Apply a Filter to a Background Image

You can apply a filter to an entire element quite easily with the filter property. But what if you want to apply a filter just to the background of an element? It's weirdly tricky.

There are CSS properties that specific to backgrounds, like background-blend-mode — but blending and filters are not the same thing. It sorta seems to be the reason we have backdrop-filter, but not quite. That does filtering as the background interacts with what is behind the element.

There is no background-filter, unfortunately. What are we to do?

Use a pseudo-element instead of a background

If you put the contents of the element in an inside wrapper, you can set that on top of a pseudo-element that is simply pretending to be a background.

.module {
  position: relative;
}
.module::before {
  content: "";
  position: absolute;
  top: 0; left: 0;
  width: 100%; height: 100%;
  background-image: url(graphic-to-be-filtered.jpg);
  filter: grayscale(100%);
}
.module-inside {
  /* This will make it stack on top of the ::before */
  position: relative;
}

See the Pen Apply Filter to Psuedo Element by Chris Coyier (@chriscoyier) on CodePen.

See how the "background" gets filtered with grayscale there? We called the grayscale filter there and applied it only to the pseudo-element, leaving the rest of the content inside unfiltered.

It depends on what kind of filtering you want... you might be able to fake it with blend modes

I ran into this issue as I was specifically trying to grayscale the background image of an element. Since, as we've covered, there is no specific property just for that, I thought about background-blend-mode, particularly how there are blending options for things like saturation and color. If I could set pure black over the top of the graphic, then I could blend them — like I can with multiple backgrounds — and essentially fake a grayscale effect.

Note that you can't use a solid color by itself when working with multiple backgrounds (that would be a background-color not background-image), so we have to fake that as well with a no-transition linear-gradient.

.module {
  background-image:
    linear-gradient(black, black),
    url(image-to-be-fake-filters.jpg);
  background-size: cover;
  background-blend-mode: saturation;
}

See the Pen Apply Fake Filter with Blend Mode by Chris Coyier (@chriscoyier) on CodePen.

Dan Wilson's explorations

Dan got way into this and made an exploratory Pen in which there are three layers:

1. Top layer: a vignette from a radial-gradient
2. Middle layer: solid color
3. Bottom layer: image graphic

You can adjust the colors used on the top two layers and apply different blend modes to each one. That was another thing I learned! Just like you can comma-separate to make multiple backgrounds (and likewise with background-size, background-position and such to apply those values to specific backgrounds) you can also comma-separate background-blend-mode to apply different blending effects to each layer.

See the Pen Multiple Backgrounds, Multiple Blend Modes by Dan Wilson (@danwilson) on CodePen.

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