How to change the color of two circles when overlapping using the D3 library

Question

Hello, I would like to know how two could be made so that two circles change color when they overlap. Preferably, the section that overlaps becomes white because it is intended to represent sets.

var canvas = d3.select("canvas"),
    context = canvas.node().getContext("2d"),
    width = canvas.property("width"),
    height = canvas.property("height"),
    radius = 32;

var circles = d3.range(4).map(function(i) {
  return {
    index: i,
    x: Math.round(Math.random() * (width - radius * 2) + radius),
    y: Math.round(Math.random() * (height - radius * 2) + radius)
  };
});

var color = d3.scaleOrdinal()
    .range(d3.schemeCategory20);

render();

canvas.call(d3.drag()
    .subject(dragsubject)
    .on("start", dragstarted)
    .on("drag", dragged)
    .on("end", dragended)
    .on("start.render drag.render end.render", render));

function render() {
  context.clearRect(0, 0, width, height);
  for (var i = 0, n = circles.length, circle; i < n; ++i) {
    circle = circles[i];
    context.beginPath();
    context.moveTo(circle.x + radius, circle.y);
    context.arc(circle.x, circle.y, radius, 0, 2 * Math.PI);
    context.fillStyle = color(circle.index);
    context.fill();
    if (circle.active) {
      context.lineWidth = 2;
      context.stroke();
    }
  }
}

function dragsubject() {
  for (var i = circles.length - 1, circle, x, y; i >= 0; --i) {
    circle = circles[i];
    x = circle.x - d3.event.x;
    y = circle.y - d3.event.y;
    if (x * x + y * y < radius * radius) return circle;
  }
}

function dragstarted() {
  circles.splice(circles.indexOf(d3.event.subject), 1);
  circles.push(d3.event.subject);
  d3.event.subject.active = true;
}

function dragged() {
  d3.event.subject.x = d3.event.x;
  d3.event.subject.y = d3.event.y;
}

function dragended() {
  d3.event.subject.active = false;
}

<canvas width="800" height="500"></canvas>
<script src="//d3js.org/d3.v4.min.js"></script>

Run code

My ideal solution would be to allow me to change the color of the overlapping section to a different color to represent the intersection between the two sets.

Thank you in advance

Edit: some updates have been made, however I only found how to make coloring for static elements instead of moving

var   x1 = 100,
      y1 = 100,
      x2 = 150,
      y2 = 150,
      r = 70;

    var svg = d3.select('svg')
      .append('svg')
      .attr('width', 500)
      .attr('height', 500);

    svg.append('circle')
      .attr('cx', x1)
      .attr('cy', y1)
      .attr('r', r)
      .style('fill', 'steelblue')
      .style("fill-opacity",0.5)
      .style("stroke","black");

    svg.append('circle')
      .attr('cx', x2)
      .attr('cy', y2)
      .attr('r', r)
      .style('fill', 'orange')
      .style("fill-opacity",0.5)
      .style("stroke","black");

    var interPoints = intersection(x1, y1, r, x2, y2, r);

    svg.append("g")
      .append("path")
      .attr("d", function() {
        return "M" + interPoints[0] + "," + interPoints[2] + "A" + r + "," + r +
          " 0 0,1 " + interPoints[1] + "," + interPoints[3]+ "A" + r + "," + r +
          " 0 0,1 " + interPoints[0] + "," + interPoints[2];
      })
      .style('fill', 'red')
      .style("fill-opacity",0.5)
      .style("stroke","black");


    function intersection(x0, y0, r0, x1, y1, r1) {
      var a, dx, dy, d, h, rx, ry;
      var x2, y2;

      /* dx and dy are the vertical and horizontal distances between
       * the circle centers.
       */
      dx = x1 - x0;
      dy = y1 - y0;

      /* Determine the straight-line distance between the centers. */
      d = Math.sqrt((dy * dy) + (dx * dx));

      /* Check for solvability. */
      if (d > (r0 + r1)) {
        /* no solution. circles do not intersect. */
        return false;
      }
      if (d < Math.abs(r0 - r1)) {
        /* no solution. one circle is contained in the other */
        return false;
      }

      /* 'point 2' is the point where the line through the circle
       * intersection points crosses the line between the circle
       * centers.  
       */

      /* Determine the distance from point 0 to point 2. */
      a = ((r0 * r0) - (r1 * r1) + (d * d)) / (2.0 * d);

      /* Determine the coordinates of point 2. */
      x2 = x0 + (dx * a / d);
      y2 = y0 + (dy * a / d);

      /* Determine the distance from point 2 to either of the
       * intersection points.
       */
      h = Math.sqrt((r0 * r0) - (a * a));

      /* Now determine the offsets of the intersection points from
       * point 2.
       */
      rx = -dy * (h / d);
      ry = dx * (h / d);

      /* Determine the absolute intersection points. */
      var xi = x2 + rx;
      var xi_prime = x2 - rx;
      var yi = y2 + ry;
      var yi_prime = y2 - ry;

      return [xi, xi_prime, yi, yi_prime];
    }

<script data-require="d3@3.5.3" data-semver="3.5.3" src="//cdnjs.cloudflare.com/ajax/libs/d3/3.5.3/d3.js"></script>
<svg width="500" height="500"></svg>

Run code

^ This works for statics

var svg = d3.select("svg"),
    width = +svg.attr("width"),
    height = +svg.attr("height"),
    radius = 32;

var circles = d3.range(4).map(function() {
  return {
    x: Math.round(Math.random() * (width - radius * 2) + radius),
    y: Math.round(Math.random() * (height - radius * 2) + radius)
  };
});

var color = d3.scaleOrdinal()
    .range(d3.schemeCategory20);

svg.selectAll("circle")
  .data(circles)
  .enter().append("circle")
  	.style("fill-opacity",0.3)
    .style("stroke","black")
    .attr("cx", function(d) { return d.x; })
    .attr("cy", function(d) { return d.y; })
    .attr("r", 60)
    .style("fill", function(d, i) { return color(i); })
    .call(d3.drag()
        .on("start", dragstarted)
        .on("drag", dragged)
        .on("end", dragended));

function dragstarted(d) {
  d3.select(this).raise().classed("active", true);
}

function dragged(d) {
  d3.select(this).attr("cx", d.x = d3.event.x).attr("cy", d.y = d3.event.y);
}

function dragended(d) {
  d3.select(this).classed("active", false);
}

<svg width="500" height="500"></svg>
<script src="//d3js.org/d3.v4.min.js"></script>

Run code

^ These are my moving circles to which I would like to add an effect.

Is there a way to combine the two codes to achieve this?

Answer 1

SVG mix-blend-mode screen normal

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