4.8. Sharpen (Unsharp Mask)
| 4. Forbetringsfiltra |

### 4.8. Sharpen (Unsharp Mask)

#### 4.8.1. Oversyn

Figur 17.44. Eksempel på bruk av filteret

Originalbiletet

Eksempel på bruk av filteret «Gjer skarpare»

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.

| Tips
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 Colors → Components → Decompose.... Make sure the Decompose to Layers box is checked. Choose HSV and click OK. You will get a new gray-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 Colors → Components → Compose.... 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. Aktivering

You can find this filter through Filters → Enhance → Sharpen (Unsharp Mask)….

#### 4.8.3. Innstillingar

Figur 17.45. Innstillingane for filteret «Gjer skarpare»

Presets, «Input Type», Clipping, Blending Options, Preview, Split view

  | Notat
  These options are described in Del 2, «Common Features».

Radius

  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.

Mengde

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

Terskel

  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.

#### 4.8.4. Korleis arbeider uskarpmaska?

Det kan høyrast underleg ut å bruke ei uskarpmaske for å gjere biletet skarpare. Her er forklaringa på den merkelege ordbruken:

Tenk deg eit bilete med ein kontrast ein eller annan staden. Intensitetskurva for ei linje med pikslar som krysser denne kontrasten viser eit tydeleg avbrot i intensiteten i kryssingspunktet. Dersom kontrasten er skarp, vil kurva ha form som eit trappesteg (blå), som ein S dersom det er litt utglatting (gul).

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.

I GIMP blir denne mørkeromsprosessen simulert i følgjande steg:

1. Opna biletet og lag ein kopi ved å gå inn på biletmenyen på Bilete → Kopier

2. I kopien opnar du ein kopi av laget ved å gå inn på menyen Lag → Kopier laget. Sjå etter at dette laget er aktivt og hent deretter farm filtermenyen og vel Sløring → Gaussisk sløring. Køyr dette filteret på biletlaget med avmerking for IIR og Radius sett til 5.

3. I lagdialogen for biletkopien endrar du modus til «Trekk frå», i menyen som kjem fram når du høgreklikkar, vel du «Flett saman nedover».

4. Klikk og dra det laget du enda opp med over i originalbiletet. Det vil bli lagt til der som eit nytt biletlag.

5. Endra modus for dette laget til «Addisjon».

Voilà. Kanskje kjekt å vite at filteret Uskarpmaske gjer akkurat det same.

Heilt i byrjinga av kurva er det ein liten dipp. Dersom sløringa er svært sterk, kan det hende at resultatet av subtraksjonen blir negativt. Dette kan tilfelle gi ei stripe i kontrastfargen langs kontrastkanten.

Figur 17.46. Fargefeil ved for mykje skjerping

4.7. Symmetric Nearest Neighbor |  | 4.9. Støvfjernar

Report a bug in GIMP Report a documentation error