Sharpening: Why is background unaffected?

Re: Real-World Example in Tutorial "Sharpening: Unsharp Mask"

First off, I must confess that I've never used Unsharp Mask before :-) I just started to study the basics of it...

My understanding (may be wrong) is that when sharpening is applied to an image, the entire image will be affected. However, this doesn't seem to be the case when I look closely at the Real-World Example image (the old man) in the tutorial. The blurry background is not affected and only the subject is sharpened. Is this a unique phenomenon in Unsharp Mask? What about other sharpening algorithm?

The out of focus background does not appear to be sharpened (relative to the sharp foreground) because an unsharp mask effectively acts as a high-pass filter. In other words, it is strongest where texture in the image changes very rapidly over a short distance, such as the fabric in the man's tweed coat or the sharp edges of his profile. Exactly which type of texture becomes emphasized is controlled by the USM radius parameter. A larger radius would mean that texture which changes over a larger distance (in pixels) will be emphasized more.

In the old man example, one also can see exactly which areas are most affected by the unsharp mask (lightest regions) by looking at the b/w mask itself:

http://cdn.cambridgeincolour.com/ima...k_man-orig.jpg http://cdn.cambridgeincolour.com/ima...sk_man-usm.jpg

So the entire image *is* effected-- it's just that some regions are impacted much more than others due to the mask effect. If we made the radius in the old man example much larger (say 10 pixels in the size shown on the web), then you would start to notice that the unsharp mask would influence the background texture a whole lot more (and the fabric in the man's shirt a whole lot less).

If you find that the radius dosn't quite select what you want and it's still grabbing some (or too much) of the background -- run the unsharp mask on a new layer, then create a layer mask and "paint-in" the sharpness.

Just a couple of other "for your info" bits ...

The way Photoshop does unsharp masking is it creates a hidden copy of the image and blurs it - from there it's a relatively simple matter to look for differences between the original copy and the blurred copy. Sharpening is all about edge detection; so if there's a difference between the blurred copy and the original then it essentially means that there must be an edge - but if there is no change then it must be a surface. Pretty cool technique eh?

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If you find that the radius dosn't quite select what you want and it's still grabbing some (or too much) of the background -- run the unsharp mask on a new layer, then create a layer mask and "paint-in" the sharpness.

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An easier way is to just sharpen everything and then use the history brush to roll back the bits that don't need as much sharpening :)

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Originally Posted by Colin Southern

An easier way is to just sharpen everything and then use the history brush to roll back the bits that don't need as much sharpening :)

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May be easier but it's a destructive edit ;)

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Originally Posted by KentDub

May be easier but it's a destructive edit ;)

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If it's a worry you can just duplicate the layer before hand :)

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Originally Posted by Colin Southern

An easier way is to just sharpen everything and then use the history brush to roll back the bits that don't need as much sharpening :)

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In Lightroom there is a very neat way of controlling what is essentially unsharp mask. You can control the amount of sharpening, the radius, the detail, and what is masked from sharpening. By doing the adjustments in 1:1 and holding down the Alt key, you can see exactly what is happening.

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Originally Posted by Bill44

In Lightroom there is a very neat way of controlling what is essentially unsharp mask. You can control the amount of sharpening, the radius, the detail, and what is masked from sharpening. By doing the adjustments in 1:1 and holding down the Alt key, you can see exactly what is happening.

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Pretty much the same as in Photoshop Bill.

With gimp, i use a layer mask set to gray scale and use edge detect. With edge detect you can adjust the intensity of the edge that is detected. I usually select around 3.0, unless the edges show up really bold.

I also use the curves tool to make the edges brighter where i want and unwanted things black(steepen the curve).................if there is any noise(shows up as white dots in the background), i just errase them with a black paint brush. Most of the time the curves tool takes care of it.

You have total control as to what gets sharpened and what doesn't. Things like eyes, you paint in with white at about 20% opacity, and anything else you want sharpened.

After that i apply a gaussen blur of about 10 and then click on the image and sharpen with an unsharp mask radius 1.0 and about 3.5 for the amount. After that adjust with opacity slider. I would imagine photoshop has something similar.

The black image to the right of the man, is very similar to the mask i make with edge detect. Notice the background is black............it won't get sharpened. Only the things that are white. What is nice about my way, is i can control the intensity and blur of the mask and make it do what 'I' want.

Yep, with you on that Steve. The beauty is that, given it seemed complex when first tried, it really does get just like a habit and you can do it pretty quickly.

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Originally Posted by Donald

Yep, with you on that Steve. The beauty is that, given it seemed complex when first tried, it really does get just like a habit and you can do it pretty quickly.

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Very quick, unless you have to remove a bunch of noise. Then it can take a couple of min.