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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?

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    It seems from your comments that you actually have an out of focus problem. You might want to update your question. Now that is a more interesting question. +1 for that. – joojaa Aug 28 '15 at 19:40
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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

  • wow, I've been using Unsharp Mask for years, never really wondering what it did exactly. Quite interesting @ACEkin – PieBie Aug 28 '15 at 15:44
  • Actually that is not what the treshold adjusts – joojaa Aug 28 '15 at 17:19
  • Well then, what does the threshold adjusts? It determines the difference between the "light" and the "dark" edge where the sharpening effect will be applied. – user45605 Aug 28 '15 at 17:23
  • Firstof the computer has no knowlege of what the edge is. Ot just knows that there is a local contrast. Theshold is simply just a teshold of this contrast In essence your saying that i dont consider it to be an edge unless it has a local contrast of x. Else eliminate the effect. – joojaa Aug 28 '15 at 17:28
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    @Rhei no you should use smart sharpen, it actually kills of the type of blur a camera makes. The kind of trick high pass uses can not really counter blur. Though you can not expect miracles as the deconvolving filter of smart sharpen will also boost noise. But it works ok for 2 px blurs. – joojaa Aug 28 '15 at 19:34
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Generally, low-pass filters blur, high-pass filters sharpen.

Unsharp Mask is BOTH a high- and low-pass filter.

So you:

  • Make a low-pass filter (radius parameter) - Gaussian blur - result in a low-frequency image

  • subtract this from original image which results in a high-pass image this is the unsharp mask

  • Make a ramped contrast image of original (increased contrast is a sort of high-pass operation)

  • Compare luminosity of the above two images (mask vs contrasty): 100%? then use contrasty pixel; 0% use original pixel; 0 < luminosity < 100 ? blend

To sum up: the sharpening is a high-pass operation blended with the original using a weighting process influenced by a low-pass filter.

  • I am in agreement with this post, which answers the thing even more technically specific than i do. Because of this i can not help to wonder if we should actually leave unsharp mask behind for more appropriate methods now that processing power is not so big a problem. – joojaa Aug 28 '15 at 19:28
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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.

  • Actually, the unsharp mask is the original image and a blurred version of it. That is why it is called "unsharp" mask. It was used in the film days, combining a negative or a slide with a slightly blurry version of it produced an "apparently" sharper image, thus the name. – user45605 Aug 28 '15 at 15:07
  • @ACEkin yes unsharp is the original but you add a "sharp mask" but the extra sharp has been omitted from the name. Incidentally highpass is original picture minus a gaussian blurred version. Offst by 128 because photoshop can not handle negatve values. – joojaa Aug 28 '15 at 17:16
  • I am not talking about the name. The unsharp mask is NOT a high pass filter, that was my point. In Photoshop you still add an "Unsharp" Mask Filter to sharpen the images, a tip of the hat to the old times. – user45605 Aug 28 '15 at 17:22
  • @ACEkin Yes it is. I can build the exact same effect with high pass oveelay and treshold. – joojaa Aug 28 '15 at 17:25
  • their workings are different, their uses are different, you may prefer one over the other but that does not make them "the same". Where is the threshold adjustment on the High Pass Filter? – user45605 Aug 28 '15 at 17:27
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Let me tell you a secret, Photoshop is doing almost the same in those cases.

Let me explain.
Classic sharpening (Using Linear Filters) is as following:

  1. Take an Image.
  2. Apply High Pass Filter on it.
  3. Add the result (Or some kind of adding operation) of 2 to the original image of 1.

More than that, in Photoshop the High Pass Filter used in Unsharp Mask is the exact one in the High Pass filter.
Actually, in Photoshop, both are created as following - Photoshop's High Pass:

  1. Take an Image.
  2. Blur it using Gaussian Blur (Which is actually Box Blur applied few times).
  3. Subtract the result of 2 from 1 and put the result as High Pass Filter result.

Now, the question is, given an High Pass of an image, how do you want to use it?
If you just want to add it, then Unsharp Mask is your friend.
If you want different application (Like Multiplication and addition, etc...) use High Pass Filter + Calculation Tool / Apply Image / Blending Mode.

If you really want a modern way to sharpen images - Use Multi Scale / Multi Frequency Sharpening.
Those are superior to any of those.

Just Google Multi Scale Sharpening Photoshop.

Another great approach to deal with the weaknesses of High Pass based Sharpening (USM included) is given by DoubleUSM.

Davide Barranca is an expert on this and he has amazing and hilarious video on the subject - Double USM 2 Panel for Adobe Photoshop.
In summary, he take care of Dark and Bright Halos in an independent approach, tackling each on its own.

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