Unsharp masking
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Unsharp masking is an image manipulation technique now familiar to many users of digital image processing software, but it seems to have been first used in Germany in the 1930s as a way of increasing the apparent sharpness of photographic images.
In the original process, a large-format glass plate negative was contact-copied on to a low contrast film or plate to create a positive. However, the positive copy was made with the copy material in contact with the BACK of the original, rather than emulsion-to-emulsion, so it was blurred. After processing this blurred positive was replaced in contact with the back of original negative. When light was passed through both negative and in-register positive (in an enlarger for example), the positive partially cancelled some of the information in the negative.
Because the positive was intentionally blurred, only the low frequency (blurred) information was cancelled. In addition, the mask effectively reduced the dynamic range of the original negative. Thus, if the resulting enlarged image is recorded on contrasty photographic paper, the partial cancellation emphasizes the high frequency (fine detail) information in the original, without loss of highlight or shadow detail. The resulting print appears sharper than one made without the unsharp mask.
In the photographic procedure the amount of blurring can be controlled by changing the softness or hardness (from point light to fully diffuse) of the light source used for the initial unsharp mask exposure, while the strength of the effect can be controlled by changing the contrast and density (i.e. exposure and development) of the unsharp mask.
In traditional photography unsharp masking was usually used on monochrome materials, however special panchromatic soft-working black and white films were available for masking photographic colour transparencies. This was especially useful to control the dynamic (density) range of a transparency intended for photomechanical reproduction.
The same differencing principle is used in the unsharp masking tool in many digital imaging software packages, e.g. Adobe Photoshop or GIMP. 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. The strength of the effect is controlled by the 'Amount' slider while the spatial extent of the blur is controlled (in Photoshop) with the 'Radius' slider. The 'Threshold' control constrains sharpening to image elements that differ from each other above a certain size threshold, so that sharpening of small image details such as photographic grain can be suppressed.
Digital unsharp masking is a flexible and powerful way to increase sharpness, especially in scanned images, however it should be used with care since it is easy to create unwanted and conspicuous edge effects. On the other hand these effects can be used creatively, especially if one channel of images in RGB or Lab colour space is selected for unsharp masking.
There should be some explanation why the name is "unsharp" while it actually sharpens the image.
My take is like this:
The name is "unsharp" because it actually uses the blurred positive to create a "mask" of the original image. The unsharped image is then combined with the negative, creating the illusion that the resulting image is sharper than the original.
External links
- Interactive Example of Unsharp Mask (http://microscope.fsu.edu/primer/java/digitalimaging/processing/unsharpmask/)
- Sharpening Using An Unsharp Mask (http://www.cambridgeincolour.com/tutorials/unsharp-mask.htm): how it works, a biological motivation and how to minimize halo artifacts