Editing Unsharp masking (section)

== Digital unsharp masking ==
[[File:Unsharped eye.jpg|thumb|250px|Source image (top),<br />sharpened image (middle),<br />highly sharpened image (bottom)]]
The same differencing principle is used in the unsharp-masking tool in many digital-imaging software packages, such as [[Adobe Photoshop]] and [[GIMP]].<ref name="gimp">[https://docs.gimp.org/2.10/en/gimp-filter-unsharp-mask.html 4.9. Unsharp Mask], esp.
4.9.4. How does an unsharp mask work?, Gimp documentation.</ref> The software applies a [[Gaussian blur]] to a copy of the original image and then compares it to the original. If the difference is greater than a user-specified threshold setting, the images are (in effect) subtracted. 

Digital unsharp masking is a flexible and powerful way to increase sharpness, especially in [[scanned images]]. Unfortunately, it may create unwanted conspicuous edge effects or increase [[image noise]]. However, these effects can be used creatively, especially if a single [[Channel (digital image)|channel]] of an [[RGB color model|RGB]] or [[CIELAB|Lab]] image is sharpened. Undesired effects can be reduced by using a mask—particularly one created by [[edge detection]]—to only apply sharpening to desired regions, sometimes termed "smart sharpen".

Typically, digital unsharp masking is controlled via the amount, radius and threshold:
* Amount is listed as a percentage and controls the magnitude of each [[overshoot (signal)|overshoot]] (how much darker and how much lighter the edge borders become). This can also be thought of as how much contrast is added at the edges. It does not affect the width of the edge rims.
* Radius affects the size of the edges to be enhanced or how wide the edge rims become, so a smaller radius enhances smaller-scale detail. Higher radius values can cause halos at the edges, a detectable faint light rim around objects. Fine detail needs a smaller radius. Radius and amount interact; reducing one allows more of the other.
* Threshold controls the minimal brightness change that will be sharpened or how far apart adjacent tonal values have to be before the filter does anything. This lack of action is important to prevent smooth areas from becoming speckled. The threshold setting can be used to sharpen more pronounced edges, while leaving subtler edges untouched. Low values should sharpen more because fewer areas are excluded. Higher threshold values exclude areas of lower contrast.

Various recommendations exist for starting values of these parameters,<ref>[http://www.cambridgeincolour.com/techniques/image-sharpening.htm Guide to Image Sharpening], Cambridge in Color.</ref> and the meaning may differ between implementations. Generally a radius of 0.5 to 2 pixels and an amount of 50–150% is recommended.

It is also possible to implement USM manually, by creating a separate layer to act as the mask;<ref name="gimp"/> this can be used to help understand how USM works or for fine customization.

The typical blending formula for unsharp masking is

{{block indent | em = 1.5 | text = sharpened = original + (original − blurred) × amount.}}

=== Local contrast enhancement ===
Unsharp masking may also be used with a large radius and a small amount (such as 30–100 pixel radius and 5–20% amount<ref name="ciclce">[http://www.cambridgeincolour.com/techniques/local-contrast-enhancement.htm Local Contrast Enhancement], Cambridge in Color.</ref>), which yields increased local contrast, a technique termed ''local contrast enhancement''.<ref name="ciclce" /><ref>[http://www.luminous-landscape.com/tutorials/contrast-enhancement.shtml Understanding Local Contrast Enhancement], The Luminous Landscape.</ref> USM can increase either sharpness or (local) contrast because these are both forms of increasing differences between values, increasing slope—sharpness referring to very small-scale (high-frequency) differences, and contrast referring to larger-scale (low-frequency) differences. More powerful techniques for improving tonality are referred to as [[tone mapping]].