Add a new webgl example with real time feature filtering

examples/filter-points-webgl.html

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+---
+layout: example.html
+title: Filtering features with WebGL
+shortdesc: Using WebGL to filter large quantities of features
+docs: >
+  This example shows how to use `ol/renderer/webgl/PointsLayer` to dynamically filter a large amount
+    of point geometries. The above map is based on a dataset from the NASA containing 45k recorded meteorite
+    landing sites. Each meteorite is marked by a circle on the map (the bigger the circle, the heavier
+    the object). A pulse effect has been added, which is slightly offset by the year of the impact.
+
+  Adjusting the sliders causes the objects outside of the date range to be filtered out of the map. This is done using
+    a custom fragment shader on the layer renderer, and by using the `v_opacity` attribute of the rendered objects
+    to store the year of impact.
+
+tags: "webgl, icon, sprite, filter, feature"
+---
+<div id="map" class="map"></div>
+<form>
+  <div id="status">Show impacts between <span class="min-year"></span> and <span class="max-year"></span></div>
+
+  <label>Minimum year:</label>
+  <input id="min-year" type="range" min="1850" max="2015" step="1" value="1850"/>
+  <label>Maximum year:</label>
+  <input id="max-year" type="range" min="1850" max="2015" step="1" value="2015"/>
+</form>

examples/filter-points-webgl.js

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+import Map from '../src/ol/Map.js';
+import View from '../src/ol/View.js';
+import TileLayer from '../src/ol/layer/Tile.js';
+import Feature from '../src/ol/Feature';
+import Point from '../src/ol/geom/Point';
+import VectorLayer from '../src/ol/layer/Vector';
+import {Vector} from '../src/ol/source';
+import {fromLonLat} from '../src/ol/proj';
+import WebGLPointsLayerRenderer from '../src/ol/renderer/webgl/PointsLayer';
+import {clamp, lerp} from '../src/ol/math';
+import Stamen from '../src/ol/source/Stamen';
+
+const features = [];
+const vectorSource = new Vector({
+  features: [],
+  attributions: 'NASA'
+});
+
+const oldColor = [180, 140, 140];
+const newColor = [255, 80, 80];
+
+const startTime = Date.now() * 0.001;
+
+// hanle input values & events
+const minYearInput = document.getElementById('min-year');
+const maxYearInput = document.getElementById('max-year');
+function updateStatusText() {
+  const div = document.getElementById('status');
+  div.querySelector('span.min-year').textContent = minYearInput.value;
+  div.querySelector('span.max-year').textContent = maxYearInput.value;
+}
+minYearInput.addEventListener('input', updateStatusText);
+minYearInput.addEventListener('change', updateStatusText);
+maxYearInput.addEventListener('input', updateStatusText);
+maxYearInput.addEventListener('change', updateStatusText);
+updateStatusText();
+
+class WebglPointsLayer extends VectorLayer {
+  createRenderer() {
+    return new WebGLPointsLayerRenderer(this, {
+      colorCallback: function(feature, vertex, component) {
+        // component at index 3 is alpha
+        if (component === 3) {
+          return 1;
+        }
+
+        // color is interpolated based on year
+        const ratio = clamp((feature.get('year') - 1800) / (2013 - 1800), 0, 1);
+        return lerp(oldColor[component], newColor[component], ratio) / 255;
+      },
+      sizeCallback: function(feature) {
+        return 18 * clamp(feature.get('mass') / 200000, 0, 1) + 8;
+      },
+      fragmentShader: [
+        'precision mediump float;',
+
+        'uniform float u_time;',
+        'uniform float u_minYear;',
+        'uniform float u_maxYear;',
+
+        'varying vec2 v_texCoord;',
+        'varying float v_opacity;',
+        'varying vec4 v_color;',
+
+        'void main(void) {',
+        '  float impactYear = v_opacity;',
+
+        // filter out pixels if the year is outside of the given range
+        '  if (impactYear < u_minYear || v_opacity > u_maxYear) {',
+        '    discard;',
+        '  }',
+
+        '  vec2 texCoord = v_texCoord * 2.0 - vec2(1.0, 1.0);',
+        '  float sqRadius = texCoord.x * texCoord.x + texCoord.y * texCoord.y;',
+        '  float value = 2.0 * (1.0 - sqRadius);',
+        '  float alpha = smoothstep(0.0, 1.0, value);',
+
+        '  vec3 color = v_color.rgb;',
+        '  float period = 8.0;',
+        '  color.g *= 2.0 * (1.0 - sqrt(mod(u_time + impactYear * 0.025, period) / period));',
+
+        '  gl_FragColor = vec4(color, v_color.a);',
+        '  gl_FragColor.a *= alpha;',
+        '  gl_FragColor.rgb *= gl_FragColor.a;',
+        '}'
+      ].join(' '),
+      opacityCallback: function(feature) {
+        // here the opacity channel of the vertices is used to store the year of impact
+        return feature.get('year');
+      },
+      uniforms: {
+        u_time: function() {
+          return Date.now() * 0.001 - startTime;
+        },
+        u_minYear: function() {
+          return parseInt(minYearInput.value);
+        },
+        u_maxYear: function() {
+          return parseInt(maxYearInput.value);
+        }
+      }
+    });
+  }
+}
+
+
+function loadData() {
+  const client = new XMLHttpRequest();
+  client.open('GET', 'data/csv/meteorite_landings.csv');
+  client.onload = function() {
+    const csv = client.responseText;
+    let curIndex;
+    let prevIndex = 0;
+    let line;
+    while ((curIndex = csv.indexOf('\n', prevIndex)) > 0) {
+      line = csv.substr(prevIndex, curIndex - prevIndex).split(',');
+      prevIndex = curIndex + 1;
+
+      // skip header
+      if (prevIndex === 0) {
+        continue;
+      }
+
+      const coords = fromLonLat([parseFloat(line[4]), parseFloat(line[3])]);
+
+      features.push(new Feature({
+        mass: parseFloat(line[1]) || 0,
+        year: parseInt(line[2]) || 0,
+        geometry: new Point(coords)
+      }));
+    }
+    vectorSource.addFeatures(features);
+  };
+  client.send();
+}
+
+loadData();
+
+const map = new Map({
+  layers: [
+    new TileLayer({
+      source: new Stamen({
+        layer: 'toner'
+      })
+    }),
+    new WebglPointsLayer({
+      source: vectorSource
+    })
+  ],
+  target: document.getElementById('map'),
+  view: new View({
+    center: [0, 0],
+    zoom: 2
+  })
+});
+
+// animate the map
+function animate() {
+  map.render();
+  window.requestAnimationFrame(animate);
+}
+animate();