Positioning to specific element on parent background - html

Question

I want to make a device-independent animation in HTML5/CSS3. That means I have a background image, specifically drawn so that its edges can be cut off, and I am using it in a div element with background-size: cover, like this:
#main-image {
background: url(intro1-1.jpg) no-repeat center center fixed;
background-size: cover;
height: 100%;
width: 100%;
overflow: hidden;
z-index: 0;
}
#propeller {
background: url(propeller2.png) no-repeat;
position: relative;
top: 265px;
left: 1080px;
z-index: 10;
background-size: 100% 100%;
width: 18%;
height: 12%;
}
<div id="main-image"><div id="propeller"></div></div>
On top of the background layer, I want to draw the animating layer. Here comes the trouble: how do I position the transparent animating parts to a specific position in the full (non-scaled) background image?
I'd also need to scale the animation layer using the same ratio as the background was scaled. But how do I do that?
SO in effect, I'm looking for a way to load the HD background image, define the HD animating layer on top of it, and then apply the cover to fill the full browser screen.
What is the simplest way to do this?

Answer 1

In my experience this is hard to do in pure CSS. I've made something similar to what you're asking here: http://jsfiddle.net/ahhcE/
Here's the propeller specific code:
#propeller {
background: url(propeller2.png) no-repeat;
background-color: blue;
position: absolute;
top: 50%;
left: 50%;
margin-left: -9%;
margin-top: -6%;
z-index: 10;
background-size: 100% 100%;
width: 18%;
height: 12%;
}
I positioned it absolute just for ease, but you're likely going to want it relative if it's positioned relative to the parent div.
(sorry for the colors, my replacement for your images)
The problem is that on the top margin, and height percentages, the browser inherits those values from the width of the window. So you'll notice that if you resize the view window the box doesn't stay perfectly centered. I've usually solved this in the past using javascript. Something like this:
function heightAdjust() {
var windowHeight = $(window).height();
totalMenuHeight = $("#menu").height();
document.getElementById('menu').style.marginTop = windowHeight / 2 - totalMenuHeight / 2 + 'px';
$('.thing').css("height", windowHeight+'px');
}
Hopefully that helps. Centering vertically is really your only issue here, you can also hack this successfully using table styling which is what a few sites use for vertical positioning. More on that, and other solutions here: http://coding.smashingmagazine.com/2013/08/09/absolute-horizontal-vertical-centering-css/