How does USM (UnSharp Mask) work, technically speaking?

AG
Posted By
Aaron_Good
Dec 30, 2003
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I understand the ways of using it properly, but how does Unsharp Mask work, technically speaking? Is there a wet/dry darkroom technique this filter is drawn from?
Thanks!
Aaron

How to Master Sharpening in Photoshop

Give your photos a professional finish with sharpening in Photoshop. Learn to enhance details, create contrast, and prepare your images for print, web, and social media.

JR
John_R_Nielsen
Dec 30, 2003
Yes. If you overlay a positive image with a negative one (or vice versa), it will reduce the contrast. If the overlay is blurred, it will have the effect of not reducing the contrast where there are hard edges in the original. Thus, edges will, in effect, have added contrast, and appear sharper.

This is a royal pain to do with film, especially without a pin-register system, since you have to align the original and the overlay EXACTLY, and pray they stay that way, as the combination is handled. Digital images are like having a built-in pin-register that doesn’t go out of alignment.
GA
George_Austin
Dec 30, 2003
Aaron,

I have always assumed that USM uses edge detection techniques,i.e., that it looks at the rate of change (gradient) of color values in each color channel and in orthogonal directions. Some prefer to call it high frequency filtering in that high spatial rates of change produce high temporal changes when "scanning" or "scrolling". This is simplistic, of course, but you can experiment on your own using Filter Factory with home-made criteria on allowable rates of change to qualify as an "edge" before invoking edge enhancement algorithms.

George
RP
Russell_Proulx
Dec 30, 2003
I think the original source of USM in Photoshop comes from prepress scanners which borrowed the trick from the old days of doing it manually.

Yes it’s a contact of a pos+neg which are separated by various thicknesses of diffusion material and this creates a un-sharp halo (Radius) where additional exposure is applied to the edges of areas where light meets dark. This artificially exaggerates edge contrast which is interpreted by the brain as being sharper. It’s only an illusion as no real detail is added.

Essentially it draws dark lines on the dark side and lighter lines on the light side of tonal transitions (dark shirt against a white wall). The thickness of the line is determined by the Radius setting, how apparent(strong) the lines are is controlled by the Amount setting, while the Threshold setting determines how many levels a tonal change there should be before the effect is applied.

Higher resolution images require a bigger Radius setting and amount is usually increased until the trick becomes apparent. It’s often different with different images. Threshold is raised to avoid unwanted ‘sharpening’ of noise or textures such as skin.

I’d LOVE to hear from an old prepress pro who could tell us how this was done with film separations which is after all what Photoshop is emulating. I think the current use of this technique described for fine art B/W printing is more complicated than what was done in a prepress shops cranking out dozens of images a day. I’m not sure if they printed a sandwich only or made a second exposure to pump up the edges. I suspect the latter technique.

BTW I credit Dan Margulis (Photoshop author extrordinaire) for helping me begin to understand Unsharp Masking 🙂

Russell
LH
Lawrence_Hudetz
Dec 30, 2003
Unsharp masks were also used for contrast control; shadows in negs, highlights in transparencies. They were very pale, ghost-like images that formed a kind of detailed dodging. The new Shadow/Highlight mimics this, with much more finesse. It’s worth the price of admission!

There also existed sharp masks as well. Color Range is the PS version.
GA
George_Austin
Dec 30, 2003
Good grief !! What an entirely different mechanism than what I assumed! And interesting.
RK
Rob_Keijzer
Dec 30, 2003
In a way, USM is related to Contrast Masking. (Dark room technique is the same), but try this in PS:

Take an image that is rather high in contrast and with darker lows than you’d like. Duplicate this on a new layer.
Desaturate this new layer.
Invert this layer.
In the layer palette select the blending to be "Overlay". Blur the dupe layer somewhat (Gaussian).
Select an opacity for this layer that looks good.

When you leave the opacity high the colours tend to look like a CMYK print where the Black plate is forgotten. In that case bring down the colour saturation down slightly.

I learned this from Michael Reichmann’s
<http://www.luminous-landscape.com/index.shtml>
Rob
RP
Russell_Proulx
Dec 30, 2003
I think it’s important to distinguish between Contrast Masking and Unsharp Masking (USM) which is only one of the many effects that can be achieved with photographic Contrast Masks. I would never use the term Unsharp Mask when referring to a Contrast Mask (ie: a Contrast Mask used to reduce the contrast of a transparency for Cibachrome printing). Type "Contrast Masking" into Google.

