High Performance Animations - HTML5 Rocks

High Performance Animations - HTML5 Rocks

4 things a browser can animate cheaply

Position transform: translate(npx, npx);

Scale transform: scale(n);

Rotation transform: rotate(ndeg);

Opacity opacity: 0...1; The process that the browser goes through is pretty simple: calculate the styles that apply to the elements (Recalculate Style), generate the geometry and position for each element (Layout), fill out the pixels for each element into layers (Paint Setup and Paint) and draw the layers out to screen (Composite Layers). To achieve silky smooth animations you need to avoid work, and the best way to do that is to only change properties that affect compositing -- transform and opacity. The higher up you start on the timeline waterfall the more work the browser has to do to get pixels on to the screen. Animating Layout Properties Here are the most popular CSS properties that, when changed, trigger layout: width, height, padding, margin, display, border-width, border, top, position, font-size, float, text-align, overflow-y, font-weight, overflow, left, font-family, line-height, vertical-align, right, clear, white-space, bottom, min-height Animating Paint Properties Changing an element may also trigger painting, and the majority of painting in modern browsers is done in software rasterizers. Depending on how the elements in your app are grouped into layers, other elements besides the one that changed may also need to be painted. There are many properties that will trigger a paint, but here are the most popular: color, border-style, visibility, background, text-decoration, background-image, background-position, background-repeat, outline-color, outline, outline-style, border-radius, outline-width, box-shadow, background-size If you animate any of the above properties the element(s) affected are repainted, and the layers they belong to are uploaded to the GPU. On mobile devices this is particularly expensive because CPUs are significantly less powerful than their desktop counterparts, meaning that the painting work takes longer; and the bandwidth between the CPU and GPU is limited, so texture uploads take a long time. Animating Composite Properties There is one CSS property, however, that you might expect to cause paints that sometimes does not: opacity. Changes to opacity can be handled by the GPU during compositing by simply painting the element texture with a lower alpha value. For that to work, however, the element must be the only one in the layer. If it has been grouped with other elements then changing the opacity at the GPU would (incorrectly) fade them too. In Blink and WebKit browsers a new layer is created for any element which has a CSS transition or animation on opacity, but many developers use translateZ(0) or translate3d(0,0,0) to manually force layer creation. Forcing layers to be created ensures both that the layer is painted and ready-to-go as soon as the animation starts (creating and painting a layer is a non-trivial operation and can delay the start of your animation), and that there's no sudden change in appearance due to antialiasing changes. Promoting layers should done sparingly, though; you can overdo it and having too many layers can cause jank. Imperative vs Declarative Animations Developers often have to decide if they will animate with JavaScript (imperative) or CSS (declarative). There are pros and cons to each, so let’s take a look: Imperative The main pro of imperative animations happens to also be its main con: it’s running in JavaScript on the browser’s main thread. The main thread is already busy with other JavaScript, style calculations, layout and painting. Often there is thread contention. This substantially increases the chance of missing animation frames, which is the very last thing you want. Animating in JavaScript does give you a lot of control: starting, pausing, reversing, interrupting and cancelling are trivial. Some effects, like parallax scrolling, can only be achieved in JavaScript. Declarative The alternative approach is to write your transitions and animations in CSS. The primary advantage is that the browser can optimize the animation. It can create layers if necessary, and run some operations off the main thread which, as you have seen, is a good thing. The major con of CSS animations for many is that they lack the expressive power of JavaScript animations. It is very difficult to combine animations in a meaningful way, which means authoring animations gets complex and error-prone.