Feat/add fvi scripts

planet/analysis_ready_planetscope/forest_vitality_change_index_gain/index.md

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+---
+title: Forest Vitality Change Index - Gain Visualization (FVCI-Gain)
+parent: Analysis Ready Planetscope
+grand_parent: Planet
+layout: script
+nav_exclude: true
+scripts:
+  - [Visualization, script.js]
+additionalQueryParams: 
+  - - themeId
+    - PLANET_SANDBOX
+---
+
+## Evaluate and visualize
+ - [EO Browser](https://apps.sentinel-hub.com/eo-browser/?zoom=14&lat=44.74116&lng=-0.68441&themeId=PLANET_SANDBOX&visualizationUrl=U2FsdGVkX18GOLfPT%2FOdeWMNJprEadk9%2FIkChCyr9gwge8vm%2BhkdI6vUYWV0tSp8HlOY%2FAeyWpXjS2z2APPpQqFJfSOhtNTTIuEJDM5zvCuN3HhjP6yDpS1%2BPAlYCZlf&evalscript=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%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%3D&datasetId=3f605f75-86c4-411a-b4ae-01c896f0e54e&fromTime=2023-03-20T00%3A00%3A00.000Z&toTime=2023-07-14T23%3A59%3A59.999Z&demSource3D=%22MAPZEN%22&dataFusion=%5B%7B%22id%22%3A%22CUSTOM%22%2C%22alias%22%3A%22arps%22%2C%22additionalParameters%22%3A%7B%22collectionId%22%3A%223f605f75-86c4-411a-b4ae-01c896f0e54e%22%2C%22subType%22%3Anull%2C%22locationId%22%3A%22aws-eu-central-1%22%7D%7D%2C%7B%22id%22%3A%22CUSTOM%22%2C%22alias%22%3A%22canopy_cover%22%2C%22additionalParameters%22%3A%7B%22collectionId%22%3A%22ca501757-cf8e-43a8-b1a4-1aa59ae22425%22%2C%22subType%22%3A%22BYOC%22%2C%22locationId%22%3A%22aws-eu-central-1%22%7D%2C%22timespan%22%3A%5B%222023-03-21T00%3A00%3A00.000Z%22%2C%222023-07-21T23%3A59%3A59.999Z%22%5D%7D%5D#custom-script){:target="_blank"}
+
+ The example data is using Planet Sandox data. This data is restricted to Sentinel Hub users with active paid plans. If you are already a Planet Customer, see [here](https://community.planet.com/sentinel-hub-81/access-new-tools-for-analyzing-your-planet-data-on-sentinel-hub-732) on how to get access.
+
+## General description
+
+The FVCI-Gain script visualizes positive changes in forest vitality between two time periods. This specialized visualization focuses exclusively on areas showing improvement in forest health, making it easier to identify regeneration, growth, or recovery patterns. The script combines Analysis Ready PlanetScope imagery with the Canopy Cover layer from the Forest Carbon Monitoring dataset to ensure analysis is focused on forest areas only.
+
+## Details of the script
+
+The FVCI-Gain processing workflow includes:
+1. Filter pixels based on canopy cover percentage (>25%) for both time periods
+2. Calculate vegetation index (users can choose between NDVI or SAVI) for both time periods
+3. Clip and scale both indices to a standardized range
+4. Calculate the difference between the time periods
+5. Identify pixels with positive change exceeding the threshold (>2.5%)
+6. Classify positive changes into 20 intensity levels
+7. Apply a color palette transitioning from yellow through green to purple to visualize different gain intensities
+
+The script utilizes the first and last PlanetScope ARD acquisitions and Canopy Cover products in the specified time range.
+
+## Description of representative images
+
+The FVCI-Gain visualization uses a color palette specifically designed to highlight forest vitality improvements:
+- Light yellow to yellow: Minor improvements in forest health
+- Yellow-green to green: Moderate improvements
+- Dark green: Significant improvements
+- Purple to magenta: Major improvements/regeneration
+
+Areas with no significant improvement or insufficient canopy cover remain transparent.
+
+A visualization of FVCI-Gain between March and July 2023 over Cestas, France:
+
+![FVCI-Gain over Cestas](fig/fig1.png)

