# Find the background position by mouse pos and its hit area

I'm trying to creating a jQuery plugin where the user can interact with a div element background image. Basically, the background-image is larger than div element container, so if the user move the mouse over the div element, the image is automatically scrolled to be seen completely.

I thought to do that by working with this relation, but it is imprecise.

mX : aW = bX(?) : bW


so

-((mX * bW) / aW) / 2 = bX


this does something near but it isn't precise, how can I find the right way to do the proportion?

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imho this is rather a stackoverflow question – Listing Feb 3 '12 at 11:52
I thought to write the question here because I've asked about a mathematical relation first – vitto Feb 3 '12 at 12:04

I'm going to give your "?" the name X. When mX = 0, you want X = 0. When mX = aW, you want X = bW - aW. So mX : aW :: X : bW - aW. Solving, we get X = (bW - aW)*mX/aW.