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HTML5 10 __Create heat map using Canvas API

黄舟Original: 2017-02-18 14:47:031977browse

You can create many applications using the Canvas API: graphics, charts, and picture editing. One of the most wonderful applications is to modify or overwrite existing content. The most popular overlay map is called a heat map. Heat maps can be used on city maps to mark traffic conditions or to show storm activity.

On the map, you only need to stack the canvas on the map to display it. In fact, it is to use canvas to cover the map, and then draw different heat levels based on the corresponding activity data.

An example is provided below.

<!DOCTYPE html>
<html>
  <title>HTML5 Canvas Example</title>
  <style type="text/css">
    @import url("styles.css");
    #heatmap {
        background-image: url("mapbg.jpg");
    }
  </style>
  <h1>HTML5 Canvas Example</h1>
  <h2>Heatmap </h2>
  <canvas id="heatmap" class="clear" style="border: 1px solid ; " height="300" width="300"> </canvas>
    <button id="resetButton">Reset</button>
  <script>
    function log() {
        console.log(arguments);
    }

    var points = {};
    var SCALE = 3;

    var x = -1;
    var y = -1;

    function loadDemo() {
        document.getElementById("resetButton").onclick = reset;
        canvas = document.getElementById("heatmap");
        context = canvas.getContext(&#39;2d&#39;);
        context.globalAlpha = 0.2;
        context.globalCompositeOperation = "lighter";

        function sample() {
            if (x != -1) {
                addToPoint(x,y)
            }
            setTimeout(sample, 100);
        }

        canvas.onmousemove = function(e) {
            x = e.clientX - e.target.offsetLeft;
            y = e.clientY - e.target.offsetTop;
            addToPoint(x,y)
        }
        sample();
    }

    function reset() {
        points = {};
        context.clearRect(0,0,300,300);
        x = -1;
        y = -1;
    }

    function getColor(intensity) {
        var colors = ["#072933", "#2E4045", "#8C593B", "#B2814E", "#FAC268", "#FAD237"];
        return colors[Math.floor(intensity/2)];
    }

    function drawPoint(x, y, radius) {
            context.fillStyle= getColor(radius);
            radius = Math.sqrt(radius)*6;

            context.beginPath();
            context.arc(x, y, radius, 0, Math.PI*2, true)

            context.closePath();
            context.fill();
    }

    function addToPoint(x, y) {
        x = Math.floor(x/SCALE);
        y= Math.floor(y/SCALE);

        if (!points[[x,y]]) {
            points[[x,y]] = 1;
        } else if (points[[x,y]]==10) {
            return
        } else {
            points[[x,y]]++;
        }
        drawPoint(x*SCALE,y*SCALE, points[[x,y]]);
    }
    window.addEventListener("load", loadDemo, true);
  </script>
</html>

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