# Custom element <progressive-img> Custom element (written in Polymer) to load img progressively and lazily. It first shows tiny blurred placeholder and loads full-sized image afterwards. It can also defer loading until placeholder is clicked or enters viewport. ## [Demo](demo/index.html) ## Basic usage You basically take all your <img> tags, change them to <progressive-img> and add placeholder attribute. It will display a blurred placeholder and load src/srcset after being clicked. ```html ``` ## Load strategy ```html ``` Load strategy controls when image starts loading. There are 3 options: | Load strategy | Behavior | | --------------- | -------- | | instant | image will start loading immediately | | on-visible | image will load if is visible or when enters viewport | Clicking the placeholder will always trigger image loading no matter what. You can completely leave out load-strategy if that's all you need. ## Intersection margin ```html ``` Intersection margin only takes effect if `load-strategy="on-visible"` is set. Default value is **200px**, so image will trigger loading when enters 200px wide area around viewport.  For this effect, progressive-img uses intersectionObserver API. Here you can read more about how it works: - [Google dev docs](https://developers.google.com/web/updates/2016/04/intersectionobserver) - [Arnelle Balane's medium blog post](https://blog.arnellebalane.com/the-intersection-observer-api-d441be0b088d) - [Mozilla docs](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API) intersection-margin attribute is assigned to rootMargin of intersection observer, so you can use any [valid value of rootMargin](https://w3c.github.io/IntersectionObserver/#dom-intersectionobserverinit-rootmargin). ## IntersectionObserver Polyfill If you need to use `load-strategy=on-visible` in [browsers that don't currently support intersection API](https://caniuse.com/#feat=intersectionobserver), you can just include official polyfill: https://github.com/w3c/IntersectionObserver/tree/master/polyfill, it's one liner. ## What about browsers with disabled javascript? Without js, progressive-img can't load. Which is great because you can easily create fallback: ```html ``` ## Styling progressive-img uses 4 CSS custom properties to customize the appearance. | CSS property | description | | --------------- | ----------- | | --placeholder-filter | Filter applied to placeholder image. Defaults to `blur(10px) saturate(1.2)` | | --placeholder-scale | Transform scale value. Defaults to `1.1` to prevent blurred placeholder from having white borders | | --transition-duration | Duration of swapping placeholder and final image. Defaults to `.2s` | | --transition-timing-function | Timing function of swapping placeholder and final image. Defaults to `ease-in` | You can find examples of this in [demo page](demo/index.html)