2.2. Dialog Kanály

Obrázek 15.9. The Channels dialog

The Channels dialog

The Channels dialog is the main interface to edit, modify and manage your channels. Channels have a double usage. This is why the dialog is divided into two parts: the first part for color channels and the second part for selection masks.

Color channels apply to the image and not to a specific layer. Basically, three primary colors are necessary to render all the wide range of natural colors. As other digital software, GIMP uses Red, Green, and Blue as primary colors. The first and primary channels display the Red, Green, and Blue values of each pixel in your image. Next to the channel name is a thumbnail displaying a grayscale representation of each channel, where white is 100% and black is 0% of the primary color. Alternatively, if your image is not a colored but a Grayscale image, there is only one primary channel called Gray. For an Indexed image with a fixed number of known colors there is also only one primary channel called Indexed. Then there is an optional channel called Alpha. This channel displays transparency values of each pixel in your image (See Alpha Channel in Glossary). In front of this channel is a thumbnail displaying a grayscale representation of the transparency where white is opaque and visible, and black is transparent and invisible. If you create your image without transparency then the Alpha channel is not present, but you can add it from the Layers dialog menu. Also, if you have more than one layer in your image, GIMP automatically creates an Alpha channel.

[Poznámka] Poznámka

GIMP nepodporuje barevné režimy CMYK a YUV.

Obrázek 15.10. Příklad kanálů v obrázku

Příklad kanálů v obrázku

Red channel

Příklad kanálů v obrázku

Green channel

Příklad kanálů v obrázku

Blue channel

Příklad kanálů v obrázku

Alpha channel

Příklad kanálů v obrázku

All channels


Obrázek vpravo je rozložen do tří barevných kanálů (červený, zelený a modrý) a alfa kanálu pro průhlednost. Na obrázku vpravo je průhlednost znázorněna jako šachovnice (jakoby prosvítající zpoza obrázku). V barevných kanálech, bílá je vždy bílá, neboť jsou v ní přítomny všechny barvy, a černá je vždy černá. Červená čepička je jasně viditelná v červeném kanálu ale v ostatních barevných kanálech není téměř patrná. Totéž platí pro čistě zelenou a čistě modrou, které jsou také viditelné jen ve svých vlastních kanálech.

2.2.1. Aktivace dialogu

The Channels dialog is a dockable dialog; see 2.3 – „Dokování dialogů“ for help on manipulating it.

You can access it:

  • from an image menu: WindowsDockable DialogsChannels;

  • from the Tab menu in any dockable dialog by clicking on and selecting Add TabChannels.

In the Windows menu, there is a list of detached windows which exists only if at least one dialog remains open. In this case, you can raise the Channels dialog from the image-menu: WindowsChannels.

2.2.2. Použití dialogu kanály

2.2.2.1. Overview

The top channels are the color channels and the optional Alpha channel. They are always organized in the same order and they cannot be erased. Selection masks are described below and displayed as a list in the dialog. Every channel appears in the list with its attributes, including a thumbnail and its name. A right-click in a channel list entry opens the channel context menu.

2.2.2.2. Atributy kanálu

Každý kanál zobrazený v seznamu má své atributy. Hlavním atributem je jméno kanálu. Jméno masek výběru můžete upravit po dvojitým kliknutím. Dvojité kliknutí na náhled otevře celý dialog, ve kterém můžete kromě jména nastavit i vizuální aspekty zobrazení kanálu v okně obrázku. Před náhledem může být ikona oka: Kliknutím na ikonu oka se přepíná viditelnost kanálu v okně obrázku. Pokud například učiníte modrá kanál neviditelným, bílá barva v obrázku zežloutne, protože žlutá barva je vůči modré doplňková. Pokud zneviditelníte alfa kanál, stane se vše zcela průhledným a uvidíte jen šachovnici představující pozadí za obrázkem. Toto pozadí lze nastavit v Předvolbách. Ikona řetězu umožňuje seskupování kanálů pro operace nad více kanály současně.

Channel visibility

By default every channel and thus every color value is visible. This is indicated by an open eye icon. Clicking on the eye-symbol (or the space if the channel is not visible) will toggle the visibility of the channel.

Chain channels

The channels representing selection masks (the new channels in the lower part of the channel list) may be grouped using the button with the chain symbol. Then these channels are all affected in the same way by operations applied to any one of them.

Primary color channels (the default channels in the upper part of the channel list) may be grouped too. By default, all color channels (and the alpha channel) are selected, their list entries are highlighted. Operations will be performed on all channels. By clicking on a channel list entry you can deactivate this channel. Operations like colorizing a layer will then be applied to the selected (grouped) channels only. Clicking again on the list entry will activate the channel.

