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>

Answer 2

This article on 24 ways about Calculating Color Contrast might be of interest to you. Ignore the first set of functions because they're wrong, but the YIQ formula will help you determine whether or not to use a light or dark foreground color.

Once you obtain the element's (or ancestor's) background color, you can use this function from the article to determine a suitable foreground color:

function getContrastYIQ(hexcolor){
    var r = parseInt(hexcolor.substring(1,3),16);
    var g = parseInt(hexcolor.substring(3,5),16);
    var b = parseInt(hexcolor.substring(5,7),16);
    var yiq = ((r*299)+(g*587)+(b*114))/1000;
    return (yiq >= 128) ? 'black' : 'white';
}

Answer 3

mix-blend-mode does the trick:

header {
  overflow: hidden;
  height: 100vh;
  background: url(https://www.w3schools.com/html/pic_mountain.jpg) 50%/cover;
}

h2 {
  color: white;
  font: 900 35vmin/50vh arial;
  text-align: center;
  mix-blend-mode: difference;
  filter: drop-shadow(0.05em 0.05em orange);
}

<header>
  <h2 contentEditable role='textbox' aria-multiline='true' >Edit me here</h2>
</header>

Addition (March 2018): Following, a nice tutorial explaining all different types of modes/implementations: https://css-tricks.com/css-techniques-and-effects-for-knockout-text/

Answer 4

Interesting question. My immediate thought was to invert the color of the background as the text. This involves simply parsing the background and inverting its RGB value.

Something like this: http://jsfiddle.net/2VTnZ/2/

var rgb = $('#test').css('backgroundColor');
var colors = rgb.match(/^rgb\((\d+),\s*(\d+),\s*(\d+)\)$/);
var brightness = 1;

var r = colors[1];
var g = colors[2];
var b = colors[3];

var ir = Math.floor((255-r)*brightness);
var ig = Math.floor((255-g)*brightness);
var ib = Math.floor((255-b)*brightness);

$('#test').css('color', 'rgb('+ir+','+ig+','+ib+')');

Answer 5

By combining the answers [ @alex-ball , @jeremyharris ] I found this to be the best way for me:

        $('.elzahaby-bg').each(function () {
            var rgb = $(this).css('backgroundColor');
            var colors = rgb.match(/^rgb\((\d+),\s*(\d+),\s*(\d+)\)$/);

            var r = colors[1];
            var g = colors[2];
            var b = colors[3];

            var o = Math.round(((parseInt(r) * 299) + (parseInt(g) * 587) + (parseInt(b) * 114)) /1000);

            if(o > 125) {
                $(this).css('color', 'black');
            }else{
                $(this).css('color', 'white');
            }
        });

*{
    padding: 9px;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.0/jquery.min.js"></script>
<div class='elzahaby-bg' style='background-color:#000'>color is white</div>

<div class='elzahaby-bg' style='background-color:#fff'>color is black</div>
<div class='elzahaby-bg' style='background-color:yellow'>color is black</div>
<div class='elzahaby-bg' style='background-color:red'>color is white</div>

Answer 6

I've found the BackgroundCheck script to be very useful.

It detects the overal brightness of the background (be it a background image or a color), and applies a class to the assigned text-element (background--light or background--dark), dependent on the brightness of the background.

It can be applied to still and moving elements.

(Source)

Answer 7

If you are using ES6, convert hex to RGB then can use this:

const hexToRgb = hex => {
    // turn hex val to RGB
    const result = /^#?([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})$/i.exec(hex)
    return result
        ? {
              r: parseInt(result[1], 16),
              g: parseInt(result[2], 16),
              b: parseInt(result[3], 16)
          }
        : null
}

// calc to work out if it will match on black or white better
const setContrast = rgb =>
    (rgb.r * 299 + rgb.g * 587 + rgb.b * 114) / 1000 > 125 ? 'black' : 'white'

const getCorrectColor = setContrast(hexToRgb(#ffffff))

Answer 8

Here's my attempt:

(function ($) {
    $.fn.contrastingText = function () {
        var el = this,
            transparent;
        transparent = function (c) {
            var m = c.match(/[0-9]+/g);
            if (m !== null) {
                return !!m[3];
            }
            else return false;
        };
        while (transparent(el.css('background-color'))) {
            el = el.parent();
        }
        var parts = el.css('background-color').match(/[0-9]+/g);
        this.lightBackground = !!Math.round(
            (
                parseInt(parts[0], 10) + // red
                parseInt(parts[1], 10) + // green
                parseInt(parts[2], 10) // blue
            ) / 765 // 255 * 3, so that we avg, then normalize to 1
        );
        if (this.lightBackground) {
            this.css('color', 'black');
        } else {
            this.css('color', 'white');
        }
        return this;
    };
}(jQuery));

Then to use it:

var t = $('#my-el');
t.contrastingText();

This will straight away, make the text either black or white as appropriate. To do the icons:

if (t.lightBackground) {
    iconSuffix = 'black';
} else {
    iconSuffix = 'white';
}

Then each icon could look like 'save' + iconSuffix + '.jpg'.

Note that this won't work where any container overflows its parent (for example, if the CSS height is 0, and overflow isn't hidden). To get that working would be a lot more complex.

Answer 9

In es6 contrast from a HEX 6-character color string (#123456) can be calculated with this one-liner:

const contrastColor = c=>["#000","#fff"][~~([.299,.587,.114].reduce((r,v,i)=>parseInt(c.substr(i*2+1,2),16)*v+r,0)<128)];

Here is broken down, readable version:

const contrastColor = color =>
{
  const lum = [.299 /*red*/,.587 /*green*/,.114 /*blue*/].reduce((result, value, index) => 
  {
    // with reduce() we can convert an array of numbers into a single number
    // result = previous result returned by this function
    // value = https://www.w3.org/TR/AERT/#color-contrast
    // index = current position index in the array
    // num = decimal number of Red, Green or Blue color
    const num = parseInt(color.substr(index * 2 + 1, 2), 16);
    return num * value + result;
  }, 0 /* result = 0 */);

  const isDark = lum < 128;
  const index = ~~isDark; // convert boolean into 0 or 1
  return ["#000","#fff"][index];
}

function setColors()
{

  for(let i = 0; i < 70; i++)
  {
    const bg = "#" + (~~(Math.random() * 16777216)).toString(16).padStart(6, 0),
          color = contrastColor(bg);
    node = test.children[i] || document.createElement("span");
    node.style.backgroundColor = bg;
    node.style.color = color;
    node.textContent = bg;
    if (!node.parentNode)
      test.appendChild(node);
  }
}

setColors();

#test
{
  display: flex;
  flex-wrap: wrap;
  font-family: monospace;
}
#test > *
{
  padding: 0.3em;
}

<button id="click" onclick="setColors()">change</button>
<div id="test"></div>