This example uses OpenLayers v10.9.0. The latest is v.
Edit

#### Raster Source

raster 6 pixel 2 maptiler 26

#####

Demonstrates pixelwise operations with a raster source.

This example uses a `ol/source/Raster` to generate data based on another source. The raster source accepts any number of input sources (tile or image based) and runs a pipeline of operations on the input pixels. The return from the final operation is used as the data for the output source.

In this case, a single tiled source of imagery is used as input. For each pixel, the Vegetation Greenness Index (VGI) is calculated from the input pixels. A second operation colors those pixels based on a threshold value (values above the threshold are green and those below are transparent).

##### main.js

    import {max} from 'd3-array';
    import {scaleLinear} from 'd3-scale';
    import {select} from 'd3-selection';
    import 'd3-transition';
    import Map from 'ol/Map.js';
    import View from 'ol/View.js';
    import ImageLayer from 'ol/layer/Image.js';
    import TileLayer from 'ol/layer/Tile.js';
    import ImageTile from 'ol/source/ImageTile.js';
    import RasterSource from 'ol/source/Raster.js';

    const minVgi = 0;
    const maxVgi = 0.5;
    const bins = 10;

    /**
     * Calculate the Vegetation Greenness Index (VGI) from an input pixel.  This
     * is a rough estimate assuming that pixel values correspond to reflectance.
     * @param {Array<number>} pixel An array of [R, G, B, A] values.
     * @return {number} The VGI value for the given pixel.
     */
    function vgi(pixel) {
      const r = pixel[0] / 255;
      const g = pixel[1] / 255;
      const b = pixel[2] / 255;
      return (2 * g - r - b) / (2 * g + r + b);
    }

    /**
     * Summarize values for a histogram.
     * @param {number} value A VGI value.
     * @param {Object} counts An object for keeping track of VGI counts.
     */
    function summarize(value, counts) {
      const min = counts.min;
      const max = counts.max;
      const num = counts.values.length;
      if (value < min) {
        // do nothing
      } else if (value >= max) {
        counts.values[num - 1] += 1;
      } else {
        const index = Math.floor((value - min) / counts.delta);
        counts.values[index] += 1;
      }
    }

    /**
     * Use aerial imagery as the input data for the raster source.
     */

    const key = 'Get your own API key at https://www.maptiler.com/cloud/';
    const attributions =
      '<a href="https://www.maptiler.com/copyright/" target="_blank">&copy; MapTiler</a> ' +
      '<a href="https://www.openstreetmap.org/copyright" target="_blank">&copy; OpenStreetMap contributors</a>';

    const aerial = new ImageTile({
      attributions: attributions,
      url: 'https://api.maptiler.com/maps/satellite/{z}/{x}/{y}.jpg?key=' + key,
      tileSize: 512,
      maxZoom: 20,
      crossOrigin: '',
    });

    /**
     * Create a raster source where pixels with VGI values above a threshold will
     * be colored green.
     */
    const raster = new RasterSource({
      sources: [aerial],
      /**
       * Run calculations on pixel data.
       * @param {Array} pixels List of pixels (one per source).
       * @param {Object} data User data object.
       * @return {Array} The output pixel.
       */
      operation: function (pixels, data) {
        const pixel = pixels[0];
        const value = vgi(pixel);
        summarize(value, data.counts);
        if (value >= data.threshold) {
          pixel[0] = 0;
          pixel[1] = 255;
          pixel[2] = 0;
          pixel[3] = 128;
        } else {
          pixel[3] = 0;
        }
        return pixel;
      },
      lib: {
        vgi: vgi,
        summarize: summarize,
      },
    });
    raster.set('threshold', 0.25);

    function createCounts(min, max, num) {
      const values = new Array(num);
      for (let i = 0; i < num; ++i) {
        values[i] = 0;
      }
      return {
        min: min,
        max: max,
        values: values,
        delta: (max - min) / num,
      };
    }

    raster.on('beforeoperations', function (event) {
      event.data.counts = createCounts(minVgi, maxVgi, bins);
      event.data.threshold = raster.get('threshold');
    });

    raster.on('afteroperations', function (event) {
      schedulePlot(event.resolution, event.data.counts, event.data.threshold);
    });

