Photoshop’s high pass filter can be used with layers to achieve some very useful image enhancements. This post discusses how to increase or decrease contrast along object edges and provides a few downloadable PS Actions. High pass edge contrast enhancement is a standard trick for adding “punch” to images: you probably see it all the time without even realizing it. Edge contrast reduction is a logical consequence of edge enhancement. It could be used as a “softening” filter, but probably qualifies as an “effect,” since it runs counter to expectations for good images. In other words, it’s just waiting for someone to exploit its potential.
Although we’ll describe everything step-by-step, you may find it useful to download the PS Actions at the end of the post. They have all the steps built in. I’ve also included a few files that you can download to experiment with.
The high pass filter is found on the Filter > Other menu. According to Adobe’s online help this filter “retains edge details in the specified radius where sharp color transitions occur and suppresses the rest of the image. (A radius of 0.1 pixel keeps only edge pixels.) The filter removes low-frequency detail from an image and has an effect opposite to that of the Gaussian Blur filter.” The illustration below shows how this works with a grayscale image of black dots of varying sizes.
If you apply the high pass filter to an RGB image, it processes each color channel separately, resulting in halos of complementary color around the dots in a color dot pattern. If you use the Lab color space, results are different.
The Overlay blending mode multiplies or screens pixels depending on the value of the base pixel. If you think of the range of black to white as 0.0 to 1.0, multiply performs the operation (blend * base) and screen performs the operation (1.0 – (1.0 – blend) * (1.0 – base)). When you overlay an image on itself, you darken the dark pixels and lighten the light pixels. If the image is a color image, screening and multiplying also increase color saturation.
Overlaying a high-pass filtered image on itself will enhance contrast and color saturation on the edges of elements at a scale selected by the radius value in the high pass filter. At low radius values, the effect is similar to an unsharp mask filter. At high radius values, it is very similar to the positive side of the “Clarity” slider in Photoshop and Bridge’s RAW conversion tool. Armed with this knowledge, we can create a wide range of enhancements and effects. The simplest way to explore the possibilities is with the overlay layer converted to a smart object. Adding a curves adjustment just for the overlay layer will permit still further experiments. I have provided a downloadable Photoshop action that will set up the layers automatically for you. It is set up to initially provide enhanced contrast and saturation.
With Photoshop layers set up as shown, you can vary quite a few things:
- Change the opacity or fill to “tone down” the effect
- Click the layer visibility (eye) icon to turn off the overlay layer
- Click the Smart Filters or High Pass visibility icon to show only the overlay
- Double-click the high pass filter to change its radius
- Increase or decrease contrast in the Curves layer
- Invert the Curves (choose Negative (RGB) from the curves menu)
When working with the Curves adjustment layer, keep in mind that 50% gray in the overlay layer has no effect on the underlying layer–most likely you’ll want to keep 50% gray constant, otherwise, you will lighten or darken the whole image, not just the edges. For example, if you move the black point until input is 96 and move the white point until input is 159, gray will remain constant because black and white points are equidistant from middle gray. Setting the curves to Negative (RGB) inverts the image. Instead of increasing contrast and saturation around images edges, it decreases them. In effect, it’s a kind of softening filter–one that you can tune to affect edges and shapes at different scales by changing the high pass radius. Here are some examples, using that staple of self-promo, a headshot:
Okay, that wraps the examples. Some credits and downloadable items will follow. I recommend you experiment to find values for opacity, fill, and high pass radius that prove useful for your own work–I have used somewhat exaggerated settings just to show things clearly. Adjusting the input and output values in the Curves adjustment layer can provide quite a range of effects, many of them actually useful. Stacking overlay layers with different high pass radiuses can provide many strange solarization effects (maybe useful for reviving 60s rock posters). I provide a PS action to do that, just for fun. Enjoy!