heatcanvas

This is a simple heatmap api based on HTML5 canvas. A heat map is a graphical representation of data where the values taken by a variable in a two-dimensional table are represented as colors, according to Wikipedia.

You can find an interactive demo at http://sunng87.github.com/heatcanvas

Available via bower

bower install heatcanvas

Usage

1. Create the HeatCanvas object

You can pass the canvas element object or its id to the constructor:

var heatmap = new HeatCanvas("canvasId");

2. Add some data

Add value to point (x,y) in canvas coordinate system.

heatmap.add(x, y, value);

3. Render the map

Call the render function on heatmap to draw it.

heatmap.render();

We use a simple formula to determine value of a pixel, by its distance to a point that holds data:

v = f(d)

The first two optional parameters of render define the formula.

• step

• degree

  v = Σ(data_i - step * d^degree)

A set of constants are predefined for degree:

• HeatCanvas.LINEAR
• HeatCanvas.QUAD
• HeatCanvas.CUBIC

For the third parameter of render, you can define a custom function to define color of pixels. For instance, we can use a mono-hue color scheme by this function:

var colorscheme = function(value){
    return [0.3, 0.75, value, 1];
}
heatmap.render(null, null, colorscheme);

The value for this function is guaranteed in (0,1].

4. Remove everything we just created

Call clear to erase the canvas and remove all data cached in heatmap instance.

heatmap.clear();

GoogleMap extension

HeatCanvas can be used as an OverlayView in GoogleMaps API V3.

Simply use the Map instance to create an HeatCanvasOverlayView

var heatmap = new HeatCanvasOverlayView(map, options);

Additional options available:

• step, same as described in HeatCanvas.render
• degree, same as described in HeatCanvas.render
• colorscheme, same as described in HeatCanvas.render
• opacity, the opacity of overlay view, [0,1]

Add data to map:

heatmap.pushData(latitude, longitude, value);

The map will be rendered automatically.

OpenLayers extension

Also we have a OpenLayer extension for you to embed heat map in your custom map application and OpenStreetMap.

The usage is still similar to GoogleMaps. First, construct your heat map layer with a name, OpenLayers map instance, layer options and HeatCanvas options:

var heatmap = new OpenLayers.Layer.HeatCanvas("HeatCanvas", map, {},
        {'step':0.3, 'degree':HeatCanvas.QUAD, 'opacity':0.8});

Add data to layer:

heatmap.pushData(latitude, longitude, value);

Add layer to map:

map.addLayer(heatmap);

Other extensions

There are also HeatCanvas extensions for:

• Baidu Map (demo)
• Cloudmade Leaflet (demo)

These extensions share similar API mentioned above. You can browse the source code of our demos to get detail.

License

HeatCanvas is released according to MIT License.

Thanks

• @lbt05 for his patches on GoogleMap extension and BaiduMap implementation
• @dazuma for his suggestion to speed up canvas rendering.