Animating line on scroll

Question

I am trying to animate a line on scroll but I am at a loss at the moment. The final result should be similar to how the lines animate on this site, http://www.teslamotors.com/goelectric#range (you have to scroll a little bit to get to the lines).

There is a static gray line, and then a red line that gets height when the user scrolls down. If the user scrolls up while part, or all of the red line is visible, height will be subtracted from the red line. BUT nothing should happen to the red line until the user has scrolled 200px down the page.

I have created a fiddle for this problem and I am pretty sure I know where my problem lies, but I do not have an answer for how to fix it. I think it is because my variables currentScrollPosition and lastScrollPosition in function countUp135 are always equal to each other.

Any help would be greatly appreciated. Here is my fiddle: http://jsfiddle.net/tripstar22/2HDVA/5/

Thanks in advance!

Answer 1

There are a lot of functions in your fiddle, where I do not understand why you should need them nor what they do. An updated version of your fiddle, seems to do what you want. There was no need for all thoses methods.

This seems to be enough:

var scrollTop = getScrollTop();
if (scrollTop > 200) {
    addHeightPath1(scrollTop - 150);
} else {
    addHeightPath1(0);
}

I'm also wondering about the function stoppedScrolling, where an asynchronous time function is beeing started, but you try to get an result from the stoppedSrolling() function, that is never passed as a return.

Answer 2

Although there is many ways to do this in JS, I'll offer an alternative css method. Instead of animating a line on scroll you can just give the illusion that the line is animating. Check out this fiddle here. As you can see The fixed red element will follow the window on scroll and fill in the transparent area between the divs, making it seem like a red line is being drawn.

You can overlay an image with a transparent line running through it instead of grey divs to minimize the code even more, or you can add js to make the animation more fancy.