A linear ease will play the animation at the same speed from beginning to end. Animations can move linearly, smoothly, or even bounce rapidly back and forth. You can use it on the element opacity instead of the display attribute. The above curve defines how the output (y. Types Of Easing Functions. easeout { animation-timing-function: ease-out; } . I need to create a Javascript-Fallback for CSS3-Animations and try to make it as close as possible to the CSS-Version. The final snippet of code would look like this:Learn how to create simple CSS animations such as rollover buttons, 3D transforms, animated graphics and logos, load screens, and more. 52. @keyframes glow { 100% { outline: none; padding: 3px 0 3px 3px; margin: 5px 1px 3px 0; box-shadow: 0 0 5px rgba (81, 203, 238, 1); } } . css URL Extension) and we'll pull the CSS from that Pen and include it. 3, an optional string naming an easing function may be used. Easing functions may be specified on individual keyframes in a @keyframes rule. 7, the easing function scales the value to produce an output progress of 0. There are six transition-timing-functions in CSS, and they include: linear; ease; ease-in; ease-out; ease-in-out; cubic-bezier()/li> These are also called easing functions. There is 1 other project in the npm registry using spring. html { --color: orange; } p { color: var(--color); } It is incredibly powerful when combined with calc (). Animations are certainly trendy, especially in web design where CSS transitions have become widely supported by modern browsers without affecting page performance too much (jQuery easing functions. It works with CSS properties, SVG, DOM attributes and JavaScript Objects. The accepted strings are the GreenSock easing functions. ease linear ease-in ease-out ease-in-out. Install. However, we expect to have more control over the animation properties like duration, easing function, delay, and so on to create even more elaborate animations or. See Also: How to Create the Perfect Image Crossfade with CSS3 Transitions. In the Easing Animation dialog that appears, select the easing you want. com - Play it CSS Property: animation-timing-function: linear. One think I've noticed is that, no matter how I ""time" things with keyframe animations, the transitions are always smooth. Animations in Svelte. To make it smoother, you just have to add more key frames. g. It may be used to describe how fast values change during animations. You can change as many CSS properties you want, as many times as you want. The CubicInOut easing function accelerates the animation at. O valor deve ser um dos disponíveis em animation-iteration-count (en-US). This function can be a very detailed value, which is awkward to calculate manually, but there are a lot of free websites which provide resources and live customisation for animation timing functions. The effects helper methods like . It must be a positive integer (greater than 0). Below is a snippet of what the shorthand and longhand variants might look like: /* shorthand */ #foo { animation: bobble 2s ease-in infinite. Keep in mind, you should always use CSS to do something instead of javascript if at any way possible. The <single-transition-timing-function> CSS data type denotes a mathematical function that describes how fast one-dimensional values change during transitions or animations. Easing functions determine the rate at which an animation progresses over time, and can make a big difference in the overall feel of an animation. 25, 1); animation. They allow you to create smooth transitions at each stage of your animations. The most popular easing functions are simple: bi, tri, and sine. 5) // === 0. You’ll see these in action in the next section. Given an input progress of 0. 4, you can set per-property easing functions within a single . easeOut. CSS Cubic Bezier Generator is a free online tool for generating CSS code for cubic bezier easing functions by getting info about the animation from time vs progress graph or directly previewing the animation. Generates a string that represents a set of values used with the linear-easing function to replicate spring animations, you can check out the linear-easing playground here. For this I created a custom transition function modal that’s going to take the element and parameters and get the existing transform styles of the element and return a css function. So far so good. Overusing animations can lead to a cluttered and distracting user experience; Timing and easing: The timing and easing functions of your animations should be carefully crafted to create a more user-friendly experience. If the animating value is set as an array of multiple values for a keyframes animation, ease can be set as an array of easing functions to set different easings between each of those values. In CSS, the transition and animation properties allow you to specify an easing function. linear animations move at a constant speed. zero or one <easing-function> value representing the easing function to use; zero, one, or two <time> values. They offer more fine-grained control as you can control different stops of the animations. Here's what you'd learn in this lesson: David adds an initial animation to illustrate how easing affects how the animation plays. Similar to native CSS transitions, jQuery UI's class animations provide a smooth transition from one state to another while allowing you to keep all the details. Easings, durations, delays, and more will all affect the value as you