Kapitel 7. Rita med GIMP
| Del II. Hur blir jag en GIMP-gud? |

## Kapitel 7. Rita med GIMP

## 1. Markeringen

När du arbetar i en bild vill du ofta bara påverka en del av denna. I GIMP gör du det genom att markera den delen. Varje bild har en markering som den associeras med. De flesta GIMP-operationerna arbetar bara på de markerade områdena i bilden.

Figur 7.1. Hur skulle du isolera trädet?

There are many, many situations where creating just the right selection is the key to getting the result you want, and often it is not easy to do. For example, in the above image, suppose I want to cut the tree out from its background, and paste it into a different image. To do this, I need to create a selection that contains the tree and nothing but the tree. It is difficult because the tree has a complex shape, and in several spots is hard to distinguish from the objects behind it.

Figur 7.2. Markering som visas som vanligt med en streckad linje.

Now here is a very important point, and it is crucial to understand this. Ordinarily when you create a selection, you see it as a dashed line enclosing a portion of the image. The common, not entirely accurate, idea you could get from this, is that the selection is a sort of container, with the selected parts of the image inside, and the unselected parts outside. Although this concept of selection is okay for many purposes, it is not entirely correct.

Actually the selection is implemented as a channel. In terms of its internal structure, it is identical to the red, green, blue, and alpha channels of an image. Thus, the selection has a value defined at each pixel of the image, ranging between 0 (unselected) and 255 (fully selected). The advantage of this approach is that it allows some pixels to be partially selected, by giving them intermediate values between 0 and 255. As you will see, there are many situations where it is desirable to have smooth transitions between selected and unselected regions.

Vad är då den streckade linjen som visas när du skapar en markering?

The dashed line is a contour line, dividing areas that are more than half selected from areas that are less than half selected.

Figur 7.3. Samma markering i snabbmaskläge.

While looking at the dashed line that represents the selection, always remember that the line only tells part of the story. If you want to see the selection in full detail, the easiest way is to click the QuickMask button in the lower left corner of the image window. This causes the selection to be shown as a translucent overlay atop the image. Selected areas are unaffected; unselected areas are reddened. The more completely selected an area is, the less red it appears.

Many operations work differently in QuickMask mode, as mentioned in the QuickMask overview. Use the QuickMask button in the lower left corner of the image window to toggle QuickMask mode on and off.

Figur 7.4. Samma markering i snabbmaskläge efter fjädring.

### 1.1. Fjädring

With the default settings, the basic selection tools, such as the Rectangle Select tool, create sharp selections. Pixels inside the dashed line are fully selected, and pixels outside completely unselected. You can verify this by toggling QuickMask: you see a clear rectangle with sharp edges, surrounded by uniform red. Use the ”Feather edges” checkbox in the Tool Options to toggle between graduated selections and sharp selections. The feather radius, which you can adjust, determines the distance over which the transition occurs.

If you are following along, try this with the Rectangle Select tool, and then toggle QuickMask. You will see that the clear rectangle has a fuzzy edge.

Feathering is particularly useful when you are cutting and pasting, so that the pasted object blends smoothly and unobtrusively with its surroundings.

It is possible to feather a selection at any time, even if it was originally created as a sharp selection. Use Select → Feather from the image menu to open the Feather Selection dialog. Set the feather radius and click OK. Use Select → Sharpen to do the opposite—sharpen a graduated selection into an all-or-nothing selection.

| Notera
For technically oriented readers: feathering works by applying a Gaussian blur to the selection channel, with the specified blurring radius.

### 1.2. Göra en markering delvis transparent

You can set layer opacity, but you cannot do that directly for a selection. It is quite useful to make the image of a glass transparent. Use the following methods to set the layer opacity:

- For simple selections, use the Eraser tool with the desired opacity.

- For complex selections: use Select → Float to create a floating selection. This creates a new layer with the selection called ”Floating Selection” (Avsnitt 4.5, ”Float”). Set the opacity slider in the Layer Dialog to the desired opacity. Then anchor the selection: outside the selection, the mouse pointer includes an anchor. When you click while the mouse pointer includes the anchor, the floating selection disappears from the Layer Dialog and the selection is at the right place and partially transparent (anchoring works this way only if a selection tool is activated : you can also use the Anchor Layer command in the context menu by right clicking on the selected layer in the layer dialog).

  And, if you use this function frequently: Ctrl-C to copy the selection, Ctrl-V to paste the clipboard as a floating selection, and Layer → New Layer to turn the selection into a new layer. You can adjust the opacity before, or after creating the new layer.

- Another way: use Layer → Mask → Add Layer Mask to add a layer mask to the layer with the selection, initializing it with the selection. Then use a brush with the desired opacity to paint the selection with black, i.e. paint it with transparency. Then Layer/Mask/Apply Layer Mask. See Avsnitt 2.1.4, ”Lagermaskar”.

- To make the solid background of an image transparent, add an Alpha channel, and use the Magic Wand to select the background. Then, use the Color Picker tool to select the background color, which becomes the foreground color in Toolbox. Use the Bucket Fill tool with the selected color. Set the Bucket Fill mode to ”Color Erase”, which erases pixels with the selected color; other pixels are partially erased and their color is changed.

  Den enklaste metoden är att använda Redigera → Töm vilket ger full transparens till en markering.

2.3. Spara bilder med transparens |  | 2. Skapa och använda markeringar

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