How much of an element is visible in viewport
Here's a snippet illustrating how you can calculate this.
I've put the % values in the boxes for readability, and it even kinda "follows" the viewport ^^ :
Fiddle version
function listVisibleBoxes() {
var results = [];
$("section").each(function () {
var screenTop = document.documentElement.scrollTop;
var screenBottom = document.documentElement.scrollTop + $(window).height();
var boxTop = $(this).offset().top;
var boxHeight = $(this).height();
var boxBottom = boxTop + boxHeight;
if(boxTop > screenTop) {
if(boxBottom < screenBottom) {
//full box
results.push(this.id + "-100%");
$(this).html("100%").css({ "line-height": "50vh" });
} else if(boxTop < screenBottom) {
//partial (bottom)
var percent = Math.round((screenBottom - boxTop) / boxHeight * 100) + "%";
var lineHeight = Math.round((screenBottom - boxTop) / boxHeight * 50) + "vh";
results.push(this.id + "-" + percent);
$(this).html(percent).css({ "line-height": lineHeight });
}
} else if(boxBottom > screenTop) {
//partial (top)
var percent = Math.round((boxBottom - screenTop) / boxHeight * 100) + "%";
var lineHeight = 100 - Math.round((boxBottom - screenTop) / boxHeight * 50) + "vh";
results.push(this.id + "-" + percent);
$(this).html(percent).css({ "line-height": lineHeight });
}
});
$("#data").html(results.join(" | "));
}
$(function () {
listVisibleBoxes();
$(window).on("scroll", function() {
listVisibleBoxes();
});
});
body {
background-color: rgba(255, 191, 127, 1);
font-family: Arial, sans-serif;
}
section {
background-color: rgba(175, 153, 131, 1);
height: 50vh;
font-size: 5vh;
line-height: 50vh;
margin: 10vh auto;
overflow: hidden;
text-align: center;
width: 50vw;
}
#data {
background-color: rgba(255, 255, 255, .5);
left: 0;
padding: .5em;
position: fixed;
top: 0;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<section id="one"></section>
<section id="two"></section>
<section id="three"></section>
<section id="four"></section>
<section id="five"></section>
<section id="six"></section>
<div id="data">data here</div>
After playing around a bit I think I've found perhaps the simplest way to do it: I basically determine how much the element extends over the viewport (doesn't matter in which direction) and based on this it can easily be calculated how much of it is visible.
// When the page is completely loaded.
$(document).ready(function() {
// Returns in percentages how much can be seen vertically
// of an element in the current viewport.
$.fn.pvisible = function() {
var eTop = this.offset().top;
var eBottom = eTop + this.height();
var wTop = $(window).scrollTop();
var wBottom = wTop + $(window).height();
var totalH = Math.max(eBottom, wBottom) - Math.min(eTop, wTop);
var wComp = totalH - $(window).height();
var eIn = this.height() - wComp;
return (eIn <= 0 ? 0 : eIn / this.height() * 100);
}
// If the page is scrolled.
$(window).scroll(function() {
// Setting the opacity of the divs.
$("div").each(function() {
$(this).css("opacity", Math.round($(this).pvisible()) / 100);
});
});
});
html,
body {
width: 100%;
height: 100%;
}
body {
background-color: rgba(255, 191, 127, 1);
}
div {
width: 60%;
height: 30%;
margin: 5% auto;
background-color: rgba(175, 153, 131, 1);
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div></div>
<div></div>
<div></div>
<div></div>
<div></div>
<div></div>
<div></div>
<div></div>
A little illustration to help understand how it works:
See one more example in fiddle: https://jsfiddle.net/1hfxom6h/3/
/*jslint browser: true*/
/*global jQuery, window, document*/
(function ($) {
'use strict';
var results = {};
function display() {
var resultString = '';
$.each(results, function (key) {
resultString += '(' + key + ': ' + Math.round(results[key]) + '%)';
});
$('p').text(resultString);
}
function calculateVisibilityForDiv(div$) {
var windowHeight = $(window).height(),
docScroll = $(document).scrollTop(),
divPosition = div$.offset().top,
divHeight = div$.height(),
hiddenBefore = docScroll - divPosition,
hiddenAfter = (divPosition + divHeight) - (docScroll + windowHeight);
if ((docScroll > divPosition + divHeight) || (divPosition > docScroll + windowHeight)) {
return 0;
} else {
var result = 100;
if (hiddenBefore > 0) {
result -= (hiddenBefore * 100) / divHeight;
}
if (hiddenAfter > 0) {
result -= (hiddenAfter * 100) / divHeight;
}
return result;
}
}
function calculateAndDisplayForAllDivs() {
$('div').each(function () {
var div$ = $(this);
results[div$.attr('id')] = calculateVisibilityForDiv(div$);
});
display();
}
$(document).scroll(function () {
calculateAndDisplayForAllDivs();
});
$(document).ready(function () {
calculateAndDisplayForAllDivs();
});
}(jQuery));
div {
height:200px;
width:300px;
border-width:1px;
border-style:solid;
}
p {
position: fixed;
left:320px;
top:4px;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="div1">div1</div>
<div id="div2">div2</div>
<div id="div3">div3</div>
<div id="div4">div4</div>
<p id="result"></p>