Raster Source

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).

<!DOCTYPE html>
<html>
  <head>
    <title>Raster Source</title>
    <link rel="stylesheet" href="https://openlayers.org/en/v4.6.5/css/ol.css" type="text/css">
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL"></script>
    <script src="https://openlayers.org/en/v4.6.5/build/ol.js"></script>
    <script src="https://d3js.org/d3.v3.min.js"></script>
    <style>
      .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>
      // this example uses d3 for which we don't have an externs file.
      var minVgi = 0;
      var maxVgi = 0.25;
      var 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) {
        var r = pixel[0] / 255;
        var g = pixel[1] / 255;
        var b = pixel[2] / 255;
        return (2 * g - r - b) / (2 * g + r + b);
      }


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


      /**
       * Use aerial imagery as the input data for the raster source.
       */
      var bing = new ol.source.BingMaps({
        key: 'Your Bing Maps Key from http://www.bingmapsportal.com/ here',
        imagerySet: 'Aerial'
      });


      /**
       * Create a raster source where pixels with VGI values above a threshold will
       * be colored green.
       */
      var raster = new ol.source.Raster({
        sources: [bing],
        /**
         * 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) {
          var pixel = pixels[0];
          var 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', 0.1);

      function createCounts(min, max, num) {
        var values = new Array(num);
        for (var 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);
      });

      var map = new ol.Map({
        layers: [
          new ol.layer.Tile({
            source: bing
          }),
          new ol.layer.Image({
            source: raster
          })
        ],
        target: 'map',
        view: new ol.View({
          center: [-9651695, 4937351],
          zoom: 13,
          minZoom: 12,
          maxZoom: 19
        })
      });


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

      var barWidth = 15;
      var plotHeight = 150;
      var chart = d3.select('#plot').append('svg')
          .attr('width', barWidth * bins)
          .attr('height', plotHeight);

      var chartRect = chart[0][0].getBoundingClientRect();

      var tip = d3.select(document.body).append('div')
          .attr('class', 'tip');

      function plot(resolution, counts, threshold) {
        var yScale = d3.scale.linear()
            .domain([0, d3.max(counts.values)])
            .range([0, plotHeight]);

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

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

        bar.attr('class', function(count, index) {
          var 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(count, index) {
          var threshold = counts.min + (index * counts.delta);
          if (raster.get('threshold') !== threshold) {
            raster.set('threshold', threshold);
            raster.changed();
          }
        });

        bar.on('mouseover', function(count, index) {
          var area = 0;
          for (var 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',
            top: (d3.event.y - 60) + 'px',
            opacity: 1
          });
        });

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

      }

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