I recently got into Web Animation, so I will record my learning process and share it with everyone.
CSS3 3D planetary rotation demo page please click: Demo. (It is recommended to use Chrome to open)
The complete code of this article, as well as more CSS3 effects, can be seen on my Github, and I hope everyone can give it a star.
Well, some people may not be able to open the demo or the page is messed up. Here are some renderings: (The pictures are a bit big, please wait patiently for a while)
CSS3 3D planetary motion effect Picture
Randomly took another screenshot:
I strongly recommend that you click on the Demo page to experience it. The charm of CSS3 3D, what pictures can show is limited after all.
Then, the production process of this CSS3 3D planetary motion animation will not be described in detail. The focus of this article is on the introduction of Web animation and Performance Optimization. For detailed CSS3 3D, you can review the previous blog: [CSS3 Advanced] Cool 3D rotating perspective. Simple idea:
1. Use the 3D photo wall produced in the previous article as a prototype and modify it;
2. I thought of many ways to make each sphere, and finally used In this compromise, each sphere is itself a CSS3 3D shape. Then using Sass to write CSS during the production process can reduce a lot of the tedious process of writing CSS animations;
3. In the Demo, Javascript was used to write a mouse follow listening event, remove this event, the entire planetary motion animation itself is implemented in pure CSS.
The following will enter the focus of this article. From the perspective of performance optimization, we will talk about the principles of browser rendering and display, browser redrawing and rearrangement, animation performance detection optimization, etc.:
Browser The principle of rendering and display and its impact on web animation
The subtitle is a bit big. We know that the cores (rendering engines) of different browsers are different, such as the most mainstream chrome browser now. The kernel is the Blink kernel (used in Chrome (version 28 and later), Opera (version 15 and later) and Yandex browser), Firefox is Gecko, and IE is Trident. The browser kernel is responsible for interpreting web page syntax and interpreting it. When rendering (displaying) web pages, the working principles of different browser kernels are not completely consistent.
So in fact, what we will mainly discuss below is the rendering principle under the chrome browser. Because there is a lot of verifiable information on chrome kernel rendering, we dare not draw conclusions about browsers with other kernels, so the discussion below is by default for the chrome browser.
First of all, I want to throw out some conclusions:
Use transform3d api instead of transform api to force the start of GPU acceleration
Here we talk about GPU acceleration. Why can GPU accelerate 3D transformation? All this must start with the underlying rendering of the browser. The process of rendering and displaying web pages in the browser is a cliché and must be asked in interviews. It can be roughly divided into:
1. Parsing HTML (HTML Parser)
2. Build DOM Tree
3. Build Render Tree
4. Draw Render Tree(Painting)
Found one A very classic picture:
# This rendering process is regarded as a basic knowledge, so continue to delve deeper.
When the page is loaded and parsed, it represents a very familiar structure in the browser: DOM (Document Object Model, Document Object Model). When the browser renders a page, it uses many intermediate representations that are not exposed to developers, the most important structure of which is the layer.
This layer is the focus of this article:
In Chrome, there are different types of layers: RenderLayer (responsible for the DOM subtree), GraphicsLayer (responsible for the RenderLayer subtree) . Next we will discuss the GraphicsLayer layer.
GraphicsLayer layer is uploaded to the GPU as a texture.
This texture is very important here. So,
What is texture?
Texture here refers to a GPU term: think of it as a bitmap image that is moved from main memory (such as RAM) to image memory (such as VRAM in the GPU). Once it's moved to the GPU, you can match it into a mesh geometry and use textures in Chrome to get large chunks of page content off the GPU. It's easy to match different positions and transformations by applying textures to a very simple rectangular grid, which is how 3D CSS works. It’s hard to understand. Just look at the example. In chrome, we can see the concept of GraphicsLayer — layer mentioned above. In the developer tools, we make the following selection to bring up the show layer borders option:
In a very simple page, we can see as follows, this page only One layer. The blue grid represents tiles. You can think of them as units of a layer (not layers). Chrome can upload them to the GPU as part of a large layer:
Creation of the element's own layer
Because the above page is very simple, no layer is generated, but in a very complex page, for example, we can set a 3D CSS property for the element to transform it , we can see what it looks like when an element has its own layer.
Pay attention to the orange border, which draws the outline of the layer in the view:
When is the creation of a layer triggered?
The layer enclosed by the yellow border in the diagram above is GraphicsLayer, which is very important for our Web animation. Usually, Chrome will draw the content of a layer before uploading it to the GPU as a texture (paint ) into a bitmap. If the content will not change, then there is no need to repaint the layer.
