What is the situation when css div is opaque?

CSS (Cascading Style Sheet) is a technology often used in front-end development, which covers many style attributes. Among them, the opacity attribute (opacity) in the style attribute is a frequently used attribute, which can change the transparency of the element to achieve various visual effects. However, when using the opacity attribute, we will also find some problems, such as directly setting the opacity of the parent element, which will affect the transparency of the child elements, etc. This article will share some knowledge and skills about CSS div opacity to help readers solve related problems.

1. Problem: The opacity of the parent element affects the child elements

When opacity is set in the parent element, the transparency of the child element will also be affected. For example, in the following HTML and CSS code:

<div class="parent">
  <div class="child"></div>
</div>

.parent {
  background-color: #000;
  opacity: 0.5;
}

.child {
  background-color: #fff;
  opacity: 0.5;
}

We will find that although the child element has an opacity of 0.5, the element is still semi-transparent. At this point, we might think of using RGBA colors to avoid this problem instead of using the opacity attribute. However, what if you want to use the opacity property to achieve a specific effect?

2. Solution: Use RGBA color

To address the above problems, we can use RGBA color values ​​instead of the original color values. RGBA color value is a color representation that includes red, green, and blue color channels plus a transparency channel. The transparency channel is represented by a number between 0 and 1, where 0 means completely transparent and 1 means Completely opaque. Therefore, we change the above code to the following method:

.parent {
  background-color: rgba(0, 0, 0, 0.5); /* 半透明黑色 */
}

.child {
  background-color: rgba(255, 255, 255, 0.5); /* 半透明白色 */
}

We can see that at this time, both the parent element and the child element have become semi-transparent, but they will not affect each other. . This is because RGBA color values ​​are applied directly to the color itself, rather than by changing the transparency.

3. Problem: The opacity of the background image cannot be changed

If we want to use the opacity attribute to control the transparency of the background image of a certain element, we may encounter a problem: The transparency of the background image of this element and its child elements will change, but the transparency of the background image itself will not change. At this time, we can use pseudo elements to achieve this effect.

<div class="bg-image">
  <div class="content"></div>
</div>

.bg-image {
  position: relative;
}

.bg-image:before {
  content: "";
  display: block;
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: url(image.jpg) no-repeat;
  background-size: cover;
  opacity: 0.5;
}

.content {
  position: relative;
  z-index: 1; /* 使内容位于伪元素上方 */
}

In the above code, we create a relatively positioned parent element and set a pseudo element (using :before) as the background image. We add a child element above the pseudo-element to place the actual content, and give it a z-index attribute value to place it above the pseudo-element. In this way, we can control the transparency of the background image by modifying the opacity attribute of the pseudo element.

4. Summary

During development, using the opacity attribute can achieve many practical effects, but you may also encounter some problems when using it. This article introduces two common problems and provides corresponding solutions. For front-end developers, being familiar with and mastering these techniques will improve the flexibility and efficiency of code writing, allowing our web pages and applications to have more cool special effects.

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