css34 CSS Opacity / Transparency

emanlee發表於2024-06-01
CSSS3

https://www.w3schools.com/css/css_image_transparency.asp

CSS Opacity / Transparency

The opacity property specifies the opacity/transparency of an element.

Transparent Image

The opacity property can take a value from 0.0 - 1.0. The lower the value, the more transparent:

Example

img {
opacity: 0.5;
} 
<!DOCTYPE html>
<html>
<head>
<style>
img {
  opacity: 0.5;
}
</style>
</head>
<body>

<h1>Image Transparency</h1>
<p>The opacity property specifies the transparency of an element. The lower the value, the more transparent:</p>

<p>Image with 50% opacity:</p>
<img src="img_forest.jpg" alt="Forest" width="170" height="100">

</body>
</html>

Transparent Hover Effect

The opacity property is often used together with the :hover selector to change the opacity on mouse-over:

Example

img {
opacity: 0.5;
}

img:hover {
opacity: 1.0;
} 
<!DOCTYPE html>
<html>
<head>
<style>
img {
  opacity: 0.5;
}

img:hover {
  opacity: 1.0;
}
</style>
</head>
<body>

<h1>Image Transparency</h1>
<p>The opacity property is often used together with the :hover selector to change the opacity on mouse-over:</p>
<img src="img_forest.jpg" alt="Forest" width="170" height="100">
<img src="img_mountains.jpg" alt="Mountains" width="170" height="100">
<img src="img_5terre.jpg" alt="Italy" width="170" height="100">

</body>
</html>

Example explained

The first CSS block is similar to the code in Example 1. In addition, we have added what should happen when a user hovers over one of the images. In this case we want the image to NOT be transparent when the user hovers over it. The CSS for this is opacity:1;.

When the mouse pointer moves away from the image, the image will be transparent again.

An example of reversed hover effect:

Example

img:hover {
opacity: 0.5;
} 
<!DOCTYPE html>
<html>
<head>
<style>
img:hover {
  opacity: 0.5;
}
</style>
</head>
<body>

<h1>Image Transparency</h1>
<p>The opacity property is often used together with the :hover selector to change the opacity on mouse-over:</p>
<img src="img_forest.jpg" alt="Forest" width="170" height="100">
<img src="img_mountains.jpg" alt="Mountains" width="170" height="100">
<img src="img_5terre.jpg" alt="Italy" width="170" height="100">

</body>
</html>

Transparent Box

When using the opacity property to add transparency to the background of an element, all of its child elements inherit the same transparency. This can make the text inside a fully transparent element hard to read:

Example

div {
opacity: 0.3;
} 
<!DOCTYPE html>
<html>
<head>
<style>
div {
  background-color: #04AA6D;
  padding: 10px;
}

div.first {
  opacity: 0.1;
}

div.second {
  opacity: 0.3;
}

div.third {
  opacity: 0.6;
}
</style>
</head>
<body>

<h1>Transparent Box</h1>
<p>When using the opacity property to add transparency to the background of an element, all of its child elements become transparent as well. This can make the text inside a fully transparent element hard to read:</p>

<div class="first"><p>opacity 0.1</p></div>
<div class="second"><p>opacity 0.3</p></div>
<div class="third"><p>opacity 0.6</p></div>
<div><p>opacity 1 (default)</p></div>

</body>
</html>

Transparency using RGBA

If you do not want to apply opacity to child elements, like in our example above, use RGBA color values. The following example sets the opacity for the background color and not the text:


You learned from our CSS Colors Chapter, that you can use RGB as a color value. In addition to RGB, you can use an RGB color value with an alpha channel (RGBA) - which specifies the opacity for a color.

An RGBA color value is specified with: rgba(red, green, blue, alpha). The alpha parameter is a number between 0.0 (fully transparent) and 1.0 (fully opaque).

Tip: You will learn more about RGBA Colors in our CSS Colors Chapter.

Example

div {
background: rgba(76, 175, 80, 0.3) /* Green background with 30% opacity */
} 
<!DOCTYPE html>
<html>
<head>
<style>
div {
  background: rgb(4, 170, 109);
  padding: 10px;
}

div.first {
  background: rgba(4, 170, 109, 0.1);
}

div.second {
  background: rgba(4, 170, 109, 0.3);
}

div.third {
  background: rgba(4, 170, 109, 0.6);
}
</style>
</head>
<body>

<h1>Transparent Box</h1>
<p>With opacity:</p>
<div style="opacity:0.1;"><p>10% opacity</p></div>
<div style="opacity:0.3;"><p>30% opacity</p></div>
<div style="opacity:0.6;"><p>60% opacity</p></div>
<div><p>opacity 1</p></div>

<p>With RGBA color values:</p>
<div class="first"><p>10% opacity</p></div>
<div class="second"><p>30% opacity</p></div>
<div class="third"><p>60% opacity</p></div>
<div><p>default</p></div>

<p>Notice how the text gets transparent as well as the background color when using the opacity property.</p>

</body>
</html>

Text in Transparent Box


Example

<!DOCTYPE html>
<html>
<head>
<style>
div.background {
  background: url(klematis.jpg) repeat;
  border: 2px solid black;
}

div.transbox {
  margin: 30px;
  background-color: #ffffff;
  border: 1px solid black;
  opacity: 0.6;
}

div.transbox p {
  margin: 5%;
  font-weight: bold;
  color: #000000;
}
</style>
</head>
<body>

<div class="background">
  <div class="transbox">
    <p>This is some text that is placed in the transparent box.</p>
  </div>
</div>

</body>
</html>


Example explained

First, we create a <div> element (class="background") with a background image, and a border.

Then we create another <div> (class="transbox") inside the first <div>.

The <div class="transbox"> have a background color, and a border - the div is transparent.

Inside the transparent <div>, we add some text inside a <p> element.

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