Advertisement


Photoshop: high pass filter or sharpening filter?


Question

I was wondering, what is the difference between the high-pass filter and the unsharpen mask one? I am used to use the first to improve the quality of the image (to make it less dreamy and more realistic, especially when doing photomanipulations). I tired using also the second one, but the only thing I have noticed is that in this way I have to work with more paramenters, but the result at the end will be the same (using the correct set of parameters), am I right?

2015/08/28
1
3
8/28/2015 11:08:00 AM

Accepted Answer

As I wrote on the comment, unsharp filter combines a blurry (unsharp) version of the image with the original, creating an apparently sharper version. High Pass filter is an algorithm which passes the high-frequency detail while blocking the low frequency areas. When applied to a copied layer of the image it results in an image of middle gray, lighter and darker areas along the high frequency edges. The blend mode of this layer is changed to one of Overlay or Soft Light to apply the crispness to the original. Unsharp mask increases the contrast along the edges by making light side lighter and dark side darker.

Whereas in High Pass filter, there is only one parameter, radius, in the Unsharp Mask there are three, radius, amount, and threshold. The first one determines the number pixels along the edge to apply the contrast enhancement, the amount determines how much to increase the contrast, and threshold controls how different the tones along an edge has to be for the filter to apply.

Some images and approaches may benefit from high pass filter, but is it generally augmented by an application of some kind of unsharp filter for the output purpose, print or screen. This is a very long subject. If you care to read more, follow the link below: http://www.keptlight.com/index.php?s=sharpening

2015/08/28
5
8/28/2015 3:17:00 PM

Unsharp mask* is a highpass filtering technique. To be more precise Unsharp mask is original image plus a highpass filter. You can use a highpass layer to accomplish the same thing, yes.

You can also use highpass filtering for other things, such as frequency separation, masking of noise, fog removal etc... So unsharp mask is just one implementation of highpass filtering.

* if we wanted to be correct we should call it unsharp sharp mask. But that kimd of name sounds tedious so we do not call it that.

A more elborate explanation

High pass filter in Photohop is implemented via the same low cut** filtering as Unsharp mask. Thus one can say that they are in effect effectively the same operation family. The high pass is a bit optimized for Photoshop but level workflow so the effect is offset and you do not end up with negative values. And operating via internals of blur certain steps can be omitted.

There is a old technique used in photolab that indeed was a single unsharp mask. However due to mathematical decomposition of this you end up with more or less the same thing as copy of image screened with what Photoshop calls high pass.

Unlike the photolab version though the computerized unsharp mask filter has a clear sharpening stage. The photolab version also may have this stage but its more implied. Therefore the naming is slightly misleading, but works well for people who come from a older workflow.

This can be easily verified by building the layers up in photoshop from a blur. If one neglects the small errors that comes from quantisation and rounding in extra operations.

** Low cut is same thing as high pass. At least theoretically. In some cases implementation details may vary a bit for this to be untrue, the aim is the same. In this case the difference is nonexistent.

2015/08/29