Thumbnail

A small preview-icon represents the effect of the channel. On a selection mask, this preview can be enlarged by holding click down on it.

Channel name

The name of the channel, which must be unique within the image. Double-clicking on the name of a selection mask channel will allow you to edit it. The names of the primary channels (Red, Green, Blue, Alpha) can not be changed.

[Výstraha] Výstraha

Aktivované kanály jsou v dialogu zvýrazněny (barevně vysvíceny). Kliknutím na kanál v seznamu přepnete jeho aktivaci. Deaktivace barevného kanálu mívá vážné důsledky. Pokud například zakážete modrý kanál, všem pixelům přidávaným do obrázku bude chybět modrá složka a proto se například bílá bude jevit jako k modré doplňková žlutá barva.

2.2.2.3. Správa kanálů

Pod seznamem kanálů je sada tlačítekumožňujících provádět s kanály základní operace.

Upravit vlastnosti kanálu

Only available for selection masks. Here you can change the Channel name. The other two parameters affect channel visibility in the image window; they control Opacity and color used for the mask in the image window. A click on the color button displays the GIMP color selector and then you can change the mask color.

Nový kanál

You can create here a new channel. The displayed dialog lets you set Opacity and mask color used in the image to represent the selection. (If you use the New Channel button in Channel Menu, you can create this new channel with the options previously used by pressing the Shift key when clicking). This new channel is a channel mask (a selection mask) applied over the image. See Selection Mask

Raise Channel

Only available for selection masks: you can here put the channel up a level in the list. Press Shift key to move channel to top of the list.

Lower Channel

Umožňuje přesunout kanál v seznamu o příčku níže. Spolu s klávesou Shift přesune kanál až naspod seznamu.

Duplicate Channel

Umožňuje vytvořit kopii aktivního kanálu. Jméno kanálu je doplněno číslem.

[Tip] Tip

Duplikovat můžete i barevný nebo alfa kanál. Je to zajímavý způsob, jak si vytvořit jejich kopii a použít ji později jako výběr v obrázku.

Channel to Selection

Here you can transform the channel to become a selection. By default the selection derived from a channel replaces any previous active selection. It's possible to change this by clicking on control keys.

  • Shift: výběr odvozený z kanálu se přičte ke stávajícímu aktivnímu výběru. Výsledkem je sjednocení výběrů.

  • Ctrl: výběr odvozený z kanálu se od stávajícího výběru odečte. Výsledkem je rozdíl.

  • Shift+Ctrl: výsledkem je průnik výběru odvozeného z kanálu a stávajícího výběru. Jsou zachovány pouze společné části.

Delete Channel

Only available for selection masks: you can here delete the active channel.

2.2.2.4. Nabídka Kanály

Obrázek 15.11. Nabídka Kanály

Nabídka Kanály

Overview

Kontextovou nabídku kanálů vyvoláte kliknutím pravého tlačítka myši na náhled kanálu. Nabídka obsahuje stejné příkazy pro práci s kanály, jaké nabízí tlačítka dialogu. Jediným rozdílem jsou operace převodů kanálu do výběru, v nabídce má každá z nich svou samostatnou položku.

Upravit vlastnosti kanálu, Nový kanál, Raise Channel, Lower Channel, Duplicate Channel, Delete Channel

See Managing channels.

Channel to Selection

Selection derived from channel replaces any previous active selection.

Add to Selection

Selection derived from channel is added to previous active selection. Final selection is merging of both.

Ubrat z výběru

Final selection is subtraction of selection derived from a channel from previous active selection.

Vrstva

Final selection is intersection of selection derived from a channel with the previous active selection. Only common parts are kept.

2.2.3. Masky výběru

Obrázek 15.12. A selection composed out of channels.

A selection composed out of channels.

Channels can be used to save and restore your selections. In the channel dialog you can see a thumbnail representing the selection. Selection Masks are a graphical way to build selections into a gray level channel where white pixels are selected and black pixels are not selected. Therefore gray pixels are partially selected. You can think of them as feathering the selection, a smooth transition between selected and not selected. This is important to avoid the ugly pixelization effect when you fill the selection or when you erase its content after isolating a subject from background.

Vytvoření masky výběru

There are several ways to initialize a selection mask.

  • From the image window menu SelectSave to Channel if there is an active selection.

  • In the image window the bottom-left button creates a Quick Mask; the content will be initialized with the active selection.

