Flying arcs with height proportional to length

Adapted from this, to avoid the second projection and make flying arc height proportional to length.

Because the 'sky' projection is just a scaled up version of the original one, the projection of a sky point in canvas coords can be easily calculated by scaling the ground projection point along the vector from the origin.

Another improvement(?) might be to avoid the swoosh/interpolate function by doing this same trick directly on the stream of points that comes from the normal projection path function.

<!DOCTYPE html>
<meta charset="utf-8">
<style>

.land {
  fill: #999;
  stroke-opacity: 1;
}

.graticule {
  fill: none;
  stroke: black;
  stroke-width:.5;
  opacity:.2;
}

.labels {
    font: 8px sans-serif;
    fill: black;
    opacity: .5;

display:none;
}

.noclicks { pointer-events:none; }

.point {  opacity:.6; }

.arcs {
  opacity:.1;
  stroke: gray;
  stroke-width: 3;
}
.flyers {
  stroke-width:1;
  opacity: .6;
  stroke: darkred; 
}
.arc, .flyer {
  stroke-linejoin: round;
  fill:none;
}
  .arc { }
  .flyer { }
  .flyer:hover { }

</style>

d3.select(window)
    .on("mousemove", mousemove)
    .on("mouseup", mouseup);

var width = 960,
    height = 500;

var proj = d3.geo.orthographic()
    .translate([width / 2, height / 2])
    .clipAngle(90)
    .scale(220);

var path = d3.geo.path().projection(proj).pointRadius(2);

var swoosh = d3.svg.line()
      .x(function(d) { return d[0] })
      .y(function(d) { return d[1] })
      .interpolate("cardinal")
      .tension(.0);

var links = [],
    arcLines = [];

var svg = d3.select("body").append("svg")
            .attr("width", width)
            .attr("height", height)
            .on("mousedown", mousedown);

queue()
    .defer(d3.json, "world-110m.json")
    .defer(d3.json, "places.json")
    .await(ready);

function ready(error, world, places) {
  var ocean_fill = svg.append("defs").append("radialGradient")
        .attr("id", "ocean_fill")
        .attr("cx", "75%")
        .attr("cy", "25%");
      ocean_fill.append("stop").attr("offset", "5%").attr("stop-color", "#fff");
      ocean_fill.append("stop").attr("offset", "100%").attr("stop-color", "#ababab");

var globe_highlight = svg.append("defs").append("radialGradient")
        .attr("id", "globe_highlight")
        .attr("cx", "75%")
        .attr("cy", "25%");
      globe_highlight.append("stop")
        .attr("offset", "5%").attr("stop-color", "#ffd")
        .attr("stop-opacity","0.6");
      globe_highlight.append("stop")
        .attr("offset", "100%").attr("stop-color", "#ba9")
        .attr("stop-opacity","0.2");

var globe_shading = svg.append("defs").append("radialGradient")
        .attr("id", "globe_shading")
        .attr("cx", "55%")
        .attr("cy", "45%");
      globe_shading.append("stop")
        .attr("offset","30%").attr("stop-color", "#fff")
        .attr("stop-opacity","0")
      globe_shading.append("stop")
        .attr("offset","100%").attr("stop-color", "#505962")
        .attr("stop-opacity","0.3")

var drop_shadow = svg.append("defs").append("radialGradient")
        .attr("id", "drop_shadow")
        .attr("cx", "50%")
        .attr("cy", "50%");
      drop_shadow.append("stop")
        .attr("offset","20%").attr("stop-color", "#000")
        .attr("stop-opacity",".5")
      drop_shadow.append("stop")
        .attr("offset","100%").attr("stop-color", "#000")
        .attr("stop-opacity","0")

svg.append("ellipse")
    .attr("cx", 440).attr("cy", 450)
    .attr("rx", proj.scale()*.90)
    .attr("ry", proj.scale()*.25)
    .attr("class", "noclicks")
    .style("fill", "url(#drop_shadow)");

