When you write on the Google homepage using handwriting mode, the drawn path is sent as a POST request to Google Input Tools IME API as an array of X,Y points. Following is a sample payload request sent to the Google IME API.

 "method": "POST",
 "url": "https://www.google.com/inputtools/request?ime=handwriting",
 "postData": { "text": "{
   "device":"Chrome/19.0.1084.46 Safari/536.5",
   "options":"enable_pre_space",
   "requests":[{"writing_guide":{
     "writing_area_width":1920,
     "writing_area_height":617},
    "ink":[[[582,582,582,581,581,580],
            [273,274,275,275,276,276],
            [0,529,537,554,569,1009]]],
     "language":"en"}]}"}

And here’s a snippet of deobfuscated JavaScript code that traces and sends your handwriting to Google IME API.

_.Qx = function () {
  if (!(0, _.Dx)()) {
    var a = (0, _.rx)(_.Mx, _.Cx.slice(0)),
      b = {
        writing_guide: {
          writing_area_width: _.yx.width,
          writing_area_height: _.yx.height,
        },
        ink: (0, _.tc)(_.Cx, function (a) {
          return [
            (0, _.tc)(a, function (a) {
              return a.x;
            }),
            (0, _.tc)(a, function (a) {
              return a.y;
            }),
            (0, _.tc)(a, function (a) {
              return a.t;
            }),
          ];
        }),
      },
      c = (0, _.Rx)();
    c && 0 < c.length && (b.pre_context = c);
    c = window.google.kHL || 'en';
    c = c.replace('-', '_');
    b.language = c;
    var b = window.JSON.stringify({
        device: window.navigator.userAgent,
        options: 'enable_pre_space',
        requests: [b],
      }),
      d = new _.jx();
    _.kx.push(d);
    c && (0, _.cn)(d, 'complete', c);
    (0, _.cn)(d, 'ready', (0, _.Ya)(_.yda, d));
    d.lK = window.Math.max(0, 6e3);
    d.send('/inputtools/request?ime=handwriting&app=gws&cs=1', 'POST', b, {
      'Content-Type': 'application/json',
    });
  }
  _.Ux.clearRect(0, 0, _.yx.width, _.yx.height);
  _.Bx = _.o;
  _.Cx = [];
  (0, _.Vx)();
};

_.Gx = function (a) {
  var b = new window.Date().getTime() - _.Jx;
  if (!a.touches) {
    var c = a.pageX - window.pageXOffset,
      a = a.pageY - window.pageYOffset;
    return {
      x: c,
      y: a,
      t: b,
    };
  }
  c = a.touches[0].pageX - window.pageXOffset;
  a = a.touches[0].pageY - window.pageYOffset;
  return {
    x: c,
    y: a,
    t: b,
  };
};

_.zx = function () {
  _.Ux.clearRect(0, 0, _.yx.width, _.yx.height);
  _.Ux.save();
  _.Ux.lineWidth = 6;
  _.Ux.fillStyle = '#4d90fe';
  _.Ux.strokeStyle = '#4d90fe';
  _.Ux.lineCap = 'round';
  _.Ux.lineJoin = 'round';
  _.Ux.shadowColor = 'rgba(0,0,0,0.3)';
  _.Ux.shadowBlur = 3;
  _.Ux.shadowOffsetY = 2;
  for (var a = 0; a < _.Cx.length; a++) (0, _.Wx)(_.Ux, _.Cx[a]);
  _.Ux.restore();
};