The significance of this is that the time spent on redrawing can be used to do other things, such as running JavaScript. If the drawing time is very long, it will also cause animation failures and delays.
So when does an element trigger the creation of a layer? For now, layers will be created if any of the following conditions are met:
3D or perspective transformation (perspective, transform) CSS properties
For static Web pages, the layer will not be changed after it is drawn for the first time, but for static Web pages In Web animation, the DOM elements of the page are constantly changing. If the content of the layer changes during the transformation process, the layer will be repainted.
The powerful chrome developer tools provide tools that allow us to see what content has been redrawn when the animated page is running:
In the old version In chrome, there is an option show paint rects. You can see which layers of the page have been redrawn and marked with a red border.
But the new version of chrome seems to have removed this option. The current option is enable paint flashing. Its function is also to identify the place where the website changes dynamically, and it is marked with a green border.
Looking at the diagram above, you can see several green boxes on the page, indicating that redrawing has occurred. Note that Chrome will not always redraw the entire layer, it will try to intelligently redraw the invalid parts of the DOM.
Logically speaking, with so many animations happening on the page, redrawing should be very frequent. However, in my planet animation in the picture above, I only see a few green redrawing boxes. My personal understanding is that one is the GPU. Optimization, the second is that if the entire animation page has only one layer, then if transform is used for transformation, the page will inevitably need to be redrawn, but using layering (GraphicsLayer) technology, that is, the elements that meet the situation are created separately. The created layer uses transform, such as rotate. At this time, the rotation transformation of the layer does not affect other layers, so the layer does not necessarily need to be redrawn. (Personal opinions, please make corrections).
Understanding layer redrawing is critical to optimizing the performance of web animations.
What causes invalidation and forced redrawing? This question is difficult to answer exhaustively because there are a large number of situations that lead to boundary failure. The most common situation is to modify the DOM or cause reflow by manipulating CSS styles.
The best way to find the root cause of repaints and reflows is to use the timeline of the developer tools and the enable paint flashing tool, and then try to find the place where the DOM was modified just before the repaint/reflow .
Summary
So how does the browser display from DOM elements to the final animation?
The browser parses the HTML, obtains the DOM and splits it into multiple layers (GraphicsLayer)
Calculate the style results for the nodes of each layer (Recalculate style–style recalculation)
Generate graphics and positions for each node (Layout–reflow and relayout)
Replace each node The nodes are drawn and filled into the layer bitmap (Paint Setup and Paint-Redraw)
The layer is uploaded to the GPU as a texture
-
Conform multiple layers to the page to generate the final screen image (Composite Layers-layer reorganization)
A large part of the cost of Web animation lies in the redrawing of the layer, with the layer as the The underlying composite model has a profound impact on rendering performance. When no drawing is required, the overhead of composite operations is negligible, so when trying to debug rendering performance issues, the first goal is to avoid layer redraws. Then this provides a direction for animation performance optimization and reduces the redrawing and reflow of elements.
Reflow and repaint
Here we must first distinguish between two concepts, redraw and reflow.
Reflow
When part (or all) of the render tree (render Tree) needs to be rebuilt due to changes in the size, layout, hiding, etc. of the elements. This is called reflow, which is relayout.
Every page needs to be reflowed at least once, which is when the page is loaded for the first time. During reflow, the browser will invalidate the affected part of the rendering tree and reconstruct this part of the rendering tree. After completing the reflow, the browser will redraw the affected part to the screen. This process is called redrawing.
Repaint
When some elements in the render tree need to update attributes, these attributes only affect the appearance and style of the elements, but will not affect the layout, such as background-color . It is called redrawing.
It is worth noting that reflow will definitely cause redrawing, but redrawing will not necessarily cause reflow.
Obviously, the cost of reflow is greater. Simply put, when operating an element will cause the element to modify its size or position, then reflow will occur.
When reflow is triggered:
Resizing the window
Changing the font)
Adding or removing a stylesheet
Content changes, such as when the user inputs Enter text in the box (Content changes, such as a user typing text in
an input box)
Activate CSS Pseudo class , such as :hover (Activation of CSS pseudo classes such as :hover (in IE the activation of the pseudo class) class of a sibling))
Manipulating the class attribute
A script manipulating the DOM
Calculating offsetWidth and offsetHeight Properties (Calculating offsetWidth and offsetHeight)
Set style The value of the attribute (Setting a property of the style attribute)
So for the page, our purpose is to minimize the reflow and redraw of the page. A simple chestnut:
// 下面这种方式将会导致回流reflow两次
var newWidth = ap.offsetWidth + 10; // Read
ap.style.width = newWidth + 'px'; // Write
var newHeight = ap.offsetHeight + 10; // Read
ap.style.height = newHeight + 'px'; // Write
// 下面这种方式更好，只会回流reflow一次
var newWidth = ap.offsetWidth + 10; // Read
var newHeight = ap.offsetHeight + 10; // Read
ap.style.width = newWidth + 'px'; // Write
ap.style.height = newHeight + 'px'; // Write
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The above four sentences, because the offsetHeight operation is involved, the browser forces reflow twice, while the following four sentences combine the offset operation, so one page reflow is reduced.