svg.append("circle")
    .attr("cx", width / 2).attr("cy", height / 2)
    .attr("r", proj.scale())
    .attr("class", "noclicks")
    .style("fill", "url(#ocean_fill)");
  
  svg.append("path")
    .datum(topojson.object(world, world.objects.land))
    .attr("class", "land noclicks")
    .attr("d", path);

svg.append("circle")
    .attr("cx", width / 2).attr("cy", height / 2)
    .attr("r", proj.scale())
    .attr("class","noclicks")
    .style("fill", "url(#globe_highlight)");

svg.append("circle")
    .attr("cx", width / 2).attr("cy", height / 2)
    .attr("r", proj.scale())
    .attr("class","noclicks")
    .style("fill", "url(#globe_shading)");

svg.append("g").attr("class","points")
      .selectAll("text").data(places.features)
    .enter().append("path")
      .attr("class", "point")
      .attr("d", path);

// spawn links between cities as source/target coord pairs
  places.features.forEach(function(a) {
    places.features.forEach(function(b) {
      if (a !== b) {
        links.push({
          source: a.geometry.coordinates,
          target: b.geometry.coordinates
        });
      }
    });
  });

// build geoJSON features from links array
  links.forEach(function(e,i,a) {
    var feature =   { "type": "Feature", "geometry": { "type": "LineString", "coordinates": [e.source,e.target] }}
    arcLines.push(feature)
  })

svg.append("g").attr("class","arcs")
    .selectAll("path").data(arcLines)
    .enter().append("path")
      .attr("class","arc")
      .attr("d",path)

svg.append("g").attr("class","flyers")
    .selectAll("path").data(links)
    .enter().append("path")
    .attr("class","flyer")
    .attr("d", function(d) { return swoosh(flying_arc(d)) })

refresh();
}

function flying_arc(pts) {
  var source = pts.source,
      target = pts.target;

// get canvas coords of arc midpoint and globe center
  var mid = proj(location_along_arc(source, target, .5));
  var ctr = proj.translate();
  
  // max length of a great circle arc is π, 
  // so 0.3 means longest path "flies" 20% of radius above the globe
  var scale = 1 + 0.3 * d3.geo.distance(source,target) / Math.PI;

mid[0] = ctr[0] + (mid[0]-ctr[0])*scale;
  mid[1] = ctr[1] + (mid[1]-ctr[1])*scale;
  
  var result = [ proj(source),
                 mid,
                 proj(target) ]
  return result;
}

function refresh() {
  svg.selectAll(".land").attr("d", path);
  svg.selectAll(".point").attr("d", path);
  
  svg.selectAll(".arc").attr("d", path)
    .attr("opacity", function(d) {
        return fade_at_edge(d)
    })

svg.selectAll(".flyer")
    .attr("d", function(d) { return swoosh(flying_arc(d)) })
    .attr("opacity", function(d) {
      return fade_at_edge(d)
    }) 
}

return fade(dist)
}

function location_along_arc(start, end, loc) {
  var interpolator = d3.geo.interpolate(start,end);
  return interpolator(loc)
}

// modified from https://bl.ocks.org/1392560
var m0, o0;
function mousedown() {
  m0 = [d3.event.pageX, d3.event.pageY];
  o0 = proj.rotate();
  d3.event.preventDefault();
}
function mousemove() {
  if (m0) {
    var m1 = [d3.event.pageX, d3.event.pageY]
      , o1 = [o0[0] + (m1[0] - m0[0]) / 6, o0[1] + (m0[1] - m1[1]) / 6];
    o1[1] = o1[1] > 30  ? 30  :
            o1[1] < -30 ? -30 :
            o1[1];
    proj.rotate(o1);
    refresh();
  }
}
function mouseup() {
  if (m0) {
    mousemove();
    m0 = null;
  }
}
</script>

Modified http://d3js.org/d3.v3.min.js to a secure url

Modified http://d3js.org/queue.v1.min.js to a secure url

Modified http://d3js.org/topojson.v0.min.js to a secure url

https://d3js.org/d3.v3.min.js

https://d3js.org/queue.v1.min.js

https://d3js.org/topojson.v0.min.js