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              // create a field of sugar concentrations
var sugar = new field2D(256);
var sugar_old = sugar.clone();

// fill it:
function reset_sugar() {
	// fill with uniform noise

// mouse adds sugar:
function mouse(e, pt) {
	sugar.deposit(10, pt);

var agents = [];
for (var i=0; i<40; i++) {
		pos: new vec2(random(), random()),
		vel: vec2.random(0.001),
		size: 0.02,
    // stored recollection of previous sense
		sense_memory: 0.5,

var source = {
  pos: new vec2(0.5, 0.5),
	vel: vec2.random(),

function update() {
  // update field:
  var tmp = sugar_old;
  sugar_old = sugar;
  sugar = tmp;
  // diffuse it:
  sugar.diffuse(sugar_old, 0.1);
  // small background noise:
  //sugar.map(function(v) { return v + 0.01*srandom(); });

  // apply source agent to sugar:
  sugar.deposit(2, source.pos);
  // and then move it (random walk):
	for (var a of agents) {
    // get the sugar level at this location:
		var sense = sugar.sample(a.pos);
    // sanity: no such thing as a negative intensity:
    sense = Math.max(0, sense);
    // update the field to show that we removed sugar here:
	sugar.deposit(-sense, a.pos);
    // compare to the previous one:
    var change = sense / a.sense_memory;
		if (change > 1) {
      // getting better
      // forward swimming behavior:
		} else {
      // getting worse
      // tumbling behavior:
		// remember for the next time:
		a.sense_memory = sense;
		// locomotion
function draw() {
  // draw field
  // draw agents
  for (var a of agents) {
  	// body
  		.circle([0.3, 0]).circle([-0.3, 0]);
    // sensor (shows current state by color)
    draw2D.color(1-a.sense_memory, a.sense_memory, 0.5)
  	  .circle([0.3, 0.], 0.7);
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