go along. Lesson Project. $ ("#map"). Given an input progress of 0. You can control the individual components of the transition with the following sub-properties: (Note that these transitions loop infinitely only for the purpose of our examples; CSS transition s only visualize a property change from start to finish. TranslateHere’s an example that shows how harsh a standard linear-gradient () can be compared to how smooth we can make it by easing it: Screencap from “The Good, the Bad and the Ugly” with gradients overlaid. from and to will be each set of values, for instance given an output range ["#000", "#f00", "#333"] two mixers will be created, one between "#000" and "#f00" and. parseEase ('power1. During the past few months, I’ve been actively teaching myself how to draw and animate SVG shapes. This function creates an easing curve for these 2 properties: animation-timing-function and transition-timing-function. The Overflow Blog The AI assistant trained on your company’s data. com. Parameters. from() - Like a backwards . Anime (/ˈæn. Let's look at the science of CSS cubic-bezier curves, and the art of using them to make the best web animations possible. transition: <duration> <property> <delay> <timing-function>,. We can apply easing to transitions and animations in CSS. Applying this easing function to an animation would cause it to progress more slowly at first but then gradually progress more quickly. This transition between two values may be applied in different situations. 6s ; } Edit on cubic-bezier. Easing. Easing is important stuff when it comes to animations and transitions. This CSS module describes a way for authors to animate the values of CSS properties over time, using keyframes. If it's using a matching preprocessor, use the appropriate URL Extension and we'll combine the. There are several pre-defined easing functions available in CSS, such as ease-in , ease-out , and ease-in-out , which can be used to create common animation effects. Easing functions are widely applied in Cascading Style Sheets (CSS) . This extension has a few settings: visubezier. 25). You may specify multiple easing functions; each one will be applied to the corresponding property as specified by. Non-linear easing is what gives animations a more natural, life-like feel. I downloaded the source code of the animations. CSS Custom Properties. A . Cette transition entre deux valeurs peut être appliquées dans différentes situations. A string indicating which easing function to use for the transition. I use css3 easing from here:. easie provides exact CSS3 easings for jQuery. We can create even more life-like effects with cubic-bezier(), like making an element overshoot its end position slightly before coming to rest. Non-linear easing is what gives animations a more natural, life-like feel. easing: represents the animation-timing-function of the transition; the default value is ease-in-out. If no animation-timing-function is specified on a keyframe, the corresponding value of animation-timing-function from the element to which the animation is applied is used for that keyframe. The cubic-bezier function specifies a cubic bezier curve. Here's the CSS3 Animation Generator tool which generates the keyframes for you, and enables 12 additional easing functions not supported by the w3c spec: The cubic-bezier() function affords us much more control over easing in our animations in CSS. CSS animations provide more control over the animation process compared to transitions. However, animations created using the Web Animations API can use both types of easing. License. ease. In CSS, you can use a timing function for an animation, however, with the Web Animations API, you can specify a timing function for each keyframe as well as for the entire animation. Libraries like GSAP and Framer Motion and React Spring have sprung up to help us add motion to the DOM. If you don’t quite like the easing, grab a handle and fix it. A 2 items array, the first value is the number of rows, the second the number of columns. Sorted by: 2. <easing-function> は CSS のデータ型で、数値が変化する速度を記述する数学的な関数を表します。 この 2 つの値の間の遷移は様々な形で適用される可能性があります。アニメーション中に値が変化する速さを記述するために使用されることがあります。1. If no other value is assigned, the function will be set at ease by default. Ease Yourself Into Animation (great preso) Understanding Easing: Explaining Penner's Equations. Given an input progress of 0. Both are applied to the same element in parallel, but run independently of each other: The Easing class defines a number of easing functions that can be consumed by animations: The BounceIn easing function bounces the animation at the beginning. For the easing function, we would supply values to a cubic-bezier() to emulate the wave-like curve. Easing functions specify the speed at which the animation progresses at different points within the animation. (The same options are available for transition-timing-function. Es decir, se aplica al comienzo del. We’ve created this complex transition with just a few lines of CSS and JavaScript, which turned out great. Here are just a few examples: You can visualize property changes. Animations with a start time set to 1000. Here’s that same animation with easing applied: To add easing to an action, simply select the action and choose the easing type from the dropdown. g. <single-easing-function> Determina o tipo de transição. We've spoken a little bit about easing functions, and the keywords we can use, but those aren't the only ones we can use. g. We can see in