Как изменить прозрачность в Javascript для объекта

;(function() {

  "use strict";

  var lava0;
  var ge1doot = {
    screen: {
      elem:     null,
      callback: null,
      ctx:      null,
      width:    0,
      height:   0,
      left:     0,
      top:      0,
      init: function (id, callback, initRes) {
        this.elem = document.getElementById(id);
        this.callback = callback || null;
        if (this.elem.tagName == "CANVAS") this.ctx = this.elem.getContext("2d");
        window.addEventListener('resize', function () {
          this.resize();
        }.bind(this), false);
        this.elem.onselectstart = function () { return false; }
        this.elem.ondrag        = function () { return false; }
        initRes && this.resize();
        return this;
      },
      resize: function () {
        var o = this.elem;
        this.width  = o.offsetWidth;
        this.height = o.offsetHeight;
        for (this.left = 0, this.top = 0; o != null; o = o.offsetParent) {
          this.left += o.offsetLeft;
          this.top  += o.offsetTop;
        }
        if (this.ctx) {
          this.elem.width  = this.width;
          this.elem.height = this.height;
        }
        this.callback && this.callback();
      }
    }
  }

  // Point constructor
  var Point = function(x, y) {
    this.x = x;
    this.y = y;
    this.magnitude = x * x + y * y;
    this.computed = 0;
    this.force = 0;
  };
  Point.prototype.add = function(p) {
    return new Point(this.x + p.x, this.y + p.y);
  };

  // Ball constructor
  var Ball = function(parent) {
    var min = .1;
    var max = 1.5;
    this.vel = new Point(
      (Math.random() > 0.5 ? 1 : -1) * (0.2 + Math.random() * 0.25), (Math.random() > 0.5 ? 1 : -1) * (0.2 + Math.random())
    );
    this.pos = new Point(
      parent.width * 0.2 + Math.random() * parent.width * 0.6,
      parent.height * 0.2 + Math.random() * parent.height * 0.6
    );
    this.size = (parent.wh / 15) + ( Math.random() * (max - min) + min ) * (parent.wh / 15);
    this.width = parent.width;
    this.height = parent.height;
  };

  // move balls
  Ball.prototype.move = function() {

    // bounce borders
    if (this.pos.x >= this.width - this.size) {
      if (this.vel.x > 0) this.vel.x = -this.vel.x;
      this.pos.x = this.width - this.size;
    } else if (this.pos.x <= this.size) {
      if (this.vel.x < 0) this.vel.x = -this.vel.x;
      this.pos.x = this.size;
    }

    if (this.pos.y >= this.height - this.size) {
      if (this.vel.y > 0) this.vel.y = -this.vel.y;
      this.pos.y = this.height - this.size;
    } else if (this.pos.y <= this.size) {
      if (this.vel.y < 0) this.vel.y = -this.vel.y;
      this.pos.y = this.size;
    }

    // velocity
    this.pos = this.pos.add(this.vel);

  };

  // lavalamp constructor
  var LavaLamp = function(width, height, numBalls, c0, c1) {
    this.step = 5;
    this.width = width;
    this.height = height;
    this.wh = Math.min(width, height);
    this.sx = Math.floor(this.width / this.step);
    this.sy = Math.floor(this.height / this.step);
    this.paint = false;
    this.metaFill = createRadialGradient(width, height, width, c0, c1);
    this.plx = [0, 0, 1, 0, 1, 1, 1, 1, 1, 1, 0, 1, 0, 0, 0, 0];
    this.ply = [0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 1, 0, 1, 0, 1];
    this.mscases = [0, 3, 0, 3, 1, 3, 0, 3, 2, 2, 0, 2, 1, 1, 0];
    this.ix = [1, 0, -1, 0, 0, 1, 0, -1, -1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 1, 1];
    this.grid = [];
    this.balls = [];
    this.iter = 0;
    this.sign = 1;

    // init grid
    for (var i = 0; i < (this.sx + 2) * (this.sy + 2); i++) {
      this.grid[i] = new Point(
        (i % (this.sx + 2)) * this.step, (Math.floor(i / (this.sx + 2))) * this.step
      )
    }

