console 命令行工具 X clear

console

window.onscroll = function(e) {
  let scrollTop = document.body.scrollTop || document.documentElement.scrollTop; // 获取已滚动的高度

  let img2_offsetTop = document.getElementsByClassName('img')[0].offsetTop; // 获取图片2距离页面顶部的距离(包含已滚动被隐藏的部分)

  if(scrollTop > 82 && scrollTop < 482) { // +200
    console.log('scrollTop: ' + scrollTop)
    changeImg(1, scrollTop - 82)
  }
}

function changeImg(index, width) {
  let img = document.getElementById('img' + index);
  img.style.width = width + 'px'
}

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>滚动效果</title>
</head>
<body>
  <div class="box">
    <div class="img">
      <img src="http://static.runoob.com/images/demo/demo1.jpg" alt="" id="img1">
      <img src="http://static.runoob.com/images/demo/demo2.jpg" alt="" id='img2'>
    </div>
  </div>
</body>
</html>

.box {
  position: relative;
  height: 180vh;
}

img {
  width: 100%;
}

.img {
  bottom: 100vh;
  position: absolute;
  width: 100%;
}

#img1 {
  position: absolute;
  width: 50%;
  z-index: 100;
  border-radius: 50%;
}

#img2 {
  position: absolute;
  width: 100%;
}