HeatmapLayer.js

/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements.  See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership.  The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License.  You may obtain a copy of the License at
*
*   http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing,
* software distributed under the License is distributed on an
* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
* KIND, either express or implied.  See the License for the
* specific language governing permissions and limitations
* under the License.
*/

/**
 * @file defines echarts Heatmap Chart
 * @author Ovilia (me@zhangwenli.com)
 * Inspired by https://github.com/mourner/simpleheat
 *
 * @module
 */

import * as zrUtil from 'zrender/src/core/util';

var GRADIENT_LEVELS = 256;

/**
 * Heatmap Chart
 *
 * @class
 */
function Heatmap() {
    var canvas = zrUtil.createCanvas();
    this.canvas = canvas;

    this.blurSize = 30;
    this.pointSize = 20;

    this.maxOpacity = 1;
    this.minOpacity = 0;

    this._gradientPixels = {};
}

Heatmap.prototype = {
    /**
     * Renders Heatmap and returns the rendered canvas
     * @param {Array} data array of data, each has x, y, value
     * @param {number} width canvas width
     * @param {number} height canvas height
     */
    update: function(data, width, height, normalize, colorFunc, isInRange) {
        var brush = this._getBrush();
        var gradientInRange = this._getGradient(data, colorFunc, 'inRange');
        var gradientOutOfRange = this._getGradient(data, colorFunc, 'outOfRange');
        var r = this.pointSize + this.blurSize;

        var canvas = this.canvas;
        var ctx = canvas.getContext('2d');
        var len = data.length;
        canvas.width = width;
        canvas.height = height;
        for (var i = 0; i < len; ++i) {
            var p = data[i];
            var x = p[0];
            var y = p[1];
            var value = p[2];

            // calculate alpha using value
            var alpha = normalize(value);

            // draw with the circle brush with alpha
            ctx.globalAlpha = alpha;
            ctx.drawImage(brush, x - r, y - r);
        }

        if (!canvas.width || !canvas.height) {
            // Avoid "Uncaught DOMException: Failed to execute 'getImageData' on
            // 'CanvasRenderingContext2D': The source height is 0."
            return canvas;
        }

        // colorize the canvas using alpha value and set with gradient
        var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);

        var pixels = imageData.data;
        var offset = 0;
        var pixelLen = pixels.length;
        var minOpacity = this.minOpacity;
        var maxOpacity = this.maxOpacity;
        var diffOpacity = maxOpacity - minOpacity;

        while(offset < pixelLen) {
            var alpha = pixels[offset + 3] / 256;
            var gradientOffset = Math.floor(alpha * (GRADIENT_LEVELS - 1)) * 4;
            // Simple optimize to ignore the empty data
            if (alpha > 0) {
                var gradient = isInRange(alpha) ? gradientInRange : gradientOutOfRange;
                // Any alpha > 0 will be mapped to [minOpacity, maxOpacity]
                alpha > 0 && (alpha = alpha * diffOpacity + minOpacity);
                pixels[offset++] = gradient[gradientOffset];
                pixels[offset++] = gradient[gradientOffset + 1];
                pixels[offset++] = gradient[gradientOffset + 2];
                pixels[offset++] = gradient[gradientOffset + 3] * alpha * 256;
            }
            else {
                offset += 4;
            }
        }
        ctx.putImageData(imageData, 0, 0);

        return canvas;
    },

    /**
     * get canvas of a black circle brush used for canvas to draw later
     * @private
     * @returns {Object} circle brush canvas
     */
    _getBrush: function() {
        var brushCanvas = this._brushCanvas || (this._brushCanvas = zrUtil.createCanvas());
        // set brush size
        var r = this.pointSize + this.blurSize;
        var d = r * 2;
        brushCanvas.width = d;
        brushCanvas.height = d;

        var ctx = brushCanvas.getContext('2d');
        ctx.clearRect(0, 0, d, d);

        // in order to render shadow without the distinct circle,
        // draw the distinct circle in an invisible place,
        // and use shadowOffset to draw shadow in the center of the canvas
        ctx.shadowOffsetX = d;
        ctx.shadowBlur = this.blurSize;
        // draw the shadow in black, and use alpha and shadow blur to generate
        // color in color map
        ctx.shadowColor = '#000';

        // draw circle in the left to the canvas
        ctx.beginPath();
        ctx.arc(-r, r, this.pointSize, 0, Math.PI * 2, true);
        ctx.closePath();
        ctx.fill();
        return brushCanvas;
    },

    /**
     * get gradient color map
     * @private
     */
    _getGradient: function (data, colorFunc, state) {
        var gradientPixels = this._gradientPixels;
        var pixelsSingleState = gradientPixels[state] || (gradientPixels[state] = new Uint8ClampedArray(256 * 4));
        var color = [0, 0, 0, 0];
        var off = 0;
        for (var i = 0; i < 256; i++) {
            colorFunc[state](i / 255, true, color);
            pixelsSingleState[off++] = color[0];
            pixelsSingleState[off++] = color[1];
            pixelsSingleState[off++] = color[2];
            pixelsSingleState[off++] = color[3];
        }
        return pixelsSingleState;
    }
};

export default Heatmap;