    // create metaballs
    for (var k = 0; k < numBalls; k++) {
      this.balls[k] = new Ball(this);
    }
  };
  // compute cell force
  LavaLamp.prototype.computeForce = function(x, y, idx) {

    var force;
    var id = idx || x + y * (this.sx + 2);

    if (x === 0 || y === 0 || x === this.sx || y === this.sy) {
      force = 0.6 * this.sign;
    } else {
      force = 0;
      var cell = this.grid[id];
      var i = 0;
      var ball;
      while (ball = this.balls[i++]) {
        force += ball.size * ball.size / (-2 * cell.x * ball.pos.x - 2 * cell.y * ball.pos.y + ball.pos.magnitude + cell.magnitude);
      }
      force *= this.sign
    }
    this.grid[id].force = force;
    return force;
  };
  // compute cell
  LavaLamp.prototype.marchingSquares = function(next) {
    var x = next[0];
    var y = next[1];
    var pdir = next[2];
    var id = x + y * (this.sx + 2);
    if (this.grid[id].computed === this.iter) {
      return false;
    }
    var dir, mscase = 0;

    // neighbors force
    for (var i = 0; i < 4; i++) {
      var idn = (x + this.ix[i + 12]) + (y + this.ix[i + 16]) * (this.sx + 2);
      var force = this.grid[idn].force;
      if ((force > 0 && this.sign < 0) || (force < 0 && this.sign > 0) || !force) {
        // compute force if not in buffer
        force = this.computeForce(
          x + this.ix[i + 12],
          y + this.ix[i + 16],
          idn
        );
      }
      if (Math.abs(force) > 1) mscase += Math.pow(2, i);
    }
    if (mscase === 15) {
      // inside
      return [x, y - 1, false];
    } else {
      // ambiguous cases
      if (mscase === 5) dir = (pdir === 2) ? 3 : 1;
      else if (mscase === 10) dir = (pdir === 3) ? 0 : 2;
      else {
        // lookup
        dir = this.mscases[mscase];
        this.grid[id].computed = this.iter;
      }
      // draw line
      var ix = this.step / (
          Math.abs(Math.abs(this.grid[(x + this.plx[4 * dir + 2]) + (y + this.ply[4 * dir + 2]) * (this.sx + 2)].force) - 1) /
          Math.abs(Math.abs(this.grid[(x + this.plx[4 * dir + 3]) + (y + this.ply[4 * dir + 3]) * (this.sx + 2)].force) - 1) + 1
        );
      ctx.lineTo(
        this.grid[(x + this.plx[4 * dir]) + (y + this.ply[4 * dir]) * (this.sx + 2)].x + this.ix[dir] * ix,
        this.grid[(x + this.plx[4 * dir + 1]) + (y + this.ply[4 * dir + 1]) * (this.sx + 2)].y + this.ix[dir + 4] * ix
      );
      this.paint = true;
      // next
      return [
        x + this.ix[dir + 4],
        y + this.ix[dir + 8],
        dir
      ];
    }
  };

  LavaLamp.prototype.renderMetaballs = function() {
    var i = 0, ball;
    while (ball = this.balls[i++]) ball.move();
    // reset grid
    this.iter++;
    this.sign = -this.sign;
    this.paint = false;
    ctx.fillStyle = this.metaFill;
    ctx.beginPath();
    // compute metaballs
    i = 0;
    //ctx.shadowBlur = 50;
    //ctx.shadowColor = "green";
    while (ball = this.balls[i++]) {
      // first cell
      var next = [
        Math.round(ball.pos.x / this.step),
        Math.round(ball.pos.y / this.step), false
      ];
      // marching squares
      do {
        next = this.marchingSquares(next);
      } while (next);
      // fill and close path
      if (this.paint) {
        ctx.fill();
        ctx.closePath();
        ctx.beginPath();
        this.paint = false;
      }
    }
  };

  // gradients
  var createRadialGradient = function(w, h, r, c0, c1) {
    var gradient = ctx.createRadialGradient(
      w / 1, h / 1, 0,
      w / 1, h / 1, r
    );
    gradient.addColorStop(0, c0);
    gradient.addColorStop(1, c1);
    return gradient;
  };

  // main loop
  var run = function() {
    requestAnimationFrame(run);
    ctx.clearRect(0, 0, screen.width, screen.height);
    lava0.renderMetaballs();
  };
  // canvas
  var screen = ge1doot.screen.init("bubble", null, true),
      ctx = screen.ctx;
  screen.resize();
  // create LavaLamps
  lava0 = new LavaLamp(screen.width, screen.height, 6, "rgba(255, 146, 152, 0.5)", "rgba(228, 0, 142, 0.5)");

  run();

})();

body {
  margin: 0;
}

.wrap {
  overflow: hidden;
  position: relative;
  height: 100vh;
}

canvas {
  width: 100%;
  height: 100%;
}

<div class = "wrap">
  <canvas id = "bubble"></canvas>
</div>