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// detect if browser firefox as it appears to be the only
//  browser that return deltaModes different than DOM_DELTA_PIXEL
//  Ref: https://stackoverflow.com/a/37474225/4146962
var FF = !(window.mozInnerScreenX == null);

// Function grabbed from the reference above
// It tries to read current line-height of document (for 'lines' deltaMode)
function getScrollLineHeight() {
    var r;
    var iframe = document.createElement('iframe');
    iframe.src = '#';
    document.body.appendChild(iframe);
    var iwin = iframe.contentWindow;
    var idoc = iwin.document;
    idoc.open();
    idoc.write('<!DOCTYPE html><html><head></head><body><span>a</span></body></html>');
    idoc.close();
    var span = idoc.body.firstElementChild;
    r = span.offsetHeight;
    document.body.removeChild(iframe);
    return r;
}

// html5 elements
var vid = document.getElementById("v"); // HTML5 video element
var canvas = document.getElementById("c"); // HTML5 canvas element
var context = canvas.getContext('2d'); // Canvas context
var momentum = document.getElementById('m'); // Current momentum display
var delta = document.getElementById('d'); // Current deltaMode display
var lineheight = document.getElementById('l'); // Current deltaMode display

// global variables
var ch = 120; // canvas with (could be window.innerHeight)
var cw = Math.round(ch * (16 / 9)); // 16/9 proportion width
var targetOffset = 0; // Video offset target position when scrolling

// deltaY to FPS coefficients (for fine tuning)
// Possible mouse scroll wheel 'event.deltaMode'
//  modes are: 0:'pixels', 1:'lines', 2:'page'
var pc = 1000; // 'pixels' deltaY coefficient
var lh = "disabled"; //getScrollLineHeight(); // get line-height of deltaMode 'lines'
lineheight.value = lh; // display current document line height
coefficient = 30;
var deltaModes = ['pixels', 'lines', 'page']; // For deltaMode display

// Sets canvas dimensions
canvas.width = cw;
canvas.height = ch;

// Pauses video (this also starts to load the video)
vid.pause();

// Listens video changes time position
vid.addEventListener('seeked', function() {
  // Updates canvas with current video frame
  context.drawImage(vid, 0, 0, cw, ch);
});

// Listens mouse scroll wheel
window.addEventListener('wheel', function(e) {

  // Don't do what scroll wheel normally does
  e.preventDefault();

  // You don't need an amount, just positive or negative value: -1, 1
  var deltabs = 1;
  if (e.deltaY<0) deltabs = -1;

  // Disable page scrolling, modes[e.deltaMode]=='page'
  if (e.deltaMode>1) return false;

	delta.value = deltaModes[e.deltaMode];
  // Normally scrolling this should be a subtraction 
  //   not a sum but "I like it like this!"
  // targetOffset = targetOffset + (e.deltaY / coefficient); // e.deltaY is the thing!!
  targetOffset = targetOffset + (deltabs/coefficient);

  // Shows current momentum
  momentum.value = targetOffset;

  return false;
});

// Updates canvas on a loop (both for play or pause state)
var renderLoop = function() {
  requestAnimationFrame(function() {

    // This parts updates canvas when video is paused
    // Needs 'seeked' listener above
    if (vid.paused || vid.ended) {

      // Reduce target offset gradually
      targetOffset = targetOffset * 0.9;
      // Show current momentum
      momentum.value = Math.round(targetOffset * 100) / 100;

      // this part joins start and end of video when scrolling
      // forward & backwards
      var vct = vid.currentTime - targetOffset;
      if (vct < 0) {
        vct = vid.duration + vct;
      } else if (vct > vid.duration) {
        vct = vct - vid.duration;
      }
      vid.currentTime = vct;

      // This parts updates canvas when video is playing
    } else {
      // update canvas with current video frame
      context.drawImage(vid, 0, 0, cw, ch);
    }

    renderLoop(); // Recursive call to loop
  });
};
renderLoop(); // Initial call to loop
Poised Puffin