Color Compare Range Slider

For this week's Codepen Challenge, I did a Color Compare Range Slider. It's using an <input type="range"> to switch between two selectable colors of your own choice.

A Codepen-user asked this:

How did you do this?

It's not super complex, but requires a lot of visuals to explain, so let's dive in!

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First, you need an image where all the pixels you do not want to color, are transparent:

The same image will be used twice: Once as a regular image in an <img>-tag, and as a custom property: --cc-mask:url(path-to-image).

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Then you style an <input type="range">:

The background is a linear-gradient with a hard stop-point behind the thumb (the grey bar):

.selector {
  background-image: linear-gradient(
    to right,
    var(--_c1) 0 var(--_x),
    var(--_c2) 0 var(--_x)
  );
}

--_c1 and --_c2 are the colors. These are updated from JavaScript, when you select a different color. The --_x is the current value of the range-slider.

Then you add the image as a mask and set mix-blend-mode to multiply:

You then add the <img> behind the range-slider, and we're almost there:

Finally, we'll add two <input type="color">.

I'm using two small helper-scripts, one for setting the scope of a CSS Custom Property and one for setting the property (see the code in the Codepen below).

On the wrapper, <fieldset>, we add an eventListener, that'll update the three CSS Custom Properties we're using:

fieldset.addEventListener('input',
  event => property(event.target))

And that's it — hope it makes sense!

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Codepen