.2*nir),usi=0.25*(g/r)-0.57*(nir/g)-0.83*(b/g)+1;
        //DEFINE WB
        if (mndwi>MNDWI_threshold||ndwi>NDWI_threshold||aweinsh>0.1879||aweish>0.1112||ndvi<-0.2||ndwi_leaves>1) {ws=1;}
        //filter urban areas [3] and bare soil [10]
        if (filter_UABS && ws==1) {
            if ((aweinsh<=-0.03)||(dbsi>0)) {ws=0;}
        }
    }catch(err){ws=0;}
    return ws;
}
let water = wbi(B04,B03,B02,B08,B11,B12);

// Baseline subtractions
function MCI (a,b,c) {return (b-a-(c-a)*(705-665)/(740-665))};
function FAI (a,b,c) {return (b-a-(c-a)*(740-665)/(842-665))};

// True color representation
var trueColor = [3*B04,3*B03,3*B02];

// Switches to FAI if FAI > MCI
function Switch (a,b) {
    if (a > b) {
        return a
    } else {
        return b
    }
}

// Bloom index
var bloom_index = Switch(MCI(B04,B05,B06),FAI(B04,B06,B08));

// HighlightCompressVisualizer
// This compresses bloom index values over 0.05 corresponding to floating algae
// 0.05 will return ~ 0.9
const compressviz = new HighlightCompressVisualizer(0, 0.05);

// Visualise on color scale between 0 and 1 (after compression)
var viz = ColorGradientVisualizer.createBlueRed(minVal=0, maxVal=1);

//Compressed index
compressed_index = compressviz.process(bloom_index);

return water==0 ? trueColor : viz.process(compressed_index);