InterpolateHeatmapLayer is a JavaScript library for rendering temperature maps (or interpolate heatmaps) with Mapbox GJ JS. This library was inspired by the temperature-map-gl library and depends on Earcut.
Currently, Mapbox provides a heatmap layer that represent the density of points in an area, like on this picture:
This library aims at providing a heatmap that can define a color to any location by making an average of the values of the surroundings points, like on this picture:
Except a JavaScript pre-processing step, all computation is made with WebGL shaders.
A live demo showing the global temperature is available here, described here.
Note that this layer only works if the Mercator projection is used on the Mapbox map.
-
Browser:
-
Copy the interpolateHeatmapLayer.js file to your project.
-
Import the library before the script using it:
<body> <div id="map"></div> <script src="interpolateHeatmapLayer.js"></script> <script src="map.js"></script> </body>
-
Create the Mapbox map and add the layer created by
interpolateHeatmapLayer.create():
// map.js const map = (window.map = new mapboxgl.Map({ container: 'map', style: 'mapbox://styles/mapbox/light-v10' })); map.on('load', () => { const layer = interpolateHeatmapLayer.create({ // parameters here }); map.addLayer(layer); });
-
-
NPM:
npm install interpolateheatmaplayer
const interpolateHeatmapLayer = require('interpolateheatmaplayer'); // or: import { create as InterpolateHeatmapLayer } from 'interpolateheatmaplayer'; const map = (window.map = new mapboxgl.Map({ container: 'map', style: 'mapbox://styles/mapbox/light-v10' })); map.on('load', () => { const layer = interpolateHeatmapLayer.create({ // parameters here }); // or: const layer = InterpolateHeatmapLayer({}); map.addLayer(layer); });
The interpolateHeatmapLayer.create() function has one object parameter containing the following properties:
-
points: An list of points, each point being an object containing a latitudelat, a longitudelon, and a valueval. Example:points = [{ lat: 62.470663, lon: 6.176846, val: 16 }, { lat: 48.094903, lon: -1.371596, val: 20 }];
Since Mapbox uses the Web Mercator projection that projects the poles at infinity, remember to define the latitude within -85° and 85°. Default value:
[]. -
layerId: string defining the unique Mapbox layer name. Default value:''. -
opacity: number between 0 and 1 describing the transparency of the layer. Default value:0.5. -
minValue: number defining the value corresponding to the blue color. When it's not defined, the lowest value ofpointsis represented by the blue color. If some value ofpointsis lower thanminValue,minValuetakes this value. Default value:Infinity. -
maxValuesame, but for the red color. Default value:-Infinity. -
p: number affecting the computation of the color. A high value makes the color uniform around each point. Take a look at the form of the IDW in the technical explanation part if you want to know more. Default value:3. -
pointRadius: number defining a radius (in meters). The color will only appear within circles of radiuspointRadiuscentered at the points defined inpoints. IfpointsRadius <= 0, this parameter is not taken into account. Default value:0. -
fasterPointRadius: boolean indicating if a faster algorithm should be used when defining apointRadius > 0. Due to precision issues, this parameter creates bad visualizations ifpointRadius < 500. Default value:false. -
roi: list of coordinates with the same format aspoints(without thevalattribute). It defines the region of interest, meaning the layer will only be displayed inside that area. If the list is empty, the entire map is the region of interest. IfpointRadius > 0and a ROI is defined, then the heatmap will only appear on locations that are within the circles AND within the ROI. Default value:[]. -
averageThreshold: number defining a threshold. For each point of the map, if the distance between the point's value and the average value of all points is below this threshold, the associated color will be transparent. The values and the average are scaled between 0 and 1 when computing their distance, soaverageThresholdis a value between 0 and 1. For example, if you havepointswith values [0, 5, 10], and you create the layer with these parameters:const layer = interpolateHeatmapLayer.create({ points: points averageThreshold: 0.1 });
Then all points with values between 4 and 6 will be transparent.
