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JS calculate current center of screen

I searched a lot but couldn't find the answer I want. I have a JS script which shows an image when a fixed element is hovered. However I would like to always have the image at the center of the screen, not matter where the user scrolls. How can I reach that?
My JS function:
$(document).ready(function($) {
$('.trigger').mouseover(function() {
// find our span
var elem = $(this).siblings('span');
// get our img url
var src = elem.attr('data-original');
// change span to img using the value from data-original
elem.replaceWith('<img src="' + src + '"width="400" style="display:block;position:absolute;top: 0; left: 0; bottom: 0; right: 0;margin: auto;"/>');
});
$('.trigger').mouseout(function() {
// find our span
var elem = $(this).siblings('img');
// get our img url
var src = elem.attr('src');
// change img to span using the value from data-original
elem.replaceWith('<span data-original="'+src+'"></span>');
});
});
JSFiddle: https://jsfiddle.net/rbmd6a39/
I can get offset from the top of page using
window.pageYOffset;
But I don't know where to put that value to have it in the center.

You could give that image styles that would make it fixed in the center of your screen. No need for JS there.
Now also works on very large images with max-width, max-height and object-fit: contain!
.always-centered {
display: block;
position: fixed;
top: 50%;
left: 50%;
max-width: 100%;
max-height: 100%;
-o-object-fit: contain;
object-fit: contain;
transform: translate(-50%, -50%);
z-index: 99;
}

You don't actually need to use JavaScript for centering it. You could use position: fixed and center using left: 50%; transform: translateX(-50%);in CSS, which would be less demanding of the client computer.
Using JS, though, it'd be pretty much the same thing: the image would need to be fixedly positioned and you'd define left as half of the viewport minus half of your image width.
I suggest using CSS and classes if possible, since this is less convoluted and all calculations will be dealt be the browser itself.

Before and After Slider with Multiple Images

I am trying to create a page that has before and after images that use a slider based on mouse movement to show both images. I need to have multiple sliders on the page and can not seem to get them to work. Below are a couple of different examples I have found and the challenges I am having.
http://codepen.io/dudleystorey/pen/JDphy - This works well with mobile but I can not seem to add a second version without adding css for every image since the background image is embedded in the css.
div#inked-painted {
position: relative; font-size: 0;
-ms-touch-action: none;
-webkit-touch-callout: none;
-webkit-user-select: none;
}
div#inked-painted img {
width: 100%; height: auto;
}
div#colored {
background-image: url(https://s3-us-west2.amazonaws.com/s.cdpn.io/4273/colored-panel.jpg);
position: absolute;
top: 0; left: 0; height: 100%;
width: 50%;
background-size: cover;
}
http://codepen.io/ace/pen/BqEer - Here is the other example that does not work as well with mobile. I can add the second image but the slider works all the images simultaneously and not individually when a second image is added.
Can anyone help with adding the second image. I am sure both of these are very workable but I am missing something in my css/javascript knowledge that is not allowing multiple images.

You need to loop though all classes to be able set the eventhandlers individual. Your codepen example could be change to this to work with individual images at once:
var blackWhiteElements= document.getElementsByClassName("black_white");
for (i = 0; i < blackWhiteElements.length; i++) {
initCode($(blackWhiteElements[i]));
}
function initCode($black_white) {
var img_width = $black_white.find('img').width();
var init_split = Math.round(img_width/2);
$black_white.width(init_split);
$black_white.parent('.before_after_slider').mousemove(function(e){
var offX = (e.offsetX || e.clientX - $black_white.offset().left);
$black_white.width(offX);
});
$black_white.parent('.before_after_slider').mouseleave(function(e){
$black_white.stop().animate({
width: init_split
},1000)
});
}
codepen here: http://codepen.io/anon/pen/mJPmKV

Your first attempt is near sufficient.
Assign the background-image inline in the html to avoid extra classes
<div id="colored" style="background-image: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/4273/colored-panel.jpg);"></div>
change background-size on #colored to background-size: auto 100%; to reduce the "shaky" effect
background-size: auto 100%;

How to resize a div to clients viewport height?

