What is the implementation principle of Bootstrap picture centering

There are three steps to center the image in Bootstrap: Use the text-center class to achieve horizontal centering, provided that the image is an inline element or use an inline-block. Using flexbox layout to achieve vertical centering requires setting the container fixed height. Considering the problems caused by inconsistent image sizes, use max-width: 100%; limit the maximum width of the image or pre-process the image size to maintain consistency.

Bootstrap Picture centered? This question seems simple, but it actually has a secret. Many novices will fall into some pits, thinking that a simple text-center can do everything, but they find that the picture is just centered horizontally and the vertical direction is missing. In this article, let’s dig into the things in the center of Bootstrap pictures, so that you can fully master this skill and say goodbye to the troubles caused by misalignment of pictures.

Let’s start with the basics. The core of Bootstrap is a CSS-based raster system that controls the layout of elements through a series of class names. To center the image, the key is to understand display attribute and the container classes provided by Bootstrap. Don't underestimate these basic concepts, they are the cornerstones of understanding everything. For example, display: inline-block; allows elements to be displayed on a line like elements in the line, and have width and height attributes like block-level elements, which is often used in image centering processing.

Now, let's get to the point. The most common way to center images is to use Bootstrap's text-center class. This can achieve horizontal centering, but only if your image is an inline element or use inline-block .

 <code class="html"><div class="text-center"> <img src="/static/imghw/default1.png" data-src="your-image.jpg" class="lazy" alt="What is the implementation principle of Bootstrap picture centering"> </div></code>

This line of code is simple, but you may have ignored that the img tag is an inline element by default. This causes text-center to only center it horizontally, and the picture in the vertical direction is still displayed according to its own height.

To achieve vertical centering, you need a highly fixed container, and then use flexbox layout or some other tricks. I personally recommend flexbox because it is simple and efficient.

 <code class="html"><div class="d-flex justify-content-center align-items-center" style="height: 200px;"> <img src="/static/imghw/default1.png" data-src="your-image.jpg" class="lazy" alt="What is the implementation principle of Bootstrap picture centering"> </div></code>

This code uses d-flex , justify-content-center and align-items-center . d-flex sets the container to the flex layout, justify-content-center implements horizontal centering, and align-items-center implements vertical centering. Remember, style="height: 200px;" is very important, it defines the height of the container so that vertical centering takes effect. Without a fixed height, vertical centering of flexbox is not possible. This is a point that many beginners are likely to ignore.

Of course, there are other methods, such as using table layout, or absolute positioning with negative margins. But these methods are relatively complex and have poor maintenance, so I generally do not recommend them. The flexbox method is simple and easy to understand and has better performance.

Let’s talk about some possible problems. For example, the size of the picture is inconsistent, resulting in unsatisfactory centering effect. At this time, you can consider using max-width: 100%; to limit the maximum width of the image to ensure that the image does not break the container. Alternatively, pre-process the image size to keep it consistent.

Finally, regarding code readability and maintenance, I suggest you use meaningful class names instead of just pile up Bootstrap's default classes. A good code style can save you a lot of trouble in future maintenance. Remember, code is written for people to see, followed by machines. Writing elegant code is also an important step to improve your programming skills.

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