## Luku 8. Kuvien yhdistäminen

## 1. Johdanto tasoihin

You can think of layers as a stack of slides. Using layers, you can construct an image of several conceptual parts, each of which can be manipulated without affecting any other part of the image. Layers are stacked on top of each other. The bottom layer is the background of the image, and the components in the foreground of the image come above it.

Kuva 8.1. An image with layers

Layers of the image

Resulting image

There is no limit to the number of layers an image can have, only the amount of memory available on the system. It is not uncommon for advanced users to work with images containing dozens of layers. You can group layers to make your work easier, and there are many commands to handle layers.

The organization of layers in an image is shown in the Layers dialog. How it works is described in detail in the Layers Dialog section, but we will touch some aspects of it here, in relation to the layer properties that they display.

Each open image has at any time a single active drawable. A ”drawable” is a GIMP concept that includes layers, but also several other items, such as channels, layer masks, and the selection mask. Basically, a ”drawable” is anything that can be drawn on with painting tools. If a layer is currently active, it is shown highlighted in the Layers dialog, and its name is shown in the status area of the image window. If not, you can activate it by clicking on it. If none of the layers are highlighted, it means the active drawable is something else than a layer.

In the menu bar, you can find a menu called Layer, containing a number of commands that affect the active layer of the image. The same menu can be accessed by right-clicking in the Layers dialog.

### 1.1. Taso-ominaisuudet

Jokaisella tasolla on monia tärkeitä ominaisuuksia:

Nimi

  Every layer has a name. This is assigned automatically when the layer is created, but you can change it. You can change the name of a layer either by double-clicking on it in the Layers dialog, or by right-clicking there, and then selecting the top entry in the menu that appears, Edit Layer Attributes.

Alpha kanavan läsnäolo tai sen puute

  An alpha channel encodes information about how transparent a layer is at each pixel. It is visible in the Channels dialog: white is complete opacity, black is complete transparency and gray levels are partial transparencies.

  The background layer is special. If you have just created a new image, it only has one layer, which is the background layer. If the image has been created with an opaque Fill type, this one layer has no Alpha channel. To get a background layer with transparency, either create your new image with a transparent Fill type, or you use the Add an Alpha Channel command.

  If you add a new layer, even with an opaque Fill type, an Alpha channel is automatically added to the layer.

  Every layer other than the bottom layer of an image automatically has an Alpha channel, but you can't see a grayscale representation of the alpha values. See Alpha in Glossary for more information.

  Esimerkki Alpha kanavasta.

  Kuva 8.2. Alpha kanava esimerkki: Tavallinen kuva

  Original image.

  The image has three layers painted with pure 100% opaque Red, Green, and Blue. The current layer is the red one.

  In the Channels dialog, the added Alpha channel is white because the image is not transparent since there is a 100% opaque (red) layer. As the current (red) layer is 100% opaque, there is no green and no blue so their corresponding channels are black.

  Kuva 8.3. Alpha kanava esimerkki: Yksi läpinäkyvä taso

  The left part of the first layer has been made transparent (via Kohta 2.2, ”Rectangle Selection”, then Edit → Clear).

  The second layer, green, is visible.

  The Alpha channel is still white, since there is an opaque layer in this part of the image.

  Kuva 8.4. Alpha kanava esimerkki: Kaksi läpinäkyvää tasoa

  The left part of the second layer has been made transparent.

  The third layer, blue, is visible through the first and second layers.

  The Alpha channel is still white, since there is an opaque layer in this part of the image.

  Kuva 8.5. Alpha kanava esimerkki: Kolme läpinäkyvää tasoa

  The left part of the third layer has been cleared. In this case, the Clear command works like the Eraser and uses the Background color of Toolbox.

  The left part of the third layer is opaque, because the background layer has no Alpha channel.

  The Alpha channel is still white.

  Kuva 8.6. Alpha kanava esimerkki: Taustaan lisätty Alpha kanava

  We used the Layer → Transparency → Add Alpha Channel command on the (blue) background layer. The left part of the image has become fully transparent and has the color of the page on which the image is shown.

