javascript - Get final size of background image

Question

is there an easy way to get the final height and width of a background image with Javascript or jQuery even if a background-size property was applied?

I mean, I know I can get the background image url and load it to an Image object and then get the width and height. But it is the size of the source image. If someone scaled it with CSS then the size changed

How can I find its final size?

@edit

it is different from the question marked as similar because it doesnt say how to get the size in pixels if someone changed the background-size

Answer 1

Using getComputedStyle, I've created this script that returns the width and height of a given element's background, in pixels. It works with:

• Dimensions (width or height) set to auto, either explicitly or because no specific value was given (width and height default to auto)
• Dimensions set to percentage %
• Dimensions set to pixels px
• Dimensions set to a combination of any of the previous. (i.e width: 100px; height: auto or width: auto; height: 32.4% or height: 100px; width: 2% or width: 21.2%)
• background-size set to cover or contain

It works if background-size is set with an external CSS file, inline CSS, inline header CSS or if it is not set at all (meaning width and height are auto).

Here's a JsFiddle (with cover example)

http://jsfiddle.net/gp4e9d3z/3/

And here's StackOverflow's code snippet (with percentage auto units)

function getBackgroundSize(elem) {
    // This:
    //       * Gets elem computed styles:
    //             - CSS background-size
    //             - element's width and height
    //       * Extracts background URL
    var computedStyle = getComputedStyle(elem),
        image = new Image(),
        src = computedStyle.backgroundImage.replace(/url((['"])?(.*?)1)/gi, '$2'),
        cssSize = computedStyle.backgroundSize,
        elemW = parseInt(computedStyle.width.replace('px', ''), 10),
        elemH = parseInt(computedStyle.height.replace('px', ''), 10),
        elemDim = [elemW, elemH],
        computedDim = [],
        ratio;
    // Load the image with the extracted URL.
    // Should be in cache already.
    image.src = src;
    // Determine the 'ratio'
    ratio = image.width > image.height ? image.width / image.height : image.height / image.width;
    // Split background-size properties into array
    cssSize = cssSize.split(' ');
    // First property is width. It is always set to something.
    computedDim[0] = cssSize[0];
    // If height not set, set it to auto
    computedDim[1] = cssSize.length > 1 ? cssSize[1] : 'auto';
    if(cssSize[0] === 'cover') {
        // Width is greater than height
        if(elemDim[0] > elemDim[1]) {
            // Elem's ratio greater than or equal to img ratio
            if(elemDim[0] / elemDim[1] >= ratio) {
                computedDim[0] = elemDim[0];
                computedDim[1] = 'auto';
            } else {
                computedDim[0] = 'auto';
                computedDim[1] = elemDim[1];
            }
        } else {
            computedDim[0] = 'auto';
            computedDim[1] = elemDim[1];
        }
    } else if(cssSize[0] === 'contain') {
        // Width is less than height
        if(elemDim[0] < elemDim[1]) {
            computedDim[0] = elemDim[0];
            computedDim[1] = 'auto';
        } else {
            // elem's ratio is greater than or equal to img ratio
            if(elemDim[0] / elemDim[1] >= ratio) {
                computedDim[0] = 'auto';
                computedDim[1] = elemDim[1];
            } else {
                computedDim[1] = 'auto';
                computedDim[0] = elemDim[0];
            }
        }
    } else {
        // If not 'cover' or 'contain', loop through the values
        for(var i = cssSize.length; i--;) {
            // Check if values are in pixels or in percentage
            if (cssSize[i].indexOf('px') > -1) {
                // If in pixels, just remove the 'px' to get the value
                computedDim[i] = cssSize[i].replace('px', '');
            } else if (cssSize[i].indexOf('%') > -1) {
                // If percentage, get percentage of elem's dimension
                // and assign it to the computed dimension
                computedDim[i] = elemDim[i] * (cssSize[i].replace('%', '') / 100);
            }
        }
    }
    // If both values are set to auto, return image's 
    // original width and height
    if(computedDim[0] === 'auto' && computedDim[1] === 'auto') {
        computedDim[0] = image.width;
        computedDim[1] = image.height;
    } else {
        // Depending on whether width or height is auto,
        // calculate the value in pixels of auto.
        // ratio in here is just getting proportions.
        ratio = computedDim[0] === 'auto' ? image.height / computedDim[1] : image.width / computedDim[0];
        computedDim[0] = computedDim[0] === 'auto' ? image.width / ratio : computedDim[0];
        computedDim[1] = computedDim[1] === 'auto' ? image.height / ratio : computedDim[1];
    }
    // Finally, return an object with the width and height of the
    // background image.
    return {
        width: computedDim[0],
        height: computedDim[1]
    };
}

// Stuff for debugging

function updateData() {
    var background = getBackgroundSize(document.body);
    document.getElementById('width').innerHTML = background.width + 'px';
    document.getElementById('height').innerHTML = background.height + 'px';
    document.getElementById('winWidth').innerHTML = getComputedStyle(document.body).width;
    document.getElementById('winHeight').innerHTML = getComputedStyle(document.body).height;
}
// Execute onload, so that the background image is already loaded.
window.onload = window.onresize = updateData;

html, body {
    width: 100%;
    height: 100%;
    margin: 0;
    padding: 0;
}
body {
    background: url('http://hdwallpapersfit.com/wp-content/uploads/2015/03/images-7.jpg');
    background-size: 80% auto;
}
div {
    background: rgba(0, 0, 0, 0.5);
    color: #fff;
}

<div id="data">
    Background width: <span id="width"></span>
    <br>
    Background height: <span id="height"></span>
    <hr>
    Body width: <span id="winWidth"></span>
    <br>
    Body height: <span id="winHeight"></span>
</div>