Chapter 8. Combining Images
1. Introduction to Layers
A good way to visualize a GIMP image is as a stack of transparencies: in
GIMP terminology, each individual transparency is called a
layer. There is no limit, in principle, 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.
The organization of layers in an image is shown by the Layers dialog,
which is the second most important type of dialog window in GIMP, after
the Main Toolbox. The appearance of the Layers dialog is shown in the
adjoining illustration. How it works is described in detail in the
section, but we will touch on 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 types of
things, such as channels, layer masks, and the selection mask. (Basically,
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 other than a layer.
In the menubar above an image window, you can find a menu called
, 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.
Each layer in an image has a number of important attributes:
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,
Presence or absence of an alpha channel
An alpha channel encodes information about how transparent a layer
is at each pixel. It is visible in the Channel Dialog: white is
complete opacity, black is complete transparency and grey levels
are partial transparencies.
The background layer is particular. If you have just created a
new image, it has still only one layer which is a background
layer. If the image has been created with an opaque Fill type,
this one layer has no Alpha channel. If you add a new layer,
even with an opaque Fill type, an Alpha channel is automatically
created, which applies to all layers apart from the background
layer. To get a background layer with transparency, either you
create your new image with a transparent Fill type, or you use
Add an Alpha Channel.
Every layer other than the bottom layer of an image has
automatically an Alpha channel, but you can't see a grayscale
representation of the alpha values. See
Alpha in Glossary for
Example for Alpha channel.
Alpha channel example: Basic image
Figure 8.2. Alpha channel example: One transparent layer
Figure 8.3. Alpha channel example: Two transparent layers
Figure 8.4. Alpha channel example: Three transparent layers
Alpha channel example: Alpha channel added to the Background
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:
The main reason this matters is that most filters (in the
menu) only accept a subset of layer types, and appear grayed out
in the menu if the active layer does not have an acceptable type.
Often you can rectify this either by changing the mode of the
image or by adding or removing an alpha channel.
It is possible to temporarily remove a layer from an image,
without destroying it, by clicking on the symbol in the Layers
dialog. This is called “toggling the visibility”
of the layer. Most operations on an image treat toggled-off layers
as if they did not exist. When you work with images containing
many layers, with varying opacity, you often can get a better
picture of the contents of the layer you want to work on by hiding
some of the other layers.
If you Shift-click on the eye symbol, this
will cause all layers except
the one you click on to be hidden.
Linkage to other layers
If you click between the eye icon and the layer thumbnail, you get
a chain icon, which enables you to group layers for operations on
multiple layers (for example with the Move tool or a transform
Size and boundaries
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 goes into its own separate layer, and
the layer is precisely sized 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: you can't act on what doesn't
exist. 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
The amount of memory that a layer consumes is determined by its
dimensions, not its contents. So, if you are working with large
images or images that contain many layers, it might pay off to
trim layers to the minimum possible size.
The opacity of a layer determines the extent to which it lets
colors from layers beneath it in the stack show through. Opacity
ranges from 0 to 100, with 0 meaning complete transparency, and
100 meaning complete opacity.
The Mode of a layer determines how colors from the layer are
combined with colors from the underlying layers to produce a
visible result. This is a sufficiently complex, and sufficiently
important, concept to deserve a section of its own, which follows.
See Section 2, “
In addition to the alpha channel, there is another way to control
the transparency of a layer: by adding a
which is an extra grayscale drawable associated with the layer.
A layer does not have a layer mask by default: it must be added
specifically. Layer masks, and how to work with them, are
described much more extensively in the
Layer Mask section.
“Lock alpha channel” setting
In the upper left corner of the Layers dialog appears a small
checkbox that controls the “Lock”
setting for the transparency of the layer (see the figure below).
If this is checked, then the alpha channel for the layer is
locked, and no manipulation has any effect on it. In particular,
nothing that you do to a transparent part of the layer will have
Figure 8.7. Lock Alpha channel