javascript - 在 d3 map 投影周围创建一个弯曲的边界

Question

我使用所示的 geoNaturalEarth1() 在 d3 中创建了一个世界地图 here .我使用带有此投影的 geojson 世界地图来获取 map ，如下面的代码所示。然而，这显示了漂浮在没有边界的太空中的国家。我想在 map 投影周围画一个边框，这样它看起来更像一张 map 。边界将是平坦的顶部/底部，弯曲的侧面，如投影图像所示。这可能吗？我该怎么做？

var projection = d3.geoNaturalEarth1()
    .translate([w/2, h/2])
    .scale(247)
    .center([0,0]);

var path = d3.geoPath().projection(projection);

d3.json('map.geojson').then(function(world) {

    svg.selectAll(".emissions_path")
        .data(world.features)
        .enter().append("path")
        .attr('fill', '#fff')
        .attr("d", path)
        .style('stroke', 'black')
        .style('stroke-width', '0.5px');

Answer 1

您可以向路径生成器提供 Sphere 类型的 geojson:

The type Sphere is also supported, which is useful for rendering the outline of the globe; a sphere has no coordinates. (docs)

这看起来像:

var outline = {type:"Sphere"}

并且可以直接传递给路径生成器:

var context = d3.select("canvas").node().getContext("2d"),
    projection = d3.geoNaturalEarth1()
      .scale(70)
      .translate([200,100])
    path = d3.geoPath()
      .context(context)
      .projection(projection);

d3.json("https://unpkg.com/world-atlas@1/world/110m.json", function(error, world) {
  if (error) throw error;

  context.beginPath();
  context.fillStyle = "lightgreen";
  path(topojson.feature(world, world.objects.land));
  context.fill();
  
  context.beginPath();
  context.strokeStyle = "#ccc";
  path({type: "Sphere"})
  context.stroke();
  
});

<canvas width="500" height="300"></canvas>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script src="https://unpkg.com/topojson-client@3"></script>

---

顺便说一句，还有 d3.geoGraticule ，它允许以固定间隔绘制经线和平行线:

var context = d3.select("canvas").node().getContext("2d"),
        projection = d3.geoNaturalEarth1()
          .scale(70)
          .translate([200,100])
        path = d3.geoPath()
          .context(context)
          .projection(projection);

    d3.json("https://unpkg.com/world-atlas@1/world/110m.json", function(error, world) {
      if (error) throw error;

      context.beginPath();
      context.fillStyle = "lightgreen";
      path(topojson.feature(world, world.objects.land));
      context.fill();
  
      context.beginPath();
      context.strokeStyle = "#eee";
      path(d3.geoGraticule10())
      context.stroke();
      
      context.beginPath();
      context.strokeStyle = "#000";
      path({type:"Sphere"})
      context.stroke();        
      
    });

<canvas width="500" height="300"></canvas>
    <script src="https://d3js.org/d3.v4.min.js"></script>
    <script src="https://unpkg.com/topojson-client@3"></script>