  The left part of the blue layer has become transparent.

  The left part of the Alpha Channel thumbnail is black (transparent) in the Channels dialog.

Tason tyyppi

  The layer type is determined by the image type (see previous section), and the presence or absence of an alpha channel. These are the possible layer types:

  - RGB

  - RGBA

  - Harmaa

  - HarmaaA

  - Indeksoitu

  - IndeksoituA

  The main reason this matters is that some filters (in the Filters menu) only accept a subset of layer types, and appear disabled in the menu if the active layer does not have a supported type. Often you can rectify this either by changing the mode of the image, or by adding or removing an alpha channel.

Näkyvyys

  Kuvasta on mahdollista poistaa taso tuhoamatta sittä napsauttamalla symbolia Tasot dialogista. Tätä kutsutaan tason ”näkyvyyden vaihtamiseksi”. Useimmat kuvan toiminnot käsittelevät pois päältä olevia tasoja, niin kuin niitä ei olisi olemassa. Kun työskentelet kuvien kanssa, joissa on useita tasoja, joissa on vaihtelevasti läpinäkymättömyyttä, saat usein paremmin selvää sen tason sisällöstä, jota haluat työstää piilottamalla osan muista tasoista.

  | Vihje
  Jos Shift-napsautat silmä symbolia saat piilotettua kaikki tasot paitsi sen, jota napsautit katoamaan.

Active layer

  Usually, you activate a layer, to work on it, clicking it in the layer list. When you have a lot of layers, finding which layer an element of the image belongs to is not easy: then, press Alt and click with Mouse wheel on this element to activate its layer. The available layers will be looped through (starting from the upper one) while the Alt is held and the picked layer will be temporarily displayed in the status bar.

Layer Lock Settings

  If you click to the right of the eye icon, you can select the lock settings for the layer.

Layer Effects

  Directly to the left of the image thumbnail, you will see the Layer Effects icon if that layer has effects added to it.

  See Using Layer Effects for more details.

Koko ja rajat

  In GIMP, the boundaries of a layer do not necessarily match the boundaries of the image that contains it. When you create text, for example, each text item belongs to its own separate layer, and the layer size is automatically adjusted to contain the text and nothing more. Also, when you create a new layer using cut-and-paste, the new layer is sized just large enough to contain the pasted item. In the image window, the boundaries of the currently active layer are shown outlined with a black-and-yellow dashed line.

  The main reason why this matters is that you cannot do anything to a layer outside of its boundaries, unless you enabled Expand Layers in the Paint Tool Options. If this causes you problems, you can alter the dimensions of the layer using any of several commands that you can find near the bottom of the Layer menu.

  | Huomaa
  Tason käyttämän muistin määrä määräytyy sen ulottuvuuksien mukaan, eikä sen sisällön mukaan. Joten, jos työskentelet suurempien kuvien tai useita tasoja sisältävien kuvien kanssa, saattaa olla parasta säätää tason pienimpään mahdolliseen kokoon.

Peittävyys

  Tason peittävyys määrittää missä määrin se päästää alempien tasojen pinoista värejä läpi. Peittävyys käyttää arvoja välillä 0-100, jossa 0 tarkoittaa täyttä läpinäkyvyyttä ja 100 tarkoitta täyttä läpinäkymättömyyttä.

Tila

  Tason tila määrittää miten tason värit yhdistetään alempien tasojen värien kanssa tuottamaan näkyvä tulos. Tämä riittävän monimutkaista ja riittävän tärkeä käsite, jotta se tarvitsee oman kappaleensa, joka on seuraava. Katso Kohta 2, ”Tason tilat”.

Tasomaski

  Alpha kanavan lisäksi on olemassa toinen tapa hallita tason läpinäkyvyyttä: lisäämällä tasomaskin, joka on ylimääräinen tasoon liittyvä piirrettävä harmaasävy. Tasolla ei ole oletuksena tasomaskia: se tulee lisätä erikseen. Tasomaskit ja miten niillä työskennellään on kuvattu paljon tarkemmin Tasomaski osiossa.

10.2. Creating a Basic Shape |  | 2. Tason tilat

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