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ソース:Unsharp masking - Wikipedia, the free encyclopedia



Unsharp masking

Unsharp masking (USM) is an image manipulation technique, often available in digital image processing software.

The "unsharp" of the name derives from the fact that the technique uses a blurred, or "unsharp," positive image to create a "mask" of the original image.^1 The unsharped mask is then combined with the negative image, creating an image that is less blurry than the original. The resulting image, although clearer, probably loses accuracy with respect to the image's subject. In the context of signal-processing, an unsharp mask is generally a linear or nonlinear filter that amplifies high-frequency components.





  1. Photographic unsharp masking
  2. Digital unsharp masking 2.1 Local contrast enhancement
  3. Comparison with deconvolution
  4. See also
  5. References
  6. External links


  1. 写真的なアンシャープマスキング
  2. ディジタルなアンシャープマスキング 2.1 ローカル・コントラスト・エンハンスメント
  3. 逆畳み込みとの比較 (以下では未訳
  4. 次も参照せよ(以下では未訳
  5. 参考文献
  6. 外部リンク(以下では未訳

Photographic unsharp masking

The technique was first used in Germany during the 1930s as a way of increasing the acutance, or apparent resolution, of photographic images.[citation needed]

For the photographic process, a large-format glass plate negative is contact-copied onto a low contrast film or plate to create a positive image. However, the positive copy is made with the copy material in contact with the back of the original, rather than emulsion-to-emulsion, so it is blurred. After processing this blurred positive is replaced in contact with the back of the original negative. When light is passed through both negative and in-register positive (in an enlarger for example), the positive partially cancels some of the information in the negative.

Because the positive has been blurred intentionally, only the low frequency (blurred) information is cancelled. In addition, the mask effectively reduces 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 more acute than one made without the unsharp mask: its acutance is increased.

In the photographic procedure, the amount of blurring can be controlled by changing the "softness" or "hardness" (from point source 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.

For traditional photography, unsharp masking is usually used on monochrome materials; special panchromatic soft-working black and white films have been available for masking photographic colour transparencies. This has been especially useful to control the density range of a transparency intended for photomechanical reproduction.







Digital unsharp masking

The same differencing principle is used in the unsharp-masking tool in many digital-imaging software packages, such as Adobe Photoshop and GIMP.^2 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 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 is easy to create unwanted and conspicuous edge effects, or increase image noise. However, these effects can be used creatively, especially if a single channel of an RGB or 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 three settings control digital unsharp masking:

  • Amount is listed as a percentage, and controls the magnitude of each 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 minimum 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 as to good starting values for these parameters,^3^4 and the meaning may differ between implementations. Generally a radius of 0.5 to 2 pixels and an amount of 50–150% is a reasonable start.

It is also possible to implement USM manually, by creating a separate layer to act as the mask;^2 this can be used to help understand how USM works, or for fine customization.

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^5) which yields increased local contrast, a technique termed local contrast enhancement.[5][6] 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.


同じ差分化原理が、 Adobe PhotoshopGIMP ^2などの多くのディジタル画像ソフトウェアパッケージにおいて利用されている。ソフトウェアはオリジナルの複製画像にガウスぼかしを適用し、次にそれをオリジナルと比較する。もし差分が、ユーザーの指定した閾値より大きいとき、これらの画像は(結果として)減算される。閾値の操作は、たとえば写真的な粒子などの画像の小さな詳細が先鋭化されてしまうのを抑制するため、あるサイズの閾値をお互いに超えて異なっている画像の要素を先鋭化するように強制する。



  • は百分率によって示され、各超過量(エッジの境界線がどのていどより暗くなるか、どのていどより明るくなるか)の大きさを操作する。この値はエッジに対してどれだけのコントラストが付けくわえられるかと思ってもよい。この値はエッジの縁の幅には影響を与えない。
  • 半径は効果の反映されるエッジのサイズやエッジの縁がどれぐらいの幅になるかに影響を与えるので、半径の値が小さいほど詳細さの尺度も小さくなる。より大きい半径の値はエッジにハロー、すなわち物体の周囲にある検知可能なほのかな光の輪郭を引きおこす可能性がある。精度の高い詳細さにはより小さい半径の値が必要である。半径と量は相互に影響をあたえる。すなわち一方の値を下げることでもう一方の効果をさらに増すことができる。
  • 閾値は、フィルターが適用される前に、先鋭化される明るさの変化の最小値や、隣接した色調の値がどれだけ離れているべきかを操作する。このなにも適用しない値を決めることは、滑らかな領域で小さな斑点になってしまうのを防ぐのに重要である。閾値の設定は、繊細なエッジには手をつけずにさらにエッジを鮮明化させるためにも用いられる。閾値を小さくするほど、効果が除外される範囲が減るので画像は先鋭化する。閾値を高くするほど、コントラストの低い領域が除外される。


USM は、マスクとしてふるまう分離されたレイヤーを作ることによって、手動でも実装することができる^2。それによって USM がどのようにはたらいているかを理解する助けにもなるし、またより効果的なカスタマイズにも役立つ。


アンシャープマスキングは大きな半径と小さな量の組み合わせ(たとえば半径は30から100ピクセル、量は5から20パーセント^5)でも用いることができ、局所的なコントラストを増すことができる。この技術はローカル・コントラスト・エンハンスメントと呼ばれる^5^6。先鋭度と(局所的な)コントラストは両者ともに値と値の差分や傾斜を増すこと––非常に小さな尺度(高周波)の差分を参照する先鋭さと、より大きな尺度(低周波)の差分を参照するコントラスト––が形をとったものなので、 USM はこのいずれの値も増すことができるのである。より強力な、色調を改善するための技術は、トーンマッピングと呼ばれている。