I don’t mean to be picky, just trying to keep the definitions straight 🙂

Russell
LH
Lawrence_Hudetz
Dec 30, 2003
Unsharp masking for contrast control goes way back, Russell. As I mentioned, both sharp and unsharp versions for contrast masks were used. The sharpening effects of USM led to it’s employment for sharpening over contrast, and you are right: the current discrimination is to call it contrast masking. But back when Columbus was a midshipman, (he graduated just ahead of me) the preferred contrast control by masking was called unsharp masking.
AG
Aaron_Good
Dec 30, 2003
Thanks for all of your intelligent responses!

I have seen that the USM filter doesn’t really "create detail," but indeed merely increases the contrast in a selective way. It does a great job of unsoftening my scans, but also brings out the grain a good bit.

Thanks again!
-Aaron
RP
Russell_Proulx
Dec 30, 2003
But back when Columbus was a midshipman, (he graduated just ahead of me) the preferred contrast control by masking was called unsharp masking.

One could also call it an ‘Out of focus mask’ or a ‘Fuzzy Mask’, but in my world (I was a Marron Carrel special effects guy for many years and have made thousands of masks) I’d never call a ‘contrast mask’ an ‘unsharp mask’. I’ve always considered USM to be part of the prepress world in which in means only one thing. Yes, in some shops they’ll call a cyan plate a ‘blue’ plate and magenta a ‘red’ plate’, but that does not mean it’s correct. I’d just be cautious about declaring it an industry accepted term that can be used in place of "Contrast Mask" (‘Contrast-Reducing Printing Mask’ to be exact). It’s confusing.

<http://www.rit.edu/~rckpph/faq/20.03.html>

Russell
RP
Russell_Proulx
Dec 30, 2003
But back when Columbus was a midshipman, (he graduated just ahead of me) the preferred contrast control by masking was called unsharp masking.

One could also call it an ‘Out of focus mask’ or a ‘Fuzzy Mask’, but in my world (I was a Marron Carrel special effects guy for many years and have made thousands of masks) I’d never call a ‘contrast mask’ an ‘unsharp mask’. I’ve always considered USM to be part of the prepress world in which in means only one thing. Yes, in some shops they’ll call a cyan plate a ‘blue’ plate and magenta a ‘red’ plate’, but that does not mean it’s correct. I’d just be cautious about declaring it an industry accepted term that can be used in place of "Contrast Mask" (‘Contrast-Reducing Printing Mask’ to be exact). It’s confusing.

<http://www.rit.edu/~rckpph/faq/20.03.html>

Russell
LH
Lawrence_Hudetz
Dec 30, 2003
So, how did you discriminate between a sharp and unsharp contrast mask?

Here’s a reference:
<http://www.largeformatphotography.info/unsharp/>
RP
Russell_Proulx
Dec 30, 2003
Here’s a reference

Like ‘Red’ being used instead of ‘Magenta’ the terminology used by your referenced writer is wrong (ok..it’s sloppy). Again.. Photoshop’s use of USM has little to do with contrast masking except that in non-prepress photographic darkroom work it is one of many things that can be done with contrast masking techinques.

Glad to see that you’re now calling it an unsharp "contrast mask" which is more precise <g>

Russell
LH
Lawrence_Hudetz
Dec 31, 2003
Words are dynamic items, especially in the English language. "Metamerism" is another one that now has a meaning other than it’s original. Unsharp mask is a description, where USM is now a name. The use in that article is not sloppy or wrong, only ancient. If I had kept my Kodak books on the subject, ca the 50’s, you would find the term referring to a type of contrast mask, and was used that way with no misunderstanding. The art of unsharp contrast masks was quite demanding, and people who made them well were in demand, especially printing from transparencies. I still have some that were made for my images. Interestingly, I scanned a transparency with it’s unsharp contrast mask and found it to be a detriment to a good scan.

The beat goes on. 🙂
JS
John_Slate
Dec 31, 2003
Interestingly, I scanned a transparency with it’s unsharp contrast mask and found it to be a detriment to a good scan.

If your output is a transparency that someone else will scan, you definitely don’t want USM in the file.

There was a time (late 80’s) when this was a fairly common practice, to have expensive digital retouching done, then distribute the image via LVT output.

Passe, to say the least.

How to Master Sharpening in Photoshop

Give your photos a professional finish with sharpening in Photoshop. Learn to enhance details, create contrast, and prepare your images for print, web, and social media.

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