this animation’s case how the rectangle grows slightly bigger than 150% and then. GIF by franciscopola via GitHub. meɪ/) is a lightweight JavaScript animation library with a simple, yet powerful API. Once it’s done, we can set the open attribute on it to make the content visible but hiding as we have an overflow: hidden and a fixed height on the element. g. The speed curve is defined using an <easing-function>, a mathematical function that describes the rate at which a numerical value changes over time. The opacity animation should have an easing curve: linear, and the movement animation should have whatever easing function looks best to you. Happy stepping!A collection of swappable functions that add flavor to motion. Step 5: Combining Animations. JavaScript animation engine | animejs. We use the value of t to set the scale of the object. python csharp easing-functions cpp unreal-engine ease easing. Element: animate () method. ) We have four keywords to choose from: Easing out causes the animation to start more quickly than linear ones, and it also has deceleration at the end. Each animation runs for 1 second with a linear easing, so halfway through the animations the resulting transform would have the value translateX(5px) translateY(-10px) translateX(150px). CSS is a specific language, which helps describing pictures and animations for documents prepared in markup languages. An animation lets an element gradually change from one style to another. This lets you establish an acceleration curve, so that the speed of the animation can vary over its duration. Discover the advantages and drawbacks of using CSS animations for SVG. Step 5: Combining Animations. com . Use a function instead of a number/string for almost any property, and GSAP will call that function once for each target when it first renders the tween. Example of an easing function that produces an ease-in effect. This function creates an easing curve for these 2 properties: animation-timing-function and transition-timing-function. One of the most used transition timing functions is a cubic bezier curve. Introduction to the CSS basic box. The above curve defines how the output (y. ease-in and ease-out describe a quadratic relation of time and property. Animations are to be used with the animate directive. Source. Rather than using cubic curves, which has a number of limitations, the CSS3 Animation Generator uses calculated keyframes. Description. 52. This can give fine-grained control over the feel of an animation. 25, 0. There is only one function specific to CSS custom properties, but it makes the whole thing tick! The var () function is used to reference a custom property declared earlier in the document. <timing-function> The <easing-function> CSS data type denotes a mathematical function that describes how fast one-dimensional values change during animations. 0. By using basic CSS transforms, such as scale and rotate, with animation delays, the keyframe animations really come to life. Using the animation-delay function, you can specify when the animation starts. Animated provides several animation types, the most commonly used one being Animated. However, I would repeat what Duopixel says about mixing easing curves and fixed checkpoints - in this case you shouldn't actually need animations, just two 0%:100% animations - one for the parent and one. css ( {opacity:0}); $ ("#map"). More easing functions are available with the use of plug-ins, most notably the jQuery UI suite. Scroll to the Top widget. Example of an easing function that produces an ease-in effect. Changing the easing function (AKA make it bouncy) Tween. Easing animation is usable on User Interface (UI) elements such as the following: Buttons. Here’s a Pen showing different easing animations, including what’s offered in GSAP. If your runtime environment does not support ES modules, you'll need a transpiler (Babel is a nice one). 15: Resistance when swiping edges of non-infinite carousels: infinite: boolean: true: Infinite loop sliding: initialSlide: integer: 0: Slide to start on: lazyLoad: string 'ondemand' Set lazy loading technique. I love the classic Penner equations with. The easing function will run forwards for half of the duration, then backwards for the rest of the duration. Is this possible? The reason why to do this is to synchornize timing in Javascript to an animation. This is the “before-open” state (where you are animating in from). First, we calculate the height of the <details> element and we apply this height with inline styles on it. The way are going to implement the jump is pretty simple, and it requires using an easing function that we don't often talk about. Here is a version with the amount of curvature as an argument, following this general solution linked to by Creak. The transition-duration CSS property sets the length of time a transition animation should take to complete. /* * applyCurve: apply an S-curve to an input value. Definition and Usage. It is a shorthand for animation-name, animation-duration, animation-timing-function, animation-delay, animation-iteration-count, animation-direction, animation-fill-mode, animation-play-state, and animation-timeline. ease-in. An easing function has typically looked something like this:As you can see, there are lots of possible options and variations for animation easing functions and we’ll take a look at them next. CSS3's transitions and animations support easing, formally called a "timing function". on mouse hover. Drop something on the floor, and it will first accelerate downwards, and then bounce back up after hitting the floor. 