javascript - Change text color based on brightness of the covered background area?

Question

I am looking for a plugin or technique that changes a text's color or switches between predefined images/icons depending on the average brightness of the covered pixels of its parent's background-image or -color.

If the covered area of it's background is rather dark, make the text white or switch the icons.

Additionally, it'd be great if the script would notice if the parent has no defined background-color or -image and then continue to search for the nearest (from parent element to its parent element..).

What do you think, know about this idea? Is there something similar out there already? Examples?

Answer 1

Interesting resources for this:

• W3C - Ensure that foreground and background color combinations provide sufficient contrast
• Calculating the Perceived Brightness of a Color

Here's the W3C algorithm (with JSFiddle demo too):

const rgb = [255, 0, 0];

// Randomly change to showcase updates
setInterval(setContrast, 1000);

function setContrast() {
  // Randomly update colours
  rgb[0] = Math.round(Math.random() * 255);
  rgb[1] = Math.round(Math.random() * 255);
  rgb[2] = Math.round(Math.random() * 255);

  // http://www.w3.org/TR/AERT#color-contrast
  const brightness = Math.round(((parseInt(rgb[0]) * 299) +
                      (parseInt(rgb[1]) * 587) +
                      (parseInt(rgb[2]) * 114)) / 1000);
  const textColour = (brightness > 125) ? 'black' : 'white';
  const backgroundColour = 'rgb(' + rgb[0] + ',' + rgb[1] + ',' + rgb[2] + ')';
  $('#bg').css('color', textColour); 
  $('#bg').css('background-color', backgroundColour);
}

#bg {
  width: 200px;
  height: 50px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<div id="bg">Text Example</div>