Example that demonstrates a color expression using variables

examples/.eslintrc

@@ -3,6 +3,7 @@
     "$": false,
     "arc": false,
     "common": false,
+    "chroma": false,
     "createMapboxStreetsV6Style": false,
     "d3": false,
     "html2canvas": false,

examples/cog-colors.css

@@ -0,0 +1,10 @@
+.data {
+  text-align: right;
+  font-family: monospace;
+}
+td {
+  padding-right: 10px;
+}
+input[type="range"] {
+  vertical-align: text-bottom;
+}

examples/cog-colors.html

@@ -0,0 +1,32 @@
+---
+layout: example.html
+title: NDVI with a Dynamic Color Ramp
+shortdesc: NDVI from a COG with a dynamic color ramp
+docs: >
+  The GeoTIFF layer in this example draws from two Sentinel 2 sources: a red band and a near infrared band.
+  The layer style includes a `color` expression that calculates the Normalized Difference Vegetation Index (NDVI)
+  from values in the two bands.  The `interpolate` expression is used to map NDVI values to colors.  The "stop" values
+  for the color ramp are derived from application provided style variables.  Using the inputs above, the min and max
+  colors and values can be adjusted.  The `layer.updateStyleVariables()` method is called to update the
+  variables used in the interpolated color expression.
+tags: "cog, ndvi"
+resources:
+  - https://cdnjs.cloudflare.com/ajax/libs/chroma-js/2.1.2/chroma.min.js
+---
+<div id="map" class="map"></div>
+<table>
+  <tbody>
+    <tr>
+      <td>Min NDVI</td>
+      <td><input type="range" id="min-value-input" min="-1.0" max="-0.1" step="0.01"></td>
+      <td class="data" id="min-value-output"></td>
+      <td><input type="color" id="min-color"></td>
+    </tr>
+    <tr>
+      <td>Max NDVI</td>
+      <td><input type="range" id="max-value-input" min="0.1" max="1.0" step="0.01"></td>
+      <td class="data" id="max-value-output"></td>
+      <td><input type="color" id="max-color"></td>
+    </tr>
+  </tbody>
+</table>

examples/cog-colors.js

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+import GeoTIFF from '../src/ol/source/GeoTIFF.js';
+import Map from '../src/ol/Map.js';
+import TileLayer from '../src/ol/layer/WebGLTile.js';
+
+const segments = 10;
+
+const defaultMinColor = '#0300AD';
+const defaultMaxColor = '#00ff00';
+
+const defaultMinValue = -0.5;
+const defaultMaxValue = 0.7;
+
+const minColorInput = document.getElementById('min-color');
+minColorInput.value = defaultMinColor;
+
+const maxColorInput = document.getElementById('max-color');
+maxColorInput.value = defaultMaxColor;
+
+const minValueOutput = document.getElementById('min-value-output');
+const minValueInput = document.getElementById('min-value-input');
+minValueInput.value = defaultMinValue.toString();
+
+const maxValueOutput = document.getElementById('max-value-output');
+const maxValueInput = document.getElementById('max-value-input');
+maxValueInput.value = defaultMaxValue.toString();
+
+function getVariables() {
+  const variables = {};
+
+  const minColor = minColorInput.value;
+  const maxColor = maxColorInput.value;
+  const scale = chroma.scale([minColor, maxColor]).mode('lab');
+
+  const minValue = parseFloat(minValueInput.value);
+  const maxValue = parseFloat(maxValueInput.value);
+  const delta = (maxValue - minValue) / segments;
+
+  for (let i = 0; i <= segments; ++i) {
+    const color = scale(i / segments).rgb();
+    const value = minValue + i * delta;
+    variables[`value${i}`] = value;
+    variables[`red${i}`] = color[0];
+    variables[`green${i}`] = color[1];
+    variables[`blue${i}`] = color[2];
+  }
+  return variables;
+}
+
+function colors() {
+  const stops = [];
+  for (let i = 0; i <= segments; ++i) {
+    stops[i * 2] = ['var', `value${i}`];
+    const red = ['var', `red${i}`];
+    const green = ['var', `green${i}`];
+    const blue = ['var', `blue${i}`];
+    stops[i * 2 + 1] = ['color', red, green, blue];
+  }
+  return stops;
+}
+
+const ndvi = [
+  '/',
+  ['-', ['band', 2], ['band', 1]],
+  ['+', ['band', 2], ['band', 1]],
+];
+
+const source = new GeoTIFF({
+  sources: [
+    {
+      // visible red, band 1 in the style expression above
+      url: 'https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/2020/S2A_36QWD_20200701_0_L2A/B04.tif',
+      max: 10000,
+    },
+    {
+      // near infrared, band 2 in the style expression above
+      url: 'https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/2020/S2A_36QWD_20200701_0_L2A/B08.tif',
+      max: 10000,
+    },
+  ],
+});
+
+const layer = new TileLayer({
+  style: {
+    variables: getVariables(),
+    color: ['interpolate', ['linear'], ndvi, ...colors()],
+  },
+  source: source,
+});
+
+function update() {
+  layer.updateStyleVariables(getVariables());
+  minValueOutput.innerText = parseFloat(minValueInput.value).toFixed(1);
+  maxValueOutput.innerText = parseFloat(maxValueInput.value).toFixed(1);
+}
+
+minColorInput.addEventListener('input', update);
+maxColorInput.addEventListener('input', update);
+minValueInput.addEventListener('input', update);
+maxValueInput.addEventListener('input', update);
+update();
+
+const map = new Map({
+  target: 'map',
+  layers: [layer],
+  view: source.getView(),
+});