    const map = new Map({
      layers: [
        new TileLayer({
          source: aerial,
        }),
        new ImageLayer({
          source: raster,
        }),
      ],
      target: 'map',
      view: new View({
        center: [-9651695, 4937351],
        zoom: 13,
        minZoom: 12,
        maxZoom: 19,
      }),
    });

    let timer = null;
    function schedulePlot(resolution, counts, threshold) {
      if (timer) {
        clearTimeout(timer);
        timer = null;
      }
      timer = setTimeout(plot.bind(null, resolution, counts, threshold), 1000 / 60);
    }

    const barWidth = 15;
    const plotHeight = 150;
    const chart = select('#plot')
      .append('svg')
      .attr('width', barWidth * bins)
      .attr('height', plotHeight);

    const chartRect = chart.node().getBoundingClientRect();

    const tip = select(document.body).append('div').attr('class', 'tip');

    function plot(resolution, counts, threshold) {
      const yScale = scaleLinear()
        .domain([0, max(counts.values)])
        .range([0, plotHeight]);

      const bar = chart.selectAll('rect').data(counts.values);

      bar.enter().append('rect');

      bar
        .attr('class', function (count, index) {
          const value = counts.min + index * counts.delta;
          return 'bar' + (value >= threshold ? ' selected' : '');
        })
        .attr('width', barWidth - 2);

      bar
        .transition()
        .attr('transform', function (value, index) {
          return (
            'translate(' +
            index * barWidth +
            ', ' +
            (plotHeight - yScale(value)) +
            ')'
          );
        })
        .attr('height', yScale);

      bar.on('mousemove', function () {
        const index = bar.nodes().indexOf(this);
        const threshold = counts.min + index * counts.delta;
        if (raster.get('threshold') !== threshold) {
          raster.set('threshold', threshold);
          raster.changed();
        }
      });

      bar.on('mouseover', function (event) {
        const index = bar.nodes().indexOf(this);
        let area = 0;
        for (let i = counts.values.length - 1; i >= index; --i) {
          area += resolution * resolution * counts.values[i];
        }
        tip.html(message(counts.min + index * counts.delta, area));
        tip.style('display', 'block');
        tip
          .transition()
          .style('left', chartRect.left + index * barWidth + barWidth / 2 + 'px')
          .style('top', event.y - 60 + 'px')
          .style('opacity', 1);
      });

      bar.on('mouseout', function () {
        tip
          .transition()
          .style('opacity', 0)
          .on('end', function () {
            tip.style('display', 'none');
          });
      });
    }

    function message(value, area) {
      const acres = (area / 4046.86)
        .toFixed(0)
        .replace(/\B(?=(\d{3})+(?!\d))/g, ',');
      return acres + ' acres at<br>' + value.toFixed(2) + ' VGI or above';
    }

##### index.html

    <!DOCTYPE html>
    <html lang="en">
      <head>
        <meta charset="UTF-8">
        <title>Raster Source</title>
        <link rel="stylesheet" href="node_modules/ol/ol.css">
        <style>
          .map {
            width: 100%;
            height: 400px;
          }
          .rel {
            position: relative
          }

          #plot {
            pointer-events: none;
            position: absolute;
            bottom: 10px;
            left: 10px;
          }

          .bar {
            pointer-events: auto;
            fill: #AFAFB9;
          }

          .bar.selected {
            fill: green;
          }

          .tip {
            position: absolute;
            background: black;
            color: white;
            padding: 6px;
            font-size: 12px;
            border-radius: 4px;
            margin-bottom: 10px;
            display: none;
            opacity: 0;
          }
        </style>
      </head>
      <body>
        <div class="rel">
          <div id="map" class="map"></div>
          <div id="plot"></div>
        </div>

        <script type="module" src="main.js"></script>
      </body>
    </html>

##### package.json

    {
      "name": "raster",
      "dependencies": {
        "ol": "10.9.0",
        "d3-array": "^3.2.4",
        "d3-scale": "^4.0.2",
        "d3-selection": "^3.0.0",
        "d3-transition": "^3.0.1"
      },
      "devDependencies": {
        "vite": "^3.2.3"
      },
      "scripts": {
        "start": "vite",
        "build": "vite build"
      }
    }