Ok, so i want to have a series of divs which are the exact width and height of the user's browser window, regardless of the screen size. I can easily make the divs stretch horizontally with "width: 100%;" but i cant work out how to make the height stretch itself. I am guessing that i need to use some bit of javascript to judge the height, and then another piece to resize the seperate divs. Unfortunately I am a complete javascript n00b and after two hours of seemingly fruitless searching and coming up with about 100 "solutions" this was as far as id gotten (Im sure that at some point I have probably been closer to the answer):
var viewportHeight = "height:" + document.documentElement.clientHeight;
getElementById('section-1').setAttribute('style', viewportHeight);
<div class="section" id="section-1"></div>
<div class="section" id="section-2"></div>
<div class="section" id="section-3"></div>
edit:
ah i should be more clear, im attempting to have all three divs take up the entire screen, so you have to scroll down to see each one - almost like seperate slides. The idea is that each one takes up the entire screen so you cant see the next section until you scroll down, rather than having three divs which take up a third of the screen.

If you haven't already tried it, you'll want to look at parent:child inheritance of elements within the DOM by way of using CSS.
What I want to STRESS is that everyone giving you JS hacks to accomplish this is not only providing you with overkill (YOU did ask for a JavaScript solution, so they gave it to you!), but it's also a deviation from standards. HTML is for structure, CSS is for presentation, and JavaScript is for behavioral aspects... setting a div to the width of the viewport on load is a PRESENTATION aspect and should be done in CSS... not JavaScript. If you were trying to change the width based on events or user interaction, then yes JavaScript is your friend... but stick with just HTML and CSS for now.
The trick is that most elements have an undefined height - and height is later defined by the content that the element holds.
If you want to 'trick' an element into having a height other than what it wants to default to, you'll have to explicitly define it. Since you want to inherit your height from the viewport, you'll have to define the height at the top and bring it down...
You might be in luck and can avoid JavaScript altogether (unnecessary). Just use CSS.
Try something like:
html, body {
height: 100%;
width: 100%;
}
Now, when you try to set your div's later on, specify width: 100% and the height gets inherited from the html --> body --> div.
Try that and see if that solves your problem - if not, point us to a website, a pastebin, or a SOMETHING with code in it that we can just show you how to do it (whereas what you posted for code was an attempt in JavaScript which is only 1 part of the code - post the full thing either to a server or temp site like pastebin).
Here is some sample code I wrote (tested in Chromium):
The HTML:
<html>
<head>
<title>Test Divs at 100%</title>
<link rel="stylesheet" type="text/css" href="divtest.css"
</head>
<body>
<div class="test1">aef</div>
<div class="test2">aef</div>
<div class="test3">aef</div>
</body>
</html>
The CSS:
html, body {
width: 100%;
height: 100%;
background-color: #793434;
padding: 0;
margin: 0;
}
div {
height: 100%;
width: 100%;
}
.test1 {
background-color: #E3C42E;
}
.test2 {
background-color: #B42626;
}
.test3 {
background-color: #19D443
}

try this
div#welcome {
height: 100vh;
background: black;
color: white;
}
div#projects {
height: 100vh;
background: yellow;
}
<div id="welcome">
your content on screen 1
</div>
<div id="projects">
your content on screen 2
</div>
it should work for you, but little support in IE

A bit of jQuery should do it:
$(document).ready(function() {
var window_height = $(window).height();
$('#section-1").height(window_height);
});
And if you want to keep 100% height on window resize:
$(document).ready(function() {
function viewport_height() {
var window_height = $(window).height();
$('#section-1").height(window_height);
}
viewport_height();
$(window).resize(function() {
viewport_height();
});
});

try this
window.onload = init;
function init()
{
var viewportHeight = "height:" + document.documentElement.clientHeight+"px;";
document.getElementById('section-1').setAttribute('style', viewportHeight);
}

Here is a script free solution, just CSS. This assumes that the divs are directly in the body element or a parent with position absolute and the parent has no padding.
#section-1 {
position: absolute;
height: 100%;
width: 100%;
background: #ff0000;
}
#section-2 {
position: absolute;
width: 100%;
top: 100%;
height: 100%;
background: #00ff00;
}
#section-3 {
position: absolute;
width: 100%;
top: 200%;
height: 100%;
background: #0000ff;
}
See fiddle: http://jsfiddle.net/QtvU5/1/

