缩放 div 以适合背景图像

Question

I have a div with a background image that I want to expand 100% width and auto scale the div to fit the required height of the image. At the moment it is not scaling the div height unless I set the height of the div to 100% but then it just stretches to the full height of the screen, whereas I want it to scale to the height of the image.

Here is the html:

<div id="mainHeaderWrapper">


</div><!--end mainHeaderWrapper-->
<br class="clear" />;

Here is the css:

    #mainHeaderWrapper{


     background: url(http://localhost/site/gallery/bg1.jpg);
     width: 100%;
     height: auto;
     -webkit-background-size: cover;
     -moz-background-size: cover;
     -o-background-size: cover;
     background-size: cover; 
     background-size: 100% 100%;

     background-repeat: no-repeat;
     background-position: center center; 

 }

 .clear { clear: both; }

Thanks for any and all help

Answer 1

Let a transparent image dictate the DIV dimensions.
Inside that div put the same image with CSS opacity: 0

jsBin demo

<div id="mainHeaderWrapper">
   <img src="path/to/image.jpg"><!-- I'm invisible! -->
</div>

set that image to

#mainHeaderWrapper {
    background: no-repeat url(path/to/image.jpg) 50% / 100%;
}
#mainHeaderWrapper img {
    vertical-align: top;
    width: 100%; /* max width */
    opacity: 0;  /* make it transparent */
}

That way the height of the DIV will be dictated by the containing invisible image, and having the background-image set to center, full ( 50% / 100% ) it will match that image's proportions.

Need some content inside that DIV?

jsBin demo Due to the containing image, you'll need an extra child element that will be set to position: absolute acting as an overlay element

<div id="mainHeaderWrapper">
   <img src="path/to/image.jpg"><!-- I'm invisible! -->
   <div>Some content...</div>
</div>

#mainHeaderWrapper{
    position: relative;
    background: no-repeat url(path/to/image.jpg) 50% / 100%;
}
#mainHeaderWrapper > img{
    vertical-align: top;
    width: 100%; /* max width */
    opacity: 0;  /* make it transparent */
}
#mainHeaderWrapper > div{
    position: absolute;
    top: 0;
    width: 100%;
    height: 100%;
}

Answer 2

If you know the proportions of the image, use percentage padding to define the height of the container. Set height:0 and set vertical padding to a percentage of the width.

They key to this method is that percentage-based vertical padding is always related to width.

According to the box model (w3.org) :

The percentage is calculated with respect to the width of the generated box's containing block, even for 'padding-top' and 'padding-bottom'.

Below, the image is 400px X 200px, so the proportion of height to width is 1:2 and padding-top is set to 50%;

 #mainHeaderWrapper { width: 100%; height: 0; padding-top: 50%; background-image: url('https://dummyimage.com/400x200/'); background-size: 100% auto; background-repeat: no-repeat; }

 <div id="mainHeaderWrapper"></div> stuff below the image

In another example, the image is 300px X 100px. The height is ⅓ of the width, so the padding-top is set to 33.33%:

 #mainHeaderWrapper { width: 100%; height: 0; padding-top:33.33%; background-image: url('https://dummyimage.com/300x100/'); background-size: 100% auto; background-repeat: no-repeat; }

 <div id="mainHeaderWrapper"></div> stuff below the image

---

Edit:

As prompted by Paulie_D, other content in the div must be positioned absolutely, demonstrated below. I suggest positioning these elements using percentages, as well.

 #mainHeaderWrapper { width: 100%; height: 0; padding-top: 33.33%; background-image: url('https://dummyimage.com/300x100/'); background-size: 100% auto; background-repeat: no-repeat; } div#inner_content { position: absolute; top: 0; width: 100%; margin-top: 10%; color: #FFF; text-align: center; font-size: 20px; }

 <div id="mainHeaderWrapper"> <div id="inner_content">Hello World</div> </div> stuff below the image

Answer 3

This can be done without using a dummy image. I will use dimensions of an image I just worked with for example.

The dimensions of my image are 2880x1410. Simplify the dimensions -> 96/47 (I used this simple ratio calculator http://andrew.hedges.name/experiments/aspect_ratio/ ). Once you have the simplified ratio, plug the height and width to the equation:

height: calc((100vw * W) / H); So mine would read: height: calc((100vw * 47) / 96);

No need to worry about the contents of the div either (unless they dont fit)

Answer 4

 body{ margin: 0; padding: 0} #box1{ background: url(http://lorempixel.com/400/200/food/); background-size: cover; background-attachment: fixed; margin: 0; width: 100%; height: 100vh; display: table; } h1{ color: #ffffff; font-family: "Gill Sans", "Gill Sans MT", "Myriad Pro", "DejaVu Sans Condensed", Helvetica, Arial, "sans-serif"; font-size: 38px; text-align: center; font-weight: normal; background: rgba(0,0,0,0.3); display: table-cell; vertical-align: middle}

 <div id="box1"> <h1>Code Bluster BILU </h1> </div>