javascript - 使用Google maps API绘制变色折线

Question

我正在开发一个项目，该项目将 GPX 文件中的轨迹绘制到谷歌地图上，并使用属性对其进行颜色编码(我们称之为 Score)。我已经做了一些编辑，现在这个 GPX 文件中的每个坐标都有一个额外的标签 Score。这是我修改后的 GPX 文件的片段。

<trkpt lat="50.834048" lon="-0.127354"> <ele>31.0</ele>
<Score>-0.64730385</Score>
<time>2016-07-07T14:31:51Z</time>
</trkpt>

<trkpt lat="50.833833" lon="-0.127600"> <ele>31.0</ele> 
<Score>-0.647203</Score>
<time>2016-07-07T14:32:09Z</time>
</trkpt>

我正在学习 Google map 的 javascript API 来完成绘图部分。我的问题是，如何创建一条根据 Score 改变颜色的多段线？我知道我可以只用两个点绘制成千上万条迷你多段线，并为每条多段线设置颜色。但是感觉很原始。有更好的方法吗？另外，我可以编写自己的函数来解析这个 GPX 文件，但为什么谷歌地图不支持 GPX？

如果它有助于说明我的目的，这就是我试图实现的目标:

Answer 1

一个选项是为每个线段制作一条单独的折线，并根据该线段第一个点的分数为其分配颜色。

for (var i=0;i<(trkpts.length-1);i++) {
  var coord1 = new google.maps.LatLng(parseFloat(trkpts[i].getAttribute("lat")),
                                      parseFloat(trkpts[i].getAttribute("lon")));
  var coord2 = new google.maps.LatLng(parseFloat(trkpts[i+1].getAttribute("lat")),
                                      parseFloat(trkpts[i+1].getAttribute("lon")));
  var score = parseFloat(nodeValue(trkpts[i].getElementsByTagName("Score")[0]));
  var polyline = new google.maps.Polyline({
    map: map,
    path: [coord1, coord2],
    strokeColor: rgb(-1,1,score),
    strokeWeight: 4,
    strokeOpacity: 1.0
  });
}

proof of concept fiddle

代码片段:

var map;
var bounds = new google.maps.LatLngBounds();

function initialize() {
  map = new google.maps.Map(
    document.getElementById("map_canvas"), {
      center: new google.maps.LatLng(34, 108),
      zoom: 13,
      mapTypeId: google.maps.MapTypeId.ROADMAP
    });
  var gpxStr = '<gpx><trk><trkseg><trkpt lat="50.834048" lon="-0.127354"> <ele>31.0</ele><Score>-0.64730385</Score><time>2016-07-07T14:31:51Z</time></trkpt><trkpt lat="50.833833" lon="-0.127600"> <ele>31.0</ele><Score>-0.647203</Score><time>2016-07-07T14:32:09Z</time></trkpt><trkpt lat="50.833715" lon="-0.127768"> <ele>31.0</ele><Score>0.647203</Score><time>2016-07-07T14:32:50Z</time></trkpt><trkpt lat="50.833171" lon="-0.128468"> <ele>31.0</ele><Score>0.99609375</Score><time>2016-07-07T14:32:50Z</time></trkpt><trkpt lat="50.832951" lon="-0.128771"> <ele>31.0</ele><Score>-0.5</Score><time>2016-07-07T14:32:50Z</time></trkpt></trkseg></trk></gpx>';

  var xml = parseXml(gpxStr);
  var trkpts = xml.getElementsByTagName("trkpt");
  var bounds = new google.maps.LatLngBounds();
  for (var i = 0; i < (trkpts.length - 1); i++) {
    var coord1 = new google.maps.LatLng(parseFloat(trkpts[i].getAttribute("lat")),
      parseFloat(trkpts[i].getAttribute("lon")));
    bounds.extend(coord1);
    var coord2 = new google.maps.LatLng(parseFloat(trkpts[i + 1].getAttribute("lat")),
      parseFloat(trkpts[i + 1].getAttribute("lon")));
    bounds.extend(coord2);

    var score = parseFloat(nodeValue(trkpts[i].getElementsByTagName("Score")[0]));
    var polyline = new google.maps.Polyline({
      map: map,
      path: [coord1, coord2],
      strokeColor: rgb(-1, 1, score),
      strokeWeight: 4,
      strokeOpacity: 1.0
    })

  }
  map.fitBounds(bounds);
}
google.maps.event.addDomListener(window, "load", initialize);

function rgb(minimum, maximum, value) {
  var ratio = 2 * (value - minimum) / (maximum - minimum);
  b = Math.floor(Math.max(0, 255 * (1 - ratio)));
  r = Math.floor(Math.max(0, 255 * (ratio - 1)));
  g = 255 - b - r;

  var hexStr = ("00" + r.toString(16)).slice(-2);
  hexStr += ("00" + g.toString(16)).slice(-2);
  hexStr += ("00" + b.toString(16)).slice(-2);
  hexStr = "#" + hexStr;
  return hexStr
}
function parseXml(str) {
    if (window.ActiveXObject) {
      var doc = new ActiveXObject('MicrosoftXMLDOM');
      doc.loadXML(str);
      return doc;
    } else if (window.DOMParser) {
      return (new DOMParser()).parseFromString(str, 'text/xml');
    }
  }
  //nodeValue: Extract the text value of a DOM node, with leading and trailing whitespace trimmed

function nodeValue(node, defVal) {
  var retStr = "";
  if (!node) {
    return (typeof defVal === 'undefined' || defVal === null) ? '' : defVal;
  }
  if (node.nodeType == 3 || node.nodeType == 4 || node.nodeType == 2) {
    retStr += node.nodeValue;
  } else if (node.nodeType == 1 || node.nodeType == 9 || node.nodeType == 11) {
    for (var i = 0; i < node.childNodes.length; ++i) {
      retStr += arguments.callee(node.childNodes[i]);
    }
  }
  return retStr;
};

html,
body,
#map_canvas {
  height: 100%;
  width: 100%;
  margin: 0px;
  padding: 0px
}

<script src="https://maps.googleapis.com/maps/api/js"></script>
<div id="map_canvas" style="border: 2px solid #3872ac;"></div>