CSS — transparent “glass” modal, everything else darkened

THE ANSWER: background-attachment

----- JSBin Example ----

The answer is to use background-attachment

ORIGINAL QUESTION

I'm working on a project where we want to display a modal that "sees through" to the backdrop, but everywhere outside of the modal panel is slightly masked.

I have successfully used border: 10000px rgba(0,0,0,0.3) with border-radius: 10010px but this is a hack, and I can't outline the modal with a box-shadow

Is there any standard way for doing this? Bonus points if you can think of a way to apply a transparency filter gradient to an image.

I think the best option you got here is to have a 3-rows where the middle one contains 3 columns and the top-bottom rows & left-right columns (of the middle row) have darkened background:

$(function() {
  $('button').click(function() {
    tpl = '<div class="modal-reverse-container"><div class="r1"></div><div class="r2"><div class="c1"></div><div class="c2"></div><div class="c3"></div></div><div class="r3"></div></div>'
    $('body').append($(tpl));
    $('.modal-reverse-container').width($(document).width());
    $('.modal-reverse-container').height($(document).height());
    $('.modal-reverse-container r1, .modal-reverse-container r2').height($(document).height());
  });
  $(document).on('click', '.modal-reverse-container', function() {
    $(this).remove();
  });
});

td {
  text-align: center;
  background: red;
}
.modal-reverse-container {
  position: absolute;
  top: 0;
  left: 0;
}
.modal-reverse-container .r1, .modal-reverse-container .r2, .modal-reverse-container .r3 {
  height: 33%;
}
.modal-reverse-container .r1, .modal-reverse-container .r3 {
  background: rgba(0, 0, 0, 0.7);
}
.modal-reverse-container .r2 .c1, .modal-reverse-container .r2 .c2, .modal-reverse-container .r2 .c3 {
  width: 33%;
  height: 100%;
  float: left;
}
.modal-reverse-container .r2 .c3 {
  float: right;
}
.modal-reverse-container .r2 .c1, .modal-reverse-container .r2 .c3 {
  background: rgba(0, 0, 0, 0.7);
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<table width="100%">
  <tr>
    <td>1</td>
    <td>2</td>
    <td>3</td>
  </tr>
</table>
<img src="https://dummyimage.com/600x400/d950d9/fff" />
<div>
  Contrary to popular belief, Lorem Ipsum is not simply random text. It has roots in a piece of classical Latin literature from 45 BC, making it over 2000 years old. Richard McClintock, a Latin professor at Hampden-Sydney College in Virginia, looked up one of the more obscure Latin words, consectetur, from a Lorem Ipsum passage, and going through the cites of the word in classical literature, discovered the undoubtable source. Lorem Ipsum comes from sections 1.10.32 and 1.10.33 of "de Finibus Bonorum et Malorum" (The Extremes of Good and Evil) by Cicero, written in 45 BC. This book is a treatise on the theory of ethics, very popular during the Renaissance. The first line of Lorem Ipsum, "Lorem ipsum dolor sit amet..", comes from a line in section 1.10.32.
</div>
<button>Set reverse-modal</button>

neaumusic

---- JSBin Example ----

The answer is to use `background-attachment`

background-attachment: fixed;
background-size: cover;
background-position: center center;
background-image: linear-gradient(rgba(0,0,0,0.4), rgba(0,0,0,0.4)), url(http://imgur.com/oVaQJ8F.png);

.modal-backdrop {
    background: url(myurl.png) center center / cover no-repeat fixed
}

.modal-panel {
    background: url(myurl.png) center center / cover no-repeat fixed
}

The best value would be fixed (not all devices support fixed) so that your backdrop and your modal can share the same viewport X and Y (0, 0) by default

You then scale with background-size percentages or cover

When using background: shorthand, make sure to use a / to separate background-position from background-scale amounts

I ran into a bug on an older device, so I used local then manually computed leftX and topY to line up backdrop and modal-panel background-position at (0, 0) on my viewport.

I then scaled both images with the same percentage, to cover the screen

I also used a gradient, credit -- How to darken a background Image

来源：https://stackoverflow.com/questions/40010761/css-transparent-glass-modal-everything-else-darkened

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