text: // NOCOMPILE
  // this example uses d3 for which we don't have an externs file.
  goog.require('ol.Map');
  goog.require('ol.View');
  goog.require('ol.layer.Image');
  goog.require('ol.layer.Tile');
  goog.require('ol.source.BingMaps');
  goog.require('ol.source.Raster');
  
  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 {ol.raster.Pixel} 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: 'AkGbxXx6tDWf1swIhPJyoAVp06H0s0gDTYslNWWHZ6RoPqMpB9ld5FY1WutX8UoF',
    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';
  }
  