0

要使用 CSS 交叉淡入淡出两个图像，我们通常会这样做（假设 CSS 中存在过渡的跨浏览器兼容性）

#crossfade img {
  position: absolute;
  left: 0;
  width: 200px;
  transition: opacity 1s ease-in-out;
}
#crossfade img.top:hover {
  opacity: 0;
}
<div id="crossfade">
  <img class="bottom" src="http://badd.ie./_images/google-logo.png" />
  <img class="top" src="http://badd.ie./_images/chrome-logo.png" />
</div>

到目前为止，很平常。

现在，如果我想使图像垂直居中，我会使用 flexbox：

CSS：

#crossfade {
  display: flex;
  align-items: center;
  justify-content: center;
}

但是出现了两个问题。首先，position: absolute;打破了 flexbox 居中，因为图像的左上角像素以 flexbox 为中心，而不是中心像素。其次，使用 flexbox 意味着图像现在是并排的，不再相互重叠。

我尝试在图像周围添加一个容器 div，将其居中，然后将图像绝对定位在其中，但没有任何区别。他们俩仍然并排出现并偏离中心。

编辑：第二个问题解决了。谢谢查兹。请参阅下面的 MCVE，以更好地了解我正在尝试做什么。除了我将 FontAwesome 的东西换成了问号之外，这几乎是原样的。

编辑：这两个问题现在都解决了。谢谢尼尔。下面的代码片段已更新以显示工作。

var minHeaderHeight = 100; // Height of shrunken header, in pixels
var header = document.querySelector("#header"); // The header object
var maxHeaderHeight = outerHeight(header, true); // Height of expanded header, in pixels

document.addEventListener("DOMContentLoaded", initHeader);

function initHeader() {
	var	landingImage = document.getElementById("landing-image");
	if (landingImage !== null) { 
		header.classList.add("expanded");
    window.addEventListener('scroll', scrollCallback);
	} else {
		header.parentNode.style.paddingTop = minHeaderHeight + "px";
	}
}

function scrollCallback() {
	var scrollOffset = windowScrollTop();
	var transitionEvent;
	if (scrollOffset > 10) {
		header.classList.remove("expanding");
		header.classList.add("shrinking");
		header.classList.remove("expanded");
	} else {
		header.classList.remove("shrinking");
		header.classList.add("expanding");
		header.classList.add("expanded");
	}
}

// THESE TWO FUNCTIONS REPLICATE SIMILAR FUNCTIONS FROM JQUERY
function outerHeight(el, withMargins) {
	withMargins = withMargins || false;
	if (withMargins) {
	  var height = el.offsetHeight;
	  var style = getComputedStyle(el);
	  height += parseInt(style.marginTop) + parseInt(style.marginBottom);
	  return height;
	} else {
		return el.offsetHeight;
	}
}

function windowScrollTop(pos) {
	if (typeof pos === 'undefined') {
		if (window.pageYOffset !== undefined) {
			return window.pageYOffset;
		} else {
			return (document.documentElement || document.body.parentNode || document.body).scrollTop;
		}
	} else {
		document.documentElement.scrollTop = pos;
		document.body.parentNode.scrollTop = pos;
		document.body.scrollTop = pos;
		window.pageYOffset = pos;
	}
}
#header {
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  display: flex;
  justify-content: space-around;
  align-items: center;
  background-color: #000;
  height: 100px;
  text-transform: uppercase;
  font-size: 2em;
  color: white;
}

#header a {
	color: white;
  text-decoration: none;
}

#header-inner {
	width: 100%;
  z-index: 1;
  display: flex;
  justify-content: space-around;
  align-items: center;
}
 
#header.expanded {
	height: 100vh;
	background-color: transparent;
}

#header.expanding {
	-webkit-transition: height 300ms ease-in-out, background 300ms ease-in;
	-moz-transition: height 300ms ease-in-out, background 300ms ease-in;
	-ms-transition: height 300ms ease-in-out, background 300ms ease-in;
	-o-transition: height 300ms ease-in-out, background 300ms ease-in;
  transition: height 300ms ease-in-out, background 300ms ease-in;
}

#header.shrinking {
	-webkit-transition: height 300ms ease-in-out, background 300ms ease-in;
	-moz-transition: height 300ms ease-in-out, background 300ms ease-in;
	-ms-transition: height 300ms ease-in-out, background 300ms ease-in;
	-o-transition: height 300ms ease-in-out, background 300ms ease-in;
  transition: height 300ms ease-in-out, background 300ms ease-in;
}

#header-logos {
	position: relative;
}

#header-logos img {
	position: absolute;
	top: 50%;
	left: 50%;
    transform: translate(-50%, -50%);
	max-height: 100px;
}

#header.shrinking img {
	-webkit-transition: opacity 300ms ease-in-out, max-height 300ms ease-in-out;
  -moz-transition: opacity 300ms ease-in-out, max-height 300ms ease-in-out;
  -ms-transition: opacity 300ms ease-in-out, max-height 300ms ease-in-out;
  -o-transition: opacity 300ms ease-in-out, max-height 300ms ease-in-out;
  transition: opacity 300ms ease-in-out, max-height 300ms ease-in-out;
}

