OpenLayers 3：偏移描边样式

Question

我有一个 LineString 特征，通过一个笔划来设置样式。其中一些特征代表两件事，所以除了初始笔划颜色之外，我还想在它旁边有一条不同颜色的平行线。如果 ol/style/Stroke 有一个偏移量属性就像 ol/style/Image 那样，这将是一个明智的选择，但遗憾的是，它没有。如果我为这些特征创建了偏移几何体，它们将不会在缩小的水平上可见。我有哪些选择？

Answer 1

您可以根据分辨率偏移几何图形

var style = function(feature, resolution) {
var colors = ['green', 'yellow', 'red'];
var width = 4;
var styles = [];
for (var line = 0; line < colors.length; line++) {
    var dist = width * resolution * (line - (colors.length-1)/2);
    var geom = feature.getGeometry();
    var coords = [];
    var counter = 0;
    geom.forEachSegment(function(from, to) {
        var angle = Math.atan2(to[1] - from[1], to[0] - from[0]);
        var newFrom = [
            Math.sin(angle) * dist + from[0],
            -Math.cos(angle) * dist + from[1]
        ];
        var newTo = [
            Math.sin(angle) * dist + to[0],
            -Math.cos(angle) * dist + to[1]
        ];
        coords.push(newFrom);
        coords.push(newTo);
        if (coords.length > 2) {
            var intersection = math.intersect(coords[counter], coords[counter+1], coords[counter+2], coords[counter+3]);
            coords[counter+1] = (intersection) ? intersection : coords[counter+1];
            coords[counter+2] = (intersection) ? intersection : coords[counter+2];
            counter += 2;
        }
    });
    styles.push(
        new ol.style.Style({
            geometry: new ol.geom.LineString(coords),
            stroke: new ol.style.Stroke({
                color: colors[line],
                width: width
            })
        })
    );
}
return styles;
};


var raster = new ol.layer.Tile({
  source:  new ol.source.OSM() 
});

var source = new ol.source.Vector();

var vector = new ol.layer.Vector({
  source: source,
  style: style
});

var map = new ol.Map({
  layers: [raster, vector],
  target: 'map',
  view: new ol.View({
center: [-11000000, 4600000],
zoom: 4
  })
});

map.addInteraction(new ol.interaction.Draw({
  source: source,
  type: 'LineString'
}));

html, body, .map {
    margin: 0;
    padding: 0;
    width: 100%;
    height: 100%;
}

<link href="https://cdn.rawgit.com/openlayers/openlayers.github.io/master/en/v5.3.0/css/ol.css" rel="stylesheet" />
<script src="https://cdn.rawgit.com/openlayers/openlayers.github.io/master/en/v5.3.0/build/ol.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/mathjs/5.4.1/math.min.js"></script>
<div id="map" class="map"></div>

相交测试可以防止锐角出现这样的效果

对于特征几何的每个段，样式函数计算平行段设置为 4 个像素，无论分辨率如何（在这种情况下，线宽和间距相同）以原始段为中心的样式几何（例如，如果三种颜色，中心样式线将跟随原始样式线并且外部样式线偏移 +/- 4，如果两种颜色，每条线将偏移 +/- 2）最初平行线段的长度与原始但需要增加外角或减少内角，因此 math.intersect 用于找到它们应该连接以形成连续线的点。最后，对于每种颜色，完整的线串用作颜色样式的几何图形，并返回包含每种样式的数组。

这是第二个示例，其中每条偏移线都有自己的宽度和笔划图案，而不是简单的颜色和固定宽度。

var white = [255, 255, 255, 1];
var blue = [0, 153, 255, 1];
var width = 3;

var pointStyle = new ol.style.Style({
image: new ol.style.Circle({
    radius: width * 2,
    fill: new ol.style.Fill({
        color: blue
    }),
    stroke: new ol.style.Stroke({
        color: white,
        width: width / 2
    })
}),
zIndex: Infinity
});

var style = function(feature, resolution) {
var widths = [10, 10];
var strokes = [
    [
        {
            width: 4,
            color: '#7af500'
         }, {
             width: 4,
             color: '#55aa00',
             lineDash: [7,9]
         }
    ], [
        {
            width: 2,
            color: 'black'
         }, {
             width: 8,
             color: 'black',
             lineDash: [2,14],
             lineCap: 'butt'
         }
    ]
];
var styles = [pointStyle];
var totalWidth = 0
for (var line = 0; line < widths.length; line++) {
    totalWidth += widths[line];
}
var width = 0;
for (var line = 0; line < widths.length; line++) {
  var dist = (width + widths[line]/2 - totalWidth/2) * resolution;
  width += widths[line];
  var geom = feature.getGeometry();
  if (geom.forEachSegment) {
    var coords = [];
    var counter = 0;
    geom.forEachSegment(function(from, to) {
        var angle = Math.atan2(to[1] - from[1], to[0] - from[0]);
        var newFrom = [
            Math.sin(angle) * dist + from[0],
            -Math.cos(angle) * dist + from[1]
        ];
        var newTo = [
            Math.sin(angle) * dist + to[0],
            -Math.cos(angle) * dist + to[1]
        ];
        coords.push(newFrom);
        coords.push(newTo);
        if (coords.length > 2) {
            var intersection = math.intersect(coords[counter], coords[counter+1], coords[counter+2], coords[counter+3]);
            coords[counter+1] = (intersection) ? intersection : coords[counter+1];
            coords[counter+2] = (intersection) ? intersection : coords[counter+2];
            counter += 2;
        }
    });
    strokes[line].forEach( function(stroke) {
        styles.push(
            new ol.style.Style({
                geometry: new ol.geom.LineString(coords),
                stroke: new ol.style.Stroke(stroke)
            })
        );
    });
  }
}
return styles;
};


var raster = new ol.layer.Tile({
  source:  new ol.source.OSM() 
});

var source = new ol.source.Vector();

var vector = new ol.layer.Vector({
  source: source,
  style: style
});

var map = new ol.Map({
  layers: [raster, vector],
  target: 'map',
  view: new ol.View({
center: [-11000000, 4600000],
zoom: 4
  })
});

map.addInteraction(new ol.interaction.Draw({
  source: source,
  style: style,
  type: 'LineString'
}));

html, body, .map {
    margin: 0;
    padding: 0;
    width: 100%;
    height: 100%;
}

<link href="https://cdn.rawgit.com/openlayers/openlayers.github.io/master/en/v5.3.0/css/ol.css" rel="stylesheet" />
<script src="https://cdn.rawgit.com/openlayers/openlayers.github.io/master/en/v5.3.0/build/ol.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/mathjs/5.4.1/math.min.js"></script>
<div id="map" class="map"></div>

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