var pixelSize = 16;

interact('.rainbow-pixel-canvas')
  .origin('self')
  .draggable({
    snap: {
        targets: [ interact.createSnapGrid({
          x: pixelSize, y: pixelSize
        }) ]
    },
    // allow multiple drags on the same element
    maxPerElement: Infinity
  })
  // draw colored squares on move
  .on('dragmove', function (event) {
    var context = event.target.getContext('2d'),
        // calculate the angle of the drag direction
        dragAngle = 180 * Math.atan2(event.dx, event.dy) / Math.PI;

    // set color based on drag angle and speed
    context.fillStyle = 'hsl(' + dragAngle + ', 86%, '
                        + (30 + Math.min(event.speed / 1000, 1) * 50) + '%)';

    // draw squares
    context.fillRect(event.pageX - pixelSize / 2, event.pageY - pixelSize / 2,
                     pixelSize, pixelSize);
  })
  // clear the canvas on doubletap
  .on('doubletap', function (event) {
    var context = event.target.getContext('2d');

    context.clearRect(0, 0, context.canvas.width, context.canvas.height);
  });

  function resizeCanvases () {
    [].forEach.call(document.querySelectorAll('.rainbow-pixel-canvas'), function (canvas) {
      canvas.width = document.body.clientWidth;
      canvas.height = window.innerHeight * 0.7;
    });
  }

  // interact.js can also add DOM event listeners
  interact(document).on('DOMContentLoaded', resizeCanvases);
  interact(window).on('resize', resizeCanvases);