5). To make it simpler for developers to create interesting and dynamic web pages, these libraries provide a variety of pre-built animations and sophisticated capabilities like timeline controls and easing functions. CSS animations will ensure smooth transitions between keyframes. Within a keyframe, animation-timing-function is an at-rule. A cubic Bézier easing function is a type of easing function defined by four real numbers that specify the two control points, P1 and P2, of a cubic Bézier curve whose end points P0 and P3 are fixed at (0, 0) and (1, 1) respectively. animate(), each property can take an array as its value: The first member of the array is the CSS. Ease starts out slow, speeds up, then slows down. A cubic bezier is defined by four points (x1, y1, x2, y2). g. The animation-timing-function specifies the speed curve of an animation. Duration refers to the amount of time that the transition plays. g. 動畫轉變的方式。 相較於傳統 script-driven 的動畫技術，CSS animations 有三種好處： 對於不複雜的動畫來說，CSS animation 是好選擇。It is possible to accomplish the same result with CSS, but it will require additional work. When I try and replace the class ease class with the rotate class. Hamburger Menu. CSS Animations and Transitions rely on the concept of animatable properties, and all CSS properties are animatable unless otherwise specified. ) correspondent à une courbe de Bézier cubique fixe avec quatre valeurs prédéfinies; La fonction cubic-bezier() permet de paramétrer une courbe. Super-powered by Google ©2010-2023. The duration of scroll animation (in seconds). When real people, animals, and objects move around, there’s no on-off. div { -webkit-transition: all 600ms easing’s Bezier curve; transition: all. , linearly or quick at the beginning, slow at the end). For CSS scroll-driven animations, auto fills the entire timeline with the animation. See an ease-out animation. MIT License. Les valeurs avec des mots-clés (ease, linear, ease-in-out, etc. g. Transitions （译） Easing Functions in CSS3 （译） Animating Movement Smoothly Using CSS （译） The CSS Animation Events（译） The transitionEnd Event（译） Get the Value of CSS Properties As They. Setting the animation-direction property to alternate provides the second half of the movement. The simplest kind of animation that we’ve added support for is called a transition. You saw a fair amount of this in my Easing Functions in CSS3 tutorial, and I won't repeat a lot of the conceptual information you saw there. The <easing-function> CSS data type denotes a mathematical function that describes the rate at which a numerical value changes. Anime. transition properties allow elements to change values over a specified duration, animating the property changes, rather than having them occur immediately. The speed curve is used to make the changes smoothly. 5. some-element { transition: 1s linear; }17. But the other aspect of the CSS animation that we're gonna be going through in this course is transforms. A function to be executed after. linear timing function. animate() using jQuery out of the box. To associate your repository with the easing-functions topic, visit your repo's landing page and select "manage topics. It is a shorthand for animation-name, animation-duration, animation-timing-function, animation-delay, animation-iteration-count, animation-direction, animation-fill-mode, animation-play-state, and animation-timeline. Here we have three segments (the green div elements) which expand (by increasing the width property) one after another. You can change as many CSS properties you want, as many times as you want. In CSS, the transition and animation properties allow you to specify an easing function. Value Definitions We've specified that the animation should last for 2 seconds (animation-duration: 2s), use an easing function to control the animation's acceleration and deceleration (animation-timing-function: ease-in), and have a 1 second delay before starting (animation-delay: 1s). EffectTiming object. css . The process is similar to working with animation software, except that keyframes in CSS are written as percentages for the timeline of animation in the code. net. ) transition-property. Easing functions can be set by setting the name of the desired function to the Animation's Easing property. 42, 0, 0. Timing functions and easing 7m 48s The steps() timing. Step 1: Position the Object to the Center. The only easing implementations in the jQuery library are the default, called swing, and one that progresses at a constant pace, called linear. In other words, t is the same as animation progress. If no animation-timing-function is specified on a keyframe, the corresponding value of animation-timing-function from the element to which the animation is applied is used for that keyframe. Here's the CSS3 Animation Generator tool which generates the keyframes for you, and enables 12 additional easing functions not supported by the w3c spec:The cubic-bezier() function affords us much more control over easing in our animations in CSS. More easing functions are available with the use of plug-ins. " Default easing. Generally speaking, an ease-out will be the right call, and certainly a good default. 