Reducing reflow and redrawing actually means reducing the operations on the rendering tree (merging multiple DOM and style modifications), reducing requests for some style information, and making full use of the browser's optimization strategy.
In fact, the browser itself has an optimization strategy. If every Javascript sentence operates on the DOM, it will be reflowed and redrawn. If so, the browser may not be able to handle it. Therefore, many browsers will optimize these operations. The browser will maintain a queue and put all operations that will cause reflow and redrawing into this queue. When the operations in the queue reach a certain number or a certain time interval, the browser The queue will be flushed and a batch will be processed. This will turn multiple reflows and redraws into one reflow and redraw.
But there are exceptions, because sometimes we need to accurately obtain certain style information, as follows:
- #scrollTop/Left /Width/Height
##clientTop/Left/Width/Height-
#width,height
requested getComputedStyle(), or IE’s
At this time, in order to feedback the most accurate information, the browser needs to immediately reflow and redraw to ensure that the information given to us is accurate, so the flush queue may be executed in advance.
display
:none and
display:none hidden elements do not occupy any space. Its width, height and other attribute values will be "lost"
- visibility:hidden The hidden element space still exists. It still has attribute values such as height and width
- From a performance perspective, that is, in terms of reflow and redrawing,
display :none will trigger reflow
- visibility:hidden will only trigger repaint because no position changes are found
- They In optimization, visibility:hidden will appear better, because we will not change the display hierarchy that has been defined in the document because of it.
Influence on child elements:
##display:none Once display:none is applied to the parent node element, the parent node and its descendant node elements are all invisible, and regardless of Its descendant elements cannot be displayed no matter how the display value is set;
visibility:hidden Once visibility:hidden is applied to the parent node element, all its descendants will also be invisible. However, there are hidden "failures". When visibility:visible is applied to its descendant element, the descendant element will appear again.
Performance detection and optimization of animation
Performance consumption styles
Different styles have different performance consumption, for example Box-shadow is very performance-intensive from a rendering perspective because their drawing code takes too long to execute compared to other styles. This means that if a performance-intensive style needs to be redrawn frequently, you will run into performance issues. Secondly, you need to know that nothing is constant. A style that performs poorly today may be optimized tomorrow, and there are differences between browsers. So the key is to use your development tools to identify performance bottlenecks and then find ways to reduce the workload of the browser.
Fortunately, the chrome browser provides many powerful functions that allow us to detect our animation performance. In addition to the above mentioned, we can also display the FPS information of the page by checking the show FPS meter below. And GPU usage:
Use will-change to improve page scrolling, animation and other rendering performance
The official document says that this is still an experimental feature, so the syntax and behavior of this feature may change in future versions of the browser. 
使用方法示例：(具体每个取值的意义，去翻翻文档)
will-change: auto
will-change: scroll-position
will-change: contents
will-change: transform // Example of <custom-ident>
will-change: opacity // Example of <custom-ident>
will-change: left, top // Example of two <animateable-feature>
will-change: unset
will-change: initial
will-change: inherit
// 示例
.example{
will-change: transform;
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will-change 为 web 开发者提供了一种告知浏览器该元素会有哪些变化的方法，这样浏览器可以在元素属性真正发生变化之前提前做好对应的优化准备工作。 这种优化可以将一部分复杂的计算工作提前准备好，使页面的反应更为快速灵敏。
值得注意的是，用好这个属性并不是很容易：
使用 transform3d api 代替 transform api，强制开始 GPU 加速
GPU 能够加速 Web 动画，这个上文已经反复提到了。
3D transform 会启用GPU加速，例如 translate3D, scaleZ 之类，当然我们的页面可能并没有 3D 变换，但是不代表我们不能启用 GPU 加速，在非 3D 变换的页面也使用 3D transform 来操作，算是一种 hack 加速法。我们实际上不需要z轴的变化，但是还是假模假样地声明了，去欺骗浏览器。
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