-
framebufferFactor: number between 0 and 1. In short, if the framebuffer factor is around 0, the computation of the Inverse Distance Weighting algorithm will be faster but with a lower resolution. Take a look at the technical explanation part if you want to know exactly what this parameter is. Default value:0.3. -
valueToColor: GLSL function (passed as a string) that maps a value to the layer color. By default, a low value is colored blue, a medium green and a high red. This parameter allows you to change this behavior. The function must be namedvalueToColorwith afloatparameter (which will take values between 0 and 1), and must return avec3(with each component between 0 and 1). Default value:vec3 valueToColor(float value) { return vec3(max((value-0.5)*2.0, 0.0), 1.0 - 2.0*abs(value - 0.5), max((0.5-value)*2.0, 0.0)); }
-
valueToColor4: Same asvalueToColor, but with alpha channel support. The function name and signature must be defined as:vec4 valueToColor4(float value, float defaultOpacity). Default value:vec4 valueToColor4(float value, float defaultOpacity) { return vec4(valueToColor(value), defaultOpacity); }
-
layerBlendingFactor: a GLEnum that describes a coefficient to apply to the heatmap layer when blending with the map. Depending on where the heatmap layer is created, it is likely that it will appear on top of the map. In that case, pixels of the heatmap layer are blended with pixels of the map. The resulting color of a pixel is given by:color = (layerColor * layerBlendingFactor) + (mapColor * mapBlendingFactor)where
layerBlendingFactorandmapBlendingFactorare parameters specifiable here.A list of possible values of these parameters can be found here:
- You have to replace
glbyWebGLRenderingContext, for exampleWebGLRenderingContext.ZERO. - The source color refers to the heatmap color, and the destination color refers to the map color.
Default value:
WebGLRenderingContext.SRC_ALPHA. - You have to replace
-
mapBlendingFactor: same aslayerBlendingFactor, but for the map. Default value:WebGLRenderingContext.ONE_MINUS_SRC_ALPHA.
The layer returned by the interpolateHeatmapLayer.create() function has also one function: layer.updatePoints(points), in which points is an array of points as described above (objects with lat, lon, and val attributes). This function allows you to change the points without creating a new layer. Usage example:
const layer = interpolateHeatmapLayer.create({
points: somePoints
});
// some code
layer.updatePoints(newPoints);The color is computed using the Inverse Distance Weighting (IDW) algorithm:
Let:
be N known data points. We want to find a continuous and once differentiable function:
such as:
The basic form of the IDW is:
where
In WebGL, the computation is done in three steps:
-
First, we determine where to display the heatmap. This can change depending on the
pointRadiusorroiparameters for example. For that, we create a texture where pixel values indicate if the heatmap should be displayed or not. Think of it as a mask in Computer Graphics. -
Then, we compute the values given by the IDW equation. For that, we render N textures, one for each known data point ui. Each pixel of each texture contains wi*ui in its red channel and wi in its green channel. These textures are added together to create a single resulting texture containing the sum of the N textures. We can get u(x) for each pixel by dividing the red channel by the green channel, since they contain respectively sum(wi*ui) and sum(ui).
-
Finally, the first (mask) and last (IDW) textures are combined to create the heatmap layer. For each pixel, if the mask pixel indicates that the heatmap should appear here, the IDW pixel is used to compute u(x), and this value is converted to a color using the
valueToColorparameter.
As you can see, N+3 textures are created, with N+1 textures for the IDW step. This step is therefore the most expensive one. To make it a bit faster, we can reduce the sizes of those N+1 textures (a smaller texture means fewer pixels to compute). This is what the framebufferFactor is for. The width/height of the IDW textures are the width/height of the drawing texture divided by the framebufferFactor. For example, if the Mapbox map has a size of 800x600 pixels and the framebufferFactor is 0.5, then the IDW textures will have a size of 400x300 pixels. This will alter the visual aspect of the heatmap but will make the computer faster, so there is a compromise to find.