  • From the channel dialog, when you click on the New channel button or from the context menu. When created, this Selection mask appears in the Channel dialog, named Selection maskcopy with a queuing number. You can change this by using the context menu that you get by right-clicking on the channel.

2.2.3.1. Používání masek výběru

Once the channel is initialized, selected (highlighted in blue), visible (eye-icon in the dialog), and displayed as you want (color and opacity attributes), you can start to work with all the paint tools. The colors used are important. If you paint with some color other than white, grey, or black, the color Value (luminosity) will be used to define a gray (medium, light, or dark). When your mask is painted, you can transform it to a selection by clicking on the button (Channel to Selection) or from the context menu.

You can work in selection masks not only with the paint tool but also with other tools. For instance, you can use the selection tools to fill areas uniformly with gradients or patterns. By adding many selection masks in your list you can easily compose very complex selections. One can say that a selection mask is to a selection as a layer is to an image.

[Výstraha] Výstraha

Dokud je maska výběru aktivní, pracujete s maskou a nikoliv s obrázkem. Chcete-li opět pracovat s obrázkem, musíte deaktivovat všechny masky výběru. Nezapomeňte také zakázat zobrazování masek v obrázku odstraněním ikon oka. Také si zkontrolujte, že jsou aktivovány a zobrazovány všechny barevné a alfa kanály.

2.2.4. Rychlá maska

Obrázek 15.13. Dialog Quick Mask

Dialog Quick Mask

A Quick Mask is a Selection Mask intended to be used temporarily to paint a selection. Temporarily means that, unlike a normal selection mask, it will be deleted from the channel list after its transformation to selection. The selection tools sometimes show their limits when they have to be used for doing complex drawing selection, as progressive. In this case, using the QuickMask is a good idea which can give very good results.

2.2.4.1. Activating the dialog

The QuickMask can be activated in different ways:

  • From the image menu: SelectToggle QuickMask.

  • By clicking the left-bottom button showed in red on the screenshot.

  • By using the Shift+Q shortcut.

2.2.4.2. Vytvoření rychlé masky

To initialize a Quick Mask, click the bottom-left button in the image window. If a selection was active in your image, then its content appears unchanged while the border is covered with a translucent red color. If no selection was active then all the image is covered with a translucent red color. Another click on the bottom-left button will deactivate the quick mask.

From the channel dialog you can double click on the name or the thumbnail to edit the QMask attributes. Then you can change the Opacity and its filling color. At every moment you can hide the mask by clicking on the eye icon in front of the QMask.

The mask is coded in gray tones, so you must use white or gray to decrease the area limited by the mask and black to increase it. The area painted in light or dark gray will be transition areas for the selection like feathering. When your mask is ready, click again on the bottom-left button in the image window and the quick mask will be removed from the channel list and converted to a selection.

Quick mask's purpose is to paint a selection and its transitions with the paint tools without worrying about managing selection masks. It's a good way to isolate a subject in a picture because once the selection is made you only have to remove its content (or inverse if the subject is in the selection).

2.2.4.3. Using Quick Mask with a gradient

Popis

  1. Screenshot of the image window with activated QuickMask. As long as the Quickmask is activated, all operations are done on it. A gradient from black (left) to white (right) has been applied to the mask.

  2. The QuickMask is now disabled. The selection occupies the right half part of the image (marching ants) because the limit of the selection is at the middle of the gradient.

  3. A stroke is now added during the enabled selection. Weird! The gradient, although not visible, remains active all over the image, in selected and non selected areas!

After the QuickMask Button is pressed, the command generates a temporary 8-bit (0-255) channel, on which the progressive selection work is stored. If a selection is already present the mask is initialized with the content of the selection. Once QuickMask has been activated, the image is covered by a red semi-transparent veil. This one represents the non-selected pixels. Any paint tool can be used to create the selection on the QuickMask. They should use only grayscale color, conforming the channel properties, white enabling to define the future selected place. The selection will be displayed as soon as the QuickMask will be toggled but its temporary channel will not be available anymore.

[Tip] Tip

Chcete-li výběr vyrobený pomocí rychlé masky uložit v trvalém kanálu, zvolte v nabídce obrázku Vybrat/Uložit do kanálu.

2.2.4.4. Použití
  1. Otevřete obrázek nebo vytvořte Nový dokument.

  2. Activate the Quickmask using the left-bottom button in the image window. If a selection is present the mask is initialized with the content of the selection.

  3. Choose a drawing tool and use it with grayscale colors on the QuickMask.

  4. Zrušte rychlou masku opětovným kliknutím na tlačítko v levém dolním rohu okna obrázku.