Zoom on a picture on resizing

I'm developing a website where I have to display a picture (not a problem).
But, I have to display it as this link: http://buildinternet.com/project/supersized/slideshow/3.2/demo.html
So, On resizing, I have to zoom on the picture to never scale it.
Does some on know how to do it?
Here is what I have:
html:
<div id="container_images">
<ul>
<li><img src="images/desktop/myimage.jpg" alt="An awesome image"></li>
</ul>
</div>
CSS:
#container_images{
display: block;
position: fixed;
left: 0;
top: 0;
overflow: hidden;
z-index: -999;
height: 100%;
width: 100%;
}
#container_images li{
display: block;
list-style: none;
z-index: -30;
position: fixed;
overflow: hidden;
top: 0;
left: 0;
width: 100%;
height: 100%;
opacity:1;
}
#container_images li img{
width: 100%;
height: 100%;
display: block;
}

If I understood correctly you don't need the browser's inbuilt zooming (ctrl+), you just want to make the picture fill in the whole window of the browser when the window is resized.
You will need 2 things for that.
First, you need some javascript to execute on window resize event, then you will need some simple maths to calculate the center of the picture taking into account the new window size and set left/top margin of the picture to new values.
You can easily do that with jQuery:
$(window).resize(function() {
var h = $(window).height();
var w = $(window).width();
var pic = $('#container_images img');
var pic_width = pic.width(), pic_height = pic.height();
$(pic).css('margin-left': (w - pic_width)/2).css('margin-top': (h - pic_height)/2);
});
One thing to remember is that if your picture's width/height is already changed from its original size, you will get that new size from width() and height() functions.
So you either should make sure the original is not touched, or use one of the solution out there for grabbing the original picture size (e.g. How do I get actual image width and height using jQuery?).
Also the above snippet is to just get you started, you should grab the picture and its parameters in some init function and only recalculate/modify css on resize event.

How to get this kind of web page effect

The little popup window appears in the middle of the original page.
The original page is covered by grey shade if not by the popup window.
The underneath original page can still be scrolled up and down.

Follow these steps:
1) Create this CSS rule:
.overlay {
width: 100%;
height: 100%;
position: fixed;
top: 0;
left: 0;
opacity: 0.5;
background: #666;
filter: alpha(opacity=50); /* opacity for IE browsers */
}
2) Add this code to your jQuery:
$("body").prepend("<div class='overlay'></div>");
3) When done, remove it like this:
$(".overlay").remove();
Didn't test this, but it should work (maybe with very minor modifications). This is one way, if you prefer doing it by yourself. You can, however, use existing solutions such as Twitter's Bootstrap lib which is cool, and I recommend it.
http://twitter.github.com/bootstrap/
Regards.

You could use the JQueryUI dialog widget http://jqueryui.com/dialog/#modal

This is easy enough to achieve with some simple CSS...
The overlay (the grey background) is fixed in place and covers everything below:
#overlay {
display: none;
position: fixed;
top: 0;
left: 0;
width: 100%;
height: 100%;
background-color: #000;
opacity: 0;
filter: alpha(opacity=0);
z-index: 2; // above content
}
The "dialog" itself is similar in style, but smaller:
#dialog {
display: none;
position: fixed;
width: 500px;
height: 250px;
background-color: #fff;
z-index: 3; // above 'overlay'
}
The top and left attributes can be calculated with simple JavaScript, so that the dialog can be positioned in the center of the browser:
positionDialog = function() {
if (typeof window.innerHeight != 'undefined') {
dialog.top = parseInt(window.innerHeight / 2) - dialog.height;
dialog.left = parseInt(window.innerWidth / 2) - dialog.height;
}
}
And also upon window resize:
$(window).resize(function() {
positionDialog();
}
Notice how the CSS sets these DIVs to display: none. They are hidden until called, which is done by setting them to display: block.
These days, I find that it's much simpler and more robust to rely on jQuery UI's excellent dialog widget.

It's called a light box. There's a way that you can do it using only CSS:
http://www.emanueleferonato.com/2007/08/22/create-a-lightbox-effect-only-with-css-no-javascript-needed/
The key for darkening the background is the CSS opacity property of a box that you cover the background with, which you can set a black background and use this CSS for transparency:
-moz-opacity: 0.8;
opacity:.80;

You could take a look at the modal included in Twitter Bootstrap: http://twitter.github.com/bootstrap/javascript.html#modals