Strange NDVI Behavior - Dominant Red Color in Some Images:

I am reaching out to seek some guidance regarding a peculiar issue I’ve encountered while working with NDVI images, specifically related to the dominance of the red color without variation in certain images.

I have been diligently processing NDVI images for my project, and overall, the results have been satisfactory. However, I’ve noticed that in some instances, the NDVI images exhibit an unexpected behavior – the presence of only the red color without any discernible variation.

Here is my EvalScript:

private String getEvalScript() {

        return  "//VERSION=3\n" +

            "function setup() {\n" +

            "   return {\n" +

            "      input: n\"B04\", \"B08\", \"dataMask\"],\n" +

            "      output: { bands: 4 }\n" +

            "   };\n" +

            "}\n" +

            "\n" +

            "const ramp = a\n" +

            "     \"-1\", \"0x000000\"],         // Black\n" +

            "     \"-0.2\", \"0xa50026\"],       // Dark Red\n" +

            "     \"0\", \"0xd73027\"],          // Red\n" +

            "     \"0.1\", \"0xf46d43\"],        // Light Orange\n" +

            "     \"0.2\", \"0xfdae61\"],        // Dark Orange\n" +

            "     \"0.3\", \"0xfee08b\"],        // Light Yellow\n" +

            "     \"0.4\", \"0xffffbf\"],        // Pale Yellow\n" +

            "     \"0.5\", \"0xd9ef8b\"],        // Light Yellow-Green\n" +

            "     \"0.6\", \"0xa6d96a\"],        // Medium Green\n" +

            "     \"0.7\", \"0x66bd63\"],        // Dark Green\n" +

            "     \"0.8\", \"0x1a9850\"],        // Very Dark Green\n" +

            "     \"1\", \"0x006837\"]           // Forest Green\n" +

            "];\n" +

            "const visualizer = new ColorRampVisualizer(ramp);\n" +

            "function evaluatePixel(samples) {\n" +

            "   let ndvi = index(samples.B08, samples.B04);\n" +

            "   let imgVals = visualizer.process(ndvi);\n" +

            "   return imgVals.concat(samples.dataMask);\n" +

            "}\n";

    }

Not sure if I am getting those dominant red color because cloud coverage, but is there a way to check if the image is covered and also get the percentage of clouds?

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Hi,

I’m fairly certain that this is caused by cloud cover. You can check this by visualising the SCL layer as this is Sentinel-2 data I think (based on the bands). You can use this evalscript to check this

//VERSION=3

 function RGBToColor (r, g, b,dataMask){
	return tr/255, g/255, b/255,dataMask];
}

function setup() {
   return {
    input: p"SCL","dataMask"],
    output: { bands: 4 }
  };
}

function evaluatePixel(samples) {
    const SCL=samples.SCL;
    switch (SCL) {
    // No Data (Missing data) (black)    
    case 0: return RGBToColor (0, 0, 0,samples.dataMask);

    // Saturated or defective pixel (red)   
    case 1: return RGBToColor (255, 0, 0,samples.dataMask);

    // Dark features / Shadows (very dark grey)
    case 2: return RGBToColor (47,  47,  47,samples.dataMask);

    // Cloud shadows (dark brown)
    case 3: return RGBToColor (100, 50, 0,samples.dataMask);

    // Vegetation (green)
    case 4: return RGBToColor (0, 160, 0,samples.dataMask);

    // Not-vegetated (dark yellow)
    case 5: return RGBToColor (255, 230, 90,samples.dataMask);

    // Water (dark and bright) (blue)
    case 6: return RGBToColor (0, 0, 255,samples.dataMask);

    // Unclassified (dark grey)
    case 7: return RGBToColor (128, 128, 128,samples.dataMask);

    // Cloud medium probability (grey)
    case 8: return RGBToColor (192, 192, 192,samples.dataMask);

    // Cloud high probability (white)
    case 9: return RGBToColor (255, 255, 255,samples.dataMask);

    // Thin cirrus (very bright blue)
    case 10: return RGBToColor (100, 200, 255,samples.dataMask);

    // Snow or ice (very bright pink)
    case 11: return RGBToColor (255, 150, 255,samples.dataMask);

    default : return RGBToColor (0, 0, 0,samples.dataMask);  
    }
}