1 Answer. I have seen people use trigonometry functions in the animation-delay; in many cases, using a magic number. CSS Code: #myDIV { animation-timing-function: linear;} Click the property values above to see the result. Type. When you’re happy, snag your code and off you go. Animation duration specifies how many seconds or milliseconds that animation takes to complete one cycle of the animation. Useless if lerp defined: easing: function (t) => Math. . Source. The behavior of these keyframe animations can be controlled by specifying their duration, number of repeats, and repeating behavior. 6s ; } Edit on cubic-bezier. 4. In CSS, if you applied animation-timing-function: ease-in-out you might assume that the start of your animation would ease in, and the end of your animation would ease out. If that property is something like a vertical distance. One such tool is the CSS Easing Animation Tool , so let's use that to calculate our timing function. The second parameter, which is optional, is either the value "start" or "end", and specifies the point at which the change of values occur within the interval. Step 2: Adding Delays and Repeating Animations. A transition function takes a reference to the DOM node where the transition directive is used and returns an object with some parameters that control the animation and, most importantly, a css or tick function. Easing. This property lets you arbitrarily scale, rotate, translate, or skew an element. css. 7, the easing function scales the value to produce an output progress of 0. Set animation using data-aos attribute: Anchor API JS Events Recipes: Adding custom animations: Adding custom easing: Customizing default animations distance Integrating external CSS animation library (e. In a CSS animation, you can specify your timing function as part of the shorthand animation property, or by setting the animation-timing-function property directly. Animation composite order. Essentially we're going to use GSAP's TweenLite library to apply easing functions to our state's changes and let Vue's. As of jQuery 1. Ease-y Breezy: A Primer on Easing Functions. An event handler function monitors all page clicks and calls. Cubic Bezier Curves. The interpolation occurs at a constant rate from beginning to end. (The same options are available for transition-timing-function. Easing functions: linear: aos-easing-linear; ease: aos-easing-ease;. e. The easing functions are the same as those described in. Timing functions are defined in the separate CSS Easing Functions module [css-easing-1]. Customizing Animation Duration And Easing Function. in_f(t * 2) * . NEW CSS Spring Easing & support for the linear() easing function. And we can see below that it functions as a hysteresis (or lag) which takes into account our previous scroll positions. This transition between two values may be applied in different situations. Easing out is typically the best for user interface work, because the fast start gives your animations a feeling of responsiveness, while still allowing for a natural slowdown at the end. The CSS animations module lets you animate the values of CSS properties, such as background-position and transform, over time by using keyframes. Easing functions specify the speed at which the animation progresses at different points within the animation. Custom and predefined easing functions, delays, durations, decay factors, spring constants, and more can all be tweaked depending on the type of animation. To make an animation start at a certain moment, you use the startTime property. The BounceOut easing function bounces the animation at the end. The frame function would at maximum be executed every frame, But of course the browser could decide to drop/skip frames if it needed, The browser itself could work out how much rounding would happen. Each keyframe describes how the animated element should render at a given time during the animation sequence. ) We have four keywords to choose from:Easing out causes the animation to start more quickly than linear ones, and it also has deceleration at the end. In this example, we defined a transition on the background-color property with a duration of 0. Description. Start Time #. Provides a cubic bezier curve, equivalent to CSS Transitions'. The value must be one of those. This function returns a css property, easing, and duration. There are 3 types of easing functions - Linear, Step and Cubic Bézier. It is one of the most performant ways to animate on the Web,. css-animations; easing-functions; or ask your own question. Each stop is a single number that ranges from 0 to 1. 5. The start of the progress of the animation is 0 and the end is 1. It typically uses a linear interpolation technique, but it also supports easing functions. Use the animation tool to learn more about easing and what arguments to pass to the easing functions. ) transition-property. We define CSS easing with either the animation-timing-function or transition-timing-function properties, depending on whether we are working with an animation or transition respectively. Then we pass customTransition to the box. We've got a method, a target and a vars object which all contain information about the animation. CSS Custom Properties. Il buono (the good): Smooth gradients in CSS that blends into their context. animation-timing-function: linear (0, 0. Note: Color-related properties cannot be animated with . The CSS animations module lets you animate the values of CSS properties, such as background-position and transform, over time by using keyframes. Easing functions specify the speed of animation to make the movement more natural. Mimic CSS animation with JavaScript Function. You can learn more about available