High Pass Sharpening with Layers

By Ctein

Author's note: I'm scheduled to do two two-day workshops at the Mesilla Digital Imaging Workshops at the end of January. One will be on photo restoration, the other on using Layers in Photoshop. This column is a taste of what participants will be getting in the latter class. We still have room for more students; I hope some of you will be inclined to sign up.

I recently stumbled on a very useful and flexible way to enhance detail and sharpen up images. I'm sure I'm not the first to hit on this; still, Martin Evening (a walking encyclopedia of Photoshop skills) gives it only a page in Adobe Photoshop CS3 for Photographers and seems not to realize how very useful it is.

In short form, it works like this. Duplicate your image layer. Apply Photoshop's "High Pass..." filter to the top layer. Set the layer blend to "Soft Light." Detail's enhanced!

Long form: I hit upon this trick while trying to improve the eclipse photo above. The original image is more than a dozen times larger than the one here. The coronal streamers I wanted to bring out are broad features. Normal edge and fine-detail boosts miss them entirely; the local contrast enhancement method I described previously improves detail only slightly.

The High Pass filter lets me set the width of the filter. In this case, the radius was about 60 pixels! Doing that got me the version below (kinda—I painted the disk of the sun black by hand). The effect of the filter is to produce an image where non-detail comes out a uniform 128-grey. Edges get brighter or darker. So what?

Well, "Soft Blend" is one of Photoshop's "weird" blending modes. When you blend a layer that way, any pixels with values of 128 have no effect on the underlying image. Pixels lighter or darker than that lighten or darken the underlying image.

See where we're going? High Pass + Soft Blend exaggerates edges. Now, so do most Photoshop sharpening tools, but the cool part of this method is how much you can control the enhancement, as well as apply it to features far too large for normal sharpening methods to notice.

By itself, the High Pass layer has a puny effect. You can make the effect stronger by blending with one of the other modes, all the way up to "Linear Light." A Curves adjustment layer associated with the High Pass layer is a much better tool. The screen shot above shows the arrangment of layers and the curve I used to get me to the second picture at the top of this post (the finished eclipse picture). What that curve does to the High Pass layer is shown here:

Lock down the 128,128 point; then you can do anything you want to the rest of the curve without altering overall image brightness. Here I just kicked the contrast way up. But, for example, if I'd wanted to enhance the dark side of the edges and not the bright side (to avoid bright haloes, perhaps), I could have drawn a curve that looked "normal" to the right of 128,128.

You can manipulate the results other ways. Want different strength enhancements in different areas? Airbrush black into the High Pass or Curves layer mask channel to reduce its effect. Want no enhancement in some areas? Paint over those parts of the High Pass layer with 128-gray. Want wildly different enhancements in different areas? Make multiple High Pass/Curves layer sets and paint in the masks to restrict the impact to the areas you want.

This approach works fabulously well with digital camera images. Try it for yourself, with a filter radius of about 2 pixels to start. You'll be amazed how much you can sharpen your photos without getting annoying edge artifacts. Better still, if 90% of the photo looks great and 10% shows artifacts or halos, just paint those areas down in the masks. It's merely my personal taste, but I feel this approach is giving me the most natural-looking sharpening I've ever gotten.

Warning: High Pass sharpening also greatly enhances noise and grain. You'll want to run a noise reduction pass on the original image before filtering and on the resulting High Pass layer to keep it under control. I used the Neat Image plug-in in this example; Noise Ninja's also good.

_______________________

Ctein

Comments

[Chris Combs]

The technique's been around for a while, but is definitely terrific. Try Overlay mode too.

[Edward Taylor]

Ctein -

Great tip. I have been using this technique for some time and it is really useful. Switching to different blend modes can give differing results as well, as you said. You can also manage noise to an extent by masking - so that only parts of the photo that really need sharpening are effected.

Ed

[fernando]

The only place I found such a discussion is in Bruce Fraser's «Image Sharpening» book (pages 135-138). Although people like to have one button, or one technique, for most processing, this shows that the preferred masking approach (in the book and in Lightroom) is not the best for all kind of photos... methinks here is one where the mask would induce a weird texture on the low dynamic range outside of the inner circle.

Very nice to see a good example put to practice as my photos are rarely with such properties.

As for noise reduction, perhaps a mask for blurring the non-detail areas could work to reduce that effect.

[Ben Rosengart]

How does this compare to running USM with a 60px radius?

[Miserere]

Ctein,

I have a friend whose job it is to edit astronomical images for public outreach; he uses the high-pass filter exclusively as a sharpening method (with various blending modes, of course). Don't talk to him about USM, he ain't buying it :-)

One of these days I'll have to ask him to explain it to me slooowly. Or wait for you to give a seminar in the Boston area ;-)

--M.

[Mike Peters]

I've been using this exclusively for about 5 years as a way to sharpen my 4000 ppi scans of 120 film. I set the dupe layer to overlay as Chris does. You can adjust the radius and opacity to suit the particular image. Nice thing is, your images look sharp, but never sharpened, as in that artificially over-sharpened look that seems ubiquitous on flickr and other sites. It works wonders on digital capture files also.

[Chris Norris]

I see fernando beat me to it. Real World Image Sharpening with Adobe Photoshop CS2 does indeed have three pages dedicated to high pass sharpening. Those pages concentrate on using it with the Overlay mode. At 60px I think you're getting into midtone contrast rather than sharpening (to pick a few nits) although that sort of depends on what resolution image we're talking about as well. Sharpening can be such big world. But then again, is it true that "Sharpness is such a bourgeois concept"? I wonder if HCB really said that.

[Greg Brophy]

I have used this for a while with portraits and it is great for selective sharpening. You use the high pass filter and then with a brush set to 50% gray paint out the areas you don't want to be sharpened like flaws in the skin. Then the areas around the eyes and nose and other facial features will be sharp. Then set it to soft light.

[Eric Rolph]

It's also worth noting that, for most images, it helps to reduce color noise artifacts by desaturating or converting the highpass layer into black and white.

[Ctein]

Dear Folks,

Which blending mode you use is a matter of personal taste. Overlay works, as does Hard Light, et cetera. They're all stronger than Soft Light. My working style is to start with a minimal effect and then increase it until I get the degree of sharpening I want. So starting with Soft Light and using a curves adjustment layer to increase the contrast of the high-pass layer suits my habits well. But one could just as easily start with a very strong blending effect and use a curves adjustment layer to reduce it to the desired level. Six of one, half a dozen of another.

High-pass filtering does not work the same as unsharp masking, Especially not with large radius filters. Large-radius unsharp masking magnifies the difference between every pixel and the average of the surrounding pixels. This is good for enhancing local contrast; it doesn't particularly select for edges. Used at a strong enough level to produce noticeable edge enhancement, it can badly distort other tones. In the case of the sample image above, a low level of unsharp masking is good for bringing out the tonality in the corona, but it does almost nothing to bring out the edges of the coronal plumes and streamers. High-pass filtering and unsharp masking are two different tools with two different purposes.

~ pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]
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[lol101]

To avoid enhancing noise when using high-pass sharpening, just run a Gaussian blur on the HP layer with a small radius (typically between 0.5 and 2 pixels) and "voilà": all details of the order of your high pass sharpening radius will be enhanced while the noise will be blurred out.