javascript - Increase element opacity when user scrolls down the page -


i've seen lot of question related modifying element opacity when user scrolls haven't found 1 helps me way need. have tried several formulas , haven't been able achieve effect want.

i have header bg image, , inside div use overlay, , want darker , darker smoothly (opacity increase) while user scrolls down.

edit: desired effect is:

opacity default set 0.2 in css. when user starts scrolling down start increasing 0.2 1. when user scrolls again decrease 1 (or whatever value was) 0.2.

fiddle: https://jsfiddle.net/z7q2qtc6/

<div class='nice-header'>   <div class='header-overlay'></div> </div>

css

.nice-header {   position: relative;   height: 300px;   background: center center;   background-size: cover;   background-image: url(http://www.boeing.com/resources/boeingdotcom/commercial/787/assets/images/marquee-787.jpg); }  .header-overlay {   position: absolute;   top: 0; left: 0;   width: 100%; height: 100%;   background: rgb(0,0,0);   opacity: 0.2; }

js

$(window).scroll(function() {   $('.header-overlay').css({     opacity: function() {       var opacity = 0;       //todo:       //set opacity higer value whilst user scrolls       return opacity;     }   }); });

you can retrieve current scrolling position using .scrolltop() method.

to calculate opacity, subtract scrolltop value height of element , divide element's height.

example here

$(window).scroll(function() {   var scrolltop = $(this).scrolltop();    $('.header-overlay').css({     opacity: function() {       var elementheight = $(this).height();       return 1 - (elementheight - scrolltop) / elementheight;     }   }); });

if want account element's initial opacity of 0.2:

updated example

$('.header-overlay').css({   opacity: function() {     var elementheight = $(this).height(),         opacity = ((1 - (elementheight - scrolltop) / elementheight) * 0.8) + 0.2;      return opacity;   } });

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