planet/analysis_ready_planetscope/forest_vitality_change_index_gain/script.js

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+//VERSION=3
+/* 
+ * Forest Vitality Change Index - Gain Visualization
+ * This script visualizes only positive changes (gains) in forest vitality
+ * between two timepoints based on vegetation indices.
+ * 
+ * Parameters:
+ * - VI_TYPE: Vegetation index type ("NDVI" or "SAVI")
+ * - CC_MIN: Minimum canopy cover percentage
+ * - VI_MIN, VI_MAX: Range for clipping vegetation index values
+ * - CHANGE_THRESHOLD: Minimum % change to classify as gain
+ * - CLASS_INTERVAL: Interval for classifying change magnitude
+ */
+
+function setup() {
+  return {
+    input: [
+      {
+        datasource: "arps",
+        bands: ["red", "nir", "dataMask", "cloud_mask"],
+      },
+      {
+        datasource: "canopy_cover",
+        bands: ["CC", "dataMask"],
+      },
+    ],
+    output: [
+      { bands: 4 }, // RGBA for gain
+    ],
+    mosaicking: "ORBIT",
+  };
+}
+
+// Configuration parameters
+const VI_TYPE = "NDVI"; // Choose "NDVI" or "SAVI"
+const CC_MIN = 25;      // Minimum canopy cover percentage to consider valid forest
+const VI_MIN = 0.15;    // Minimum vegetation index value for clipping
+const VI_MAX = 0.85;    // Maximum vegetation index value for clipping
+const CHANGE_THRESHOLD = 2.5; // Minimum % change to classify as gain
+const CLASS_INTERVAL = 5; // Interval for classifying change into palettes
+const INVALID_VI = -9999;  // Value for invalid vegetation index
+const VI_SCALE = 99;      // Scale factor for VI values (resulting in range 1-100)
+const SAVI_L = 0.5;       // Soil adjustment factor for SAVI
+
+// Color palette - for vitality gains (from small to significant)
+const gainPalette = [
+  [1.0, 1.0, 0.6],  // 0 - smallest gain
+  [1.0, 1.0, 0.5],
+  [1.0, 1.0, 0.4],
+  [0.9, 1.0, 0.3],
+  [0.8, 1.0, 0.2],
+  [0.7, 1.0, 0.1],
+  [0.6, 1.0, 0.0],
+  [0.4, 1.0, 0.0],
+  [0.2, 1.0, 0.0],
+  [0.0, 1.0, 0.0],
+  [0.0, 0.9, 0.0],
+  [0.0, 0.8, 0.0],
+  [0.0, 0.6, 0.0],
+  [0.0, 0.4, 0.0],
+  [0.0, 0.2, 0.0],
+  [0.6, 0.0, 0.6],
+  [0.7, 0.0, 0.7],
+  [0.8, 0.0, 0.8],
+  [0.9, 0.0, 0.9],
+  [1.0, 0.0, 1.0],  // 19 - significant gain
+];
+
+// Calculate vegetation index (VI) based on type
+function calculateVI(nir, red) {
+  if (nir === undefined || red === undefined) return INVALID_VI;
+  if (nir + red === 0) return INVALID_VI; // Avoid division by zero
+
+  if (VI_TYPE === "NDVI") {
+    return (nir - red) / (nir + red);
+  } else if (VI_TYPE === "SAVI") {
+    return ((nir - red) / (nir + red + SAVI_L)) * (1 + SAVI_L);
+  } else {
+    return INVALID_VI;
+  }
+}
+
+// Clip VI to min/max range and scale to 1-100 range
+function clipAndScaleVI(vi) {
+  const clippedVI = Math.max(VI_MIN, Math.min(vi, VI_MAX));
+  return Math.round(((clippedVI - VI_MIN) / (VI_MAX - VI_MIN)) * VI_SCALE) + 1;
+}
+
+// Filter scenes to only include first and last of specified time range
+function preProcessScenes(collections) {
+  // Helper to keep only first and last orbit if possible
+  function filterOrbits(orbits, label) {
+    if (Array.isArray(orbits) && orbits.length >= 2) {
+      return [orbits[0], orbits[orbits.length - 1]];
+    } else {
+      throw new Error(`FVCI requires at least 2 ${label} scenes for change detection`);
+    }
+  }
+
+  collections.arps.scenes.orbits = filterOrbits(collections.arps.scenes.orbits, "arps");
+  collections.canopy_cover.scenes.orbits = filterOrbits(collections.canopy_cover.scenes.orbits, "canopy cover");
+
+  return collections;
+}
+
+// Main function to evaluate each pixel
+function evaluatePixel(samples) {
+  // Check for undefined inputs
+  if (!samples.arps || !samples.canopy_cover || 
+      samples.arps.length < 2 || samples.canopy_cover.length < 2) {
+    return [0, 0, 0, 0]; // Transparent
+  }
+
+  const ps_ard_T1 = samples.arps[1];
+  const ps_ard_T2 = samples.arps[0];
+  const canopy_T1 = samples.canopy_cover[1];
+  const canopy_T2 = samples.canopy_cover[0];
+
+  // Check data masks
+  if (ps_ard_T1.dataMask === 0 || ps_ard_T2.dataMask === 0) {
+    return [0, 0, 0, 0];
+  }
+
+  // Check cloud masks
+  if (ps_ard_T1.cloud_mask !== 1 || ps_ard_T2.cloud_mask !== 1) {
+    return [0, 0, 0, 0];
+  }
+
+  // Check canopy cover thresholds
+  if (canopy_T1.CC <= CC_MIN || canopy_T2.CC <= CC_MIN) {
+    return [0, 0, 0, 0];
+  }
+
+  // Calculate VI for T1 and T2
+  const vi_T1 = calculateVI(ps_ard_T1.nir, ps_ard_T1.red);
+  const vi_T2 = calculateVI(ps_ard_T2.nir, ps_ard_T2.red);
+
+  // Check for invalid VI values
+  if (vi_T1 === INVALID_VI || vi_T2 === INVALID_VI) {
+    return [0, 0, 0, 0];
+  }
+
+  // Clip and scale VI values
+  const vi100_T1 = clipAndScaleVI(vi_T1);
+  const vi100_T2 = clipAndScaleVI(vi_T2);
+
+  // Calculate percentage change directly
+  const changePercent = vi100_T2 - vi100_T1;
+
+  // Classify change into gain
+  if (changePercent > CHANGE_THRESHOLD) {
+    const classIndex = Math.min(
+      19,
+      Math.floor((changePercent - CHANGE_THRESHOLD) / CLASS_INTERVAL)
+    );
+    const color = gainPalette[classIndex];
+    return [...color, 1]; // Fully opaque gain
+  } else {
+    return [0, 0, 0, 0]; // Fully transparent if no significant change
+  }
+}