easing functions here; onStart: represents a function to be called when an element is about to animate; onEnd: represents a function to be called when an element has finished animatingThe transition-timing-function property, normally used as part of transition shorthand, is used to define a function that describes how a transition will proceed over its duration, allowing a transition to change speed during its course. It is worth considering the appropriate connecting curve used in computer graphics and animation-curves derived from the set of easing functions [9, 11]. duration: 5 seconds. }); }); Easing is the primary way to change the timing of your tweens. slide-down { height : 60px; } #a{ transition: height 200ms }I can get a custom easing function as a function (as numbers), but I don't know how to convert named easing functions such as ease-in into numbers. js ( /ˈæn. Svelte provides a flip animation function that can be. 7, the easing function scales the value to produce an output progress of 0. Transitions in CSS are applied to an element and specify that when a property changes it should do so over gradually over over a period of time. linear: The easing curve for an animation that starts and ends at the same speed. HTML Structure. The animation-direction CSS property sets whether an animation should play forward, backward, or alternate back and forth between playing the sequence forward and backward. This transition between two values. Sets a stepping function, with two parameters. This can be used to inject more advanced mixers than Framer Motion's default, for instance Flubber for morphing SVG paths. If this is your. In this example, using groups both on enter and leave allows for. The Web Animations API opens the browser's animation engine to developers and manipulation by JavaScript. The x1 and y2 are the start and end points of the curve. Specify which CSS properties to listen for changes on; Set timing (easing) functions to alter the rate of going from a one property value to another; Specify a duration to control how long the animation or transition will take; Programmatically listen to animation and transition-specific events that you can then do with as you wishYou can apply CSS to your Pen from any stylesheet on the web. Seamless workflow Fine-tune the timing, path, and speed of your SVG animations. The first value that can be parsed as a time is assigned to the transition-duration, and the second value that can be parsed as a time is assigned to transition-delay. Check out the section on transforms in our Functions module. The main problem with this approach is that the animation may look rough. 1. About easing. CSS or JavaScript. params − A map of CSS properties that the animation will move toward. Animations ease-in by starting slow and then speeding up, and ease-out by slowing. A keyframes object or null. It supports multiple programming languages and game development frameworks. All of these functions can be used on CSS to a more natural feel to your keyframe transitions. Hopefully these demos inspire you to use steps() effortlessly in your CSS animations. But adjusting the keyframes animations or transitions manually can be quite time-consuming. Property refers to a transition's target for listening and animating values upon. The linear() function creates a piecewise linear easing, allowing the approximation of complex animations and transitions by interpolating linearly between the specified points. 1000, easing. Animations can seem complex and difficult to create, but you can create suprisingly impressive animations with very little work. The value t is. This function isn’t exactly an easing function, as there’s no variation from start to finish. Next to the predefined functions, you can make your own easing function. PI/2)) + b; } but I need some help how to combine the two. When doing animations in CSS, most commonly, one would either use the default CSS timing functions provided by CSS, or go to cubic-bezier. Easing functions specify the speed at which the animation progresses at different points within the animation. 動畫的最初及結尾 2. You can use the properties in the animations module to control the duration,. Here is the official. 1. O valor deve ser um dos disponíveis em <easing-function> (en-US). There are three types of easing. easein { animation-timing-function: ease-in; } . If multiple keyframe sets exist for a given name, the last one encountered by the parser is used. You can use predefined easing functions or create custom ones with cubic-bezier. This section describes the status of this document at the time of its publication. In this article, I’ll share my interesting findings on Bézier curves. must add this value to the cascade if and only if that property is not currently undergoing a CSS Animation ([CSS3-ANIMATIONS]) on the same element. CSS properties transition and animation allow you to pick the easing function. Vue will detect if any CSS animations or transitions are being used and will automatically toggle classes on the transitioned content, allowing for a perfectly timed transition system and complete control. The function takes four values, which correspond to two control points of a cubic bézier curve. linear { animation-timing-function: linear; } . CSS Elastic ease generator. Position he object you want to move to the center of the circular path. This, in essence, lets you establish an acceleration curve so that the speed of the transition can vary over its duration.