How to get ratio of scale image, when it's autoscaled by background-cover?

Tags: jquery,css,scale

Problem :

I have some problems:

I need to bind some text to positions om image (you know, like marks on an other services). Text is binded to big image, well, like 1900x1200px.

Then on other pages I add this image to background of element, stretched to all window:

#backg {
    width: 100%;
    height: 100%;
    background-image: 'url-to-big-image.jpg'
    background-repeat: no-repeat;
    background-size: cover;

And then i have troubles: I can't bind my saved marks to this element! First, I divide original height and width of element to window height and width: Qh and Qw. I take original top and left css of mark, then multiply top*Qh and Qw*left.

It works for width, but not for height! It's somehow scaled by browser, and position of elements on image doesn't match.

How can I get ratio of height scaling, to position my marks?

Thanks in advance!

Solution :

Despite the 'Hard' issue, the answer was quite simple, though not obvious :) All the matter between scales:

First, we need to find coefficients of ratio between "real" height and width of picture and shown in the background:

var imgSize = new Image();
imgSize.onload = function() {
    $('#ourdiv').css('background-image', 'url("' + imgSize.src + '")');
    h_real = this.height;
    w_real = this.width;

    q_h = h_real / $(window).height();
    q_w = w_real / $(window).width();
imgSize.src = 'path_to_img.jpg';

Then, divide our "real" positions of element to coef's. However,depending on whether the coefficient is greater, we should divide on another:

if (q_w > q_h)
        top_d = Math.round(top_d / q_h);   
        left_d = Math.round(left_d / q_h);
        top_d = Math.round(top_d / q_w);   
        left_d = Math.round(left_d / q_w);        

And voila! - all elements on same places!

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