Image brightness / luminosity normalizer for text over image

In a work project, the UI guys had defined large parts of the interface as text over image. To improve legibility, I experimented with ways to automatically darken an image based on its original brightness.

Demo

See the plugin in action : https://antoningrele.github.io/img-brightness-normalization/

How to use

Step 1

Add a data-background-image attribute to the element, with the image URL as value.

Example : <div data-background-image="images/photo-1446770145316-10a05382c470.jpg"></div>

Step 2

Add the CSS and JS files to your page :

<link rel="stylesheet" href="https://antoningrele.github.io/img-brightness-normalization/normalizeBrightness.css" type="text/css">
<script onload="normalizeBrightness();" src="https://antoningrele.github.io/img-brightness-normalization/normalizeBrightness.js"></script>

Step 3 (optional)

You can configure the following attributes when calling normalizeBrightness() :

coeff (default=1.25) : the dark overlay's opacity is equal to the images brightness divided by this value. A value of "1" means the image brightness is compensated for entirely (a pure white image would become black). Higher values reduce the intensity of the filter.
callback (default=null) : called at the end of the brightness calculations, is given the darkened element as an argument. It's used in the demo to display the source luminosity, for example

Credits

Credits to matkl for the "average" color calculation and to Franci Penov for the brightness calculation based on that color.