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If specific div is in viewport fade in audio, if div is out of viewport fade out

I have a one page scrolling website, and what I would like to do is fade in and out ambient sounds per section. I have found a few libs that fade sound in and out on pixels but being that this site scales I cant get them to trigger properly.
Basically it would be for each section:
<div class="section_content" id="1"> /// this is the section that would need to trigger audio file 1
<div class="main_image">
</div>
<div class="main_content">
Text content and information here
</div>
</div>
<div class="section_content" id="2"> /// this is the section that would need to trigger audio file 2
<div class="main_image">
</div>
<div class="main_content">
Text content and information here
</div>
</div>
https://github.com/kamblack/AudioFade/blob/master/README.md
This was the library I tied but since its based off of pixels it wont quite work.

Adding background color in TurnJS

I have experienced problem I have never faced before and even googling it did not help.
I have basic code from the official page of script TurnJS, which is the problem I have with.
<div id="flipbook">
<div class="hard"> Turn.js </div>
<div class="hard"></div>
<div> Page 1 </div>
<div> Page 2 </div>
<div> Page 3 </div>
<div> Page 4 </div>
<div class="hard"></div>
<div class="hard"></div>
This works as it should but is has transparent background so when you have any text on pages it overlays themselves so you cannot read the text properly.
When I add
#flipbook div{
background: white;
}
It somehow breaks. Pages skew when hovered etc.
I cannot insert a jsfiddle example because the link for turnjs script is not https and I am not allowed to insert http links. And here it shows me error in code.
The skewed page is shown on image below.
Does someone know how to add a bg-colorto that flipbook?
I would aprreciate any comments.

You can add another class to your pages which specifies a background colour:
<div id="flipbook">
<div class="hard"> Turn.js </div>
<div class="hard"></div>
<div class="pageStyle"> Page 1 </div>
<div class="pageStyle"> Page 2 </div>
<div class="pageStyle"> Page 3 </div>
...
And then specify everything you need in that class. The other method is to load background images for each page. This is generally how a magazine would be created in TurnJS as it's easier to automate.
Although I just went over their docs looking for this (I know it works, I've done it before) and couldn't find much about it.

How to set a div (#jumbotron) the be the height of the viewport minus the height of another div (#header) which chnges dynamically

Background/skill: I am starting to test out using bootstrap 4 (first time using bootstrap) for a custom Wordpress theme. I have the basic understanding of HTML and CSS, but with the newer CSS and with any jQuery or javascript, I am fairly clueless and usually just hacking code I find online to try and make it work for me, with little understanding of how/why it works if I succeed.
Missing out a bunch of page header info link links to scripts and stuff which I do understand may eventually affect any suggested method, and just to keep it simple. For now I have the following HTML so far:
<div id="page" class="site">
<div id="header" class="container-fluid">
<div id="masthead" class="site-header row row-eq-height">
<div id="logo" class="col-sm-4 col-md-6 col-lg-3"></div>
<!--logo-->
<div id="fruits" class="align-self-center d-none d-md-block col-md-3 col-lg-6"></div>
<!--fruits-->
<div id="social" class="d-none d-md-block col-md-3 col-lg-3"></div>
<!--social-->
</div>
<!-- #masthead -->
</div>
<!--header-->
<div id="jumbotron" class="row">
<div id="notice" class="align-self-center col-md-3"></div>
</div>
<!--jumbotron-->
<div id="content" class="site-content"></div>
<!-- #content -->
<div id="footer" class="container-fluid">
<footer id="colophon" class="row"></footer>
<!-- #colophon -->
</div>
<!--footer-->
</div>
<!-- #page -->
The jumbotron has a background image and a div (#notice) inside it which will float to the left with some text inside. I want the div to take up all remaining viewport space after the header. I have managed to get the jumbotron div be the height of the viewport minus the header using the following CSS:
height:calc(100vh - 155px);
But this only works if I know the height of the header is 155px. In reality, it changes at different screen widths and depending on content.
Is there a way to set the CSS to use a calculation for the dynamic header?
If not, do I need to use some kind of jquery or javascript? If so, I would be very grateful for some fairly detailed instructions.
Note: I do not want the #jumbotron to be the height of the page and sit behind the header div as a solution because the background image display is important.
Thank you in advance :)

Since you can't know easily the height of the header in every device because it is a percentage of the height... And because it depends on a big bunch of CSS rules form BootStrap and your Wordpress theme... You will just measure it to deduct the remaining space.
<script>
$(document).ready(function(){
function adjustJumbotronHeight(){
var headerHeight = $("#header").outerHeight();
var viewportHeight = $(window).innerHeight();
var remains = viewportHeight - headerHeight;
$("#jumbotron").css({"height":remains});
}
// On page load
adjustJumbotronHeight();
// On resize (or mobile orientation change)
$(window).on("resize",function(){
adjustJumbotronHeight();
});
});
</script>

Setting the background of a nav using toDataURL?

So I've been working on a site that uses a JavaScript animation for the background of some of the sections. Right now, the section in question is broken down like this:
<section id="intro">
<div class="info">
<div class="container">
<div class="row">
<div class="col-full"><h1>My Name</h1></div>
</div>
<div class="row"><div class="col-1-4 centered line"></div></div>
<div class="row">
<div class="col-full"><h4>Video<span class="cool">s</span> and stuff</h4></div>
</div>
</div>
</div>
<nav id="nav">
<!-- The content between <span> </span> tags will be hidden on mobile version -->
<ul class="clearfix">
<li>Pro<span>file</span></li>
<li>Ski<span>lls</span></li>
<li>Exp<span>erience</span></li>
<li>Edu<span>cation</span></li>
<li>Por<span>tfolio</span></li>
<li>Con<span>tact</span></li>
</ul>
</nav>
</section>
Originally, I wanted to set the background of the entire section using
dataUrl = canvas.toDataURL();
document.getElementById("intro")[0].style.background='url('+dataUrl+')'
but that didn't work, so I ended up going with
dataUrl = canvas.toDataURL();
document.getElementsByClassName("info")[0].style.background='url('+dataUrl+')'
which did. However, the Nav bar scrolls with the page as you move down, and as soon as the script for scrolling it down kicks in, the background reverts to its default blue color.
My first thought was to simply add
document.getElementById("nav")[0].style.background='url('+dataUrl+')'
but that didn't work. I had some success with
document.getElementByClassName("clearfix")[0].style.background='url('+dataUrl+')'
but only the area immediately surrounding the text used the animated background; the rest was still blue.
So I'm not sure if it's some issue with getElementById, or if you can't use style.background with <section> and <nav>.
Anyone have any thoughts?

I dont think you need the array selector with getElementById so try it on the whole section again and see if that will fix it for you.
So do document.getElementById("intro").style.background='url('+dataUrl+')'

Jquery scroller based on mouse position

So I have looked at jquery hover and this is all well and great but how can I use a list into this? The example that the previous user uses is all image backgrounds.
Im not sure if the is in the right place.
<div class="backdrop">
<ul id="year"></ul>
<div class="direction left"></div>
<div class="direction right"></div>
jsfiddle

Since I could not change background position on <ul><li> I decided to use margin here if my solution.