#header.expanding img {
	-webkit-transition: opacity 300ms ease-in-out, max-height 300ms ease-in-out;
  -moz-transition: opacity 300ms ease-in-out, max-height 300ms ease-in-out;
  -ms-transition: opacity 300ms ease-in-out, max-height 300ms ease-in-out;
  -o-transition: opacity 300ms ease-in-out, max-height 300ms ease-in-out;
  transition: opacity 300ms ease-in-out, max-height 300ms ease-in-out;
}

#header-logo-top {
	opacity: 0;
}

#header.expanded #header-logo-top {
	opacity: 1;
}

#header.expanded img {
  max-height: 500px;
}

#landing-image {
	height: 100vh;
	background-position: center;
	background-repeat: no-repeat;
	background-size: cover;
}

#landing-image.home-page {
	background-image: url("https://upload.wikimedia.org/wikipedia/commons/2/2f/KANTHALLOOR%2CEruvikulam%26Anamalais_in_the_background.jpg");
}

#page-content {
    max-width: 750px;
}
<div id="header">
  <div id="header-inner">
	  <div class="header-button menu-bars">
	      <a href="#">?</a>
	  </div>
    <div id="header-link-1" class="header-button">
        <a href="#" class="btn">Link 1</a>
    </div>
    <div id="header-link-2" class="header-button">
        <a href="#" class="btn">Link 2</a>
    </div>
    <div id ="header-logos" class="header-button">
			<a href="#"><img id="header-logo-bottom" src="https://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Google_%22G%22_Logo.svg/1000px-Google_%22G%22_Logo.svg.png"></a>
			<a href="#"><img id="header-logo-top" src="https://upload.wikimedia.org/wikipedia/commons/thumb/e/e2/Google_Chrome_icon_%282011%29.svg/2000px-Google_Chrome_icon_%282011%29.svg.png"></a>
	  </div>
    <div id="header-dates" class="header-button">
        1 | 2 | 3 JAN 2018
    </div>
    <div id="header-socials" class="header-button">
			<a href="#">?</a>
			<span>|</span>
			<a href="#">?</a>
			<span>|</span>
			<a href="#">?</a>
			<span>|</span>
			<a href="#">?</a>
			<span>|</span>
			<a href="#">?</a>
    </div>
	  <div class="header-button spacer">
	  </div>
  </div>
</div>
<div id="landing-image" class="home-page"></div> 
<div id="page-content">
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Nulla fermentum enim quis sapien accumsan, sit amet consectetur justo laoreet. Integer tortor nibh, dapibus quis nisi nec, euismod sagittis arcu. Ut vehicula nisi vitae ante efficitur congue. Vivamus malesuada facilisis tortor. Maecenas maximus felis at justo finibus volutpat vitae sed dolor. Suspendisse quis nulla massa. Aliquam dignissim leo ut arcu viverra fringilla. Morbi convallis dignissim augue, at pharetra purus. Cras neque elit, dictum et eros eget, faucibus varius ligula. In tincidunt dolor quis accumsan ornare. Ut eget efficitur sapien. Maecenas nunc justo, malesuada quis porta quis, dignissim ac enim. Nulla vestibulum odio ac sem egestas, et lobortis metus aliquam. Mauris at tincidunt erat. Fusce ut dictum diam. 
Sed luctus sem vel euismod imperdiet. Fusce non tincidunt elit, id aliquet ipsum. Aliquam quam libero, tincidunt id diam eu, pretium porta velit. Mauris placerat efficitur ipsum non fringilla. In suscipit ipsum quis leo faucibus, ut porttitor diam porta. Donec luctus, mauris non posuere aliquam, neque velit euismod ipsum, vitae ornare erat sapien in nulla. Interdum et malesuada fames ac ante ipsum primis in faucibus. Nunc consequat tortor sed dui placerat lacinia. Vivamus tempus ultrices massa, vitae sagittis enim convallis sed. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. 
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</div>

4

3 回答 3

0

不要使用 flexbox 将图像居中，使用 left、top 和 transform 的组合： translate；

#crossfade {
 position:relative;
}
#crossfade img {
  position: absolute;
  left: 50%;
  top: 50%;
  transfrom: translate(-50%, -50%);
}
于 2017-10-23T14:01:44.217 回答
0

您没有为使用另一个容器的第二次尝试提供代码，因此我们只能假设您做了什么；然而，这是一个使用带有 flexbox 的嵌套容器的工作示例：

.box {
  width: 700px;
  height: 400px;
  display: flex;
  align-items: center;
  justify-content: center;
  background: lightblue;
}

.images {
  width: 350px;
  height: 150px;
  position: relative;
}

.images img {
  position: absolute;
  top: 0;
  left: 0;
  transition: opacity 1s ease 0s;
}

.img2 {
  filter: sepia(50%);
  opacity: 0;
}

.images:hover img.img2 {
  opacity: 1;
}
<div class="box">
  <div class="images">
    <img src="http://via.placeholder.com/350x150">
    <img class="img2" src="http://via.placeholder.com/350x150">
  </div>
</div>

外部 flexbox 容器居中内部容器，内部容器的位置相对允许图像是absolute. 如果您事先知道图像的尺寸，则此方法有效。

于 2017-10-23T14:04:21.203 回答
0

感谢下面的 Neil 和 Chaz。答案如下：

在 flexbox 项目中，您有一个图像容器。然后以下样式适用于容器和图像：

.image-container {
  position: relative;
}

.image-container img {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

然后可以将图像容器放置在 flexbox 项目容器中。

我已经更新了上面的代码片段以显示它可以正常工作。

于 2017-10-25T10:23:37.743 回答