Scratch

$(window).scroll(function () {
 if (window.innerHeight + window.pageYOffset < 1020) {
  document.getElementsByTagName('body')[0].style.backgroundAttachment = "scroll";
  document.getElementsByTagName('body')[0].style.backgroundPosition = "0% 0%"
 }
 if ((window.innerHeight + window.pageYOffset > 1021) && (window.innerHeight + window.pageYOffset < document.getElementById('portfolio').offsetTop)) {
  if (window.backgroundExtra === undefined) { window.backgroundExtra = 1021 - window.innerHeight; }
  document.getElementsByTagName('body')[0].style.backgroundAttachment = "fixed";
  document.getElementsByTagName('body')[0].style.backgroundPosition = "0% -" + window.backgroundExtra + "px"
 }
 if (window.innerHeight + window.pageYOffset > document.getElementById('portfolio').offsetTop) {
  document.getElementsByTagName('body')[0].style.backgroundAttachment = "scroll";
  document.getElementsByTagName('body')[0].style.backgroundPosition = "0% " + (document.getElementById('portfolio').offsetTop - window.innerHeight - window.backgroundExtra) + "px"
 }
});