High-pass filter, because you’re worth it

The familiar Gaussian blur filter eliminates sharp detail, leaving only large shapes of color. If we think of the image pixels as a waveform, the blurred image contains only the low frequencies of the original image – the Gaussian blur is therefore a low-pass filter. Sometimes we might want the opposite: an image filter that eliminates large scale color variation and leaves only sharp details. This kind of high-pass filter would therefore be the functional inverse of a Gaussian blur.

We can in fact build a high-pass filter simply by subtracting a blurred image from the original:

Let’s try using the high-pass filter for smoothing skin. Here’s a DV image of some wrinkles… (Yeah, I didn’t need to go very far to get this image.)

The result of the above high-pass filter looks like this:

And with a couple more nodes….

….my hand looks 20 years younger. (Of course what really matters is the way it makes me feel. More self-confidence thanks to the new, improved formula!) Roll over to compare with original image.

Quick overview of how the effect is constructed: the high-pass matte is slightly expanded and blurred, then used as a mask to composite a blur over the original image. (The blurred image is marked as “wrinkle fill image” in the node screenshot above.) The result is then masked using a Color Range Key into the shadows and midtones of the hand. This is because I didn’t want the blur to smoothen out those illuminated hairs on the hand – the result would have looked too artificial.

What is this effect good for? I would imagine that certain segments of the video production industry have an interest in making skin look airbrushed… But I wouldn’t know anything about that.

Of course you don’t have to settle for smoothing out details. The “wrinkle fill” image could just as well be a sharpened version of the original, in case you want to make those wrinkles really pop out – for example if you’ve shot a documentary in Afghanistan, but the 97-year old refugee doesn’t look quite old enough.
(Disclaimer: you may lose your Oscar if the trick is exposed. Please don’t take documentary production advice from me.)