4.8. Sharpen (Unsharp Mask)

4.8.1. 概観

図17.44 シャープ化フィルターの適用例

シャープ化フィルターの適用例

元画像

シャープ化フィルターの適用例

シャープフィルター適用後


Out-of-focus photographs and most digitized images often need a sharpness correction. This is due to the digitizing process that has to divide a color continuum in points with slightly different colors. Elements smaller than the sampling frequency will be averaged into a uniform color. So sharp borders will be rendered a little blurred. The same phenomenon appears when printing color dots on paper.

The Sharpen filter (previously called Unsharp Mask) sharpens edges of the elements without increasing noise or blemish. It is the king of the sharpen filters.

[ヒント] ヒント

Some imaging devices like digital cameras or scanners offer to sharpen the created images for you. We strongly recommend you disable this means of sharpening and use GIMP filters instead. This way you regain the full control over the sharpening of your images.

To prevent color distortion while sharpening, you can Decompose your image to HSV and work only on Value. Go to ColorsComponentsDecompose.... Make sure the Decompose to Layers box is checked. Choose HSV and click OK. You will get a new grey-level image with three layers, one for Hue, one for Saturation, and one for Value. (Close the original image so you won't get confused). Select the Value layer and apply your sharpening to it. When you are done, with that same layer selected, reverse the process by using Compose. Go to ColorsComponentsCompose.... Again choose HSV and click OK. You will get back your original image except that it will have been sharpened in the Value component.

4.8.2. フィルターの呼び出し方

You can find this filter through FiltersEnhanceSharpen (Unsharp Mask)….

4.8.3. オプション

図17.45 シャープフィルターのオプション

「シャープ」フィルターのオプション

プレビュー

プレビュー オプションを有効にしておれば画像に実際にフィルターをかける前からダイアログ上で調節したとおりに即座に効果のようすが見て判るようになっています。

半径

The slider and input boxes (0.0-1500.0) allow you to set how many pixels on either side of an edge will be affected by sharpening. It is better to always sharpen an image at its final resolution.

This slider and input boxes (0.0-300.0) allow you to set the strength of sharpening.

しきい値

This slider and input boxes (0.0-1.0) allow you to set the minimum difference in pixel values that indicates an edge where sharpen must be applied. So you can protect areas of smooth tonal transition from sharpening, and avoid creation of blemishes in face, sky or water surface.

Clipping

The result of this filter can be larger than the original image. With the default Adjust option, the layer will be automatically resized as necessary when the filter is applied. With the Clip option the result will be clipped to the layer boundary.

4.8.4. アンシャープマスクが効くわけ

アンシャープマスクを用いて画像をシャープ化するというのも何だかおかしな話です。 説明しましょう。

Think of an image with a contrast in some place. The intensity curve of the pixels on a line going through this contrast will show an abrupt increase of intensity: like a stair if contrast is perfectly sharp (blue), like an S if there is some blur (yellow).

Now, we have an original image with some blur (black curve) we want to sharpen. We apply some more blur: the intensity variation will be more gradual (green curve).

Let us subtract the difference between blurredness intensity (green curve) and the intensity of the original image (black curve) to the intensity of the original image (black curve). We get the red curve, which is more abrupt: contrast and sharpness are increased. QED.

Unsharp mask was first used in silver photography. The photographer first creates a copy of the original negative by contact, on a film, placing a thin glass plate between both; that will produce a blurred copy because of light diffusion. Then both films are placed in a photo enlarger, to reproduce them on paper. The dark areas of the positive blurred film, opposed to the clear areas of the original negative will prevent light to go through and so will be subtracted from the light going through the original film.

ディジタル化写真では GIMP だとつぎのような工程を経ます。

  1. 画像を開いたら複製をとります。 画像複製

  2. 複製側でレイヤーのコピーをとります。 レイヤーレイヤーの複製 つぎに フィルターぼかしガウスぼかし... を初期設定の IIR オプションと半径が 5 の設定でコピーのレイヤーにかけます。

  3. In the layer dialog of the duplicated image, change Mode to Subtract, and in the right-click menu, select Merge down.

  4. こうしてできたレイヤーをレイヤーダイアログでクリックして元の画像のウィンドウ上までドラッグして放つと、 新しいレイヤーになります。

  5. この新たなレイヤーのモードをレイヤーダイアログで 加算 に設定します。

ほらこのとおり、 アンシャープマスクプラグインと同じ処理のできあがりです。

グラフ上で曲線のはじまりがくぼんでいますね。 ぼかし処理が重くなると、 このくぼみがとても深くなります。 結果的に減算が負の値を生み、 コントラストのある部分に沿って補色のすじができたり、 明るめの星雲を背景に見える星のまわりに黒い暈 (ハロー) ができるのです (黒目効果)。

図17.46 黒目効果

黒目効果