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
A “drawable” is a GIMP concept that includes layers, but
other types of things, 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 other than a layer.
In the menubar above an image window, 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.
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,
Edit Layer Attributes.
Presence or absence of an alpha channel
As explained in the previous section, 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.
Not every layer is required to have an alpha channel, though. In
many cases, the bottom layer of an image (often named
“Background”) lacks any alpha channel: this means
that it is
completely opaque at every point. Alphaless layers are created
when you open an image from a file format that does not support
transparency, or when you create a new image using
with a transparent Fill Type, or when you flatten an image into a
Every layer other than the bottom layer of an image must have an
alpha channel. For the bottom layer, it is optional. Many
operations cannot be performed on layers that lack an alpha
channel. Moving the layer to a different position in the layer
stack is one obvious example (since only bottom layers are allowed
not to have an alpha channel), but any operation involving
transparency would also be included. You can add an alpha channel
to a layer that lacks one using the menu command
Layer->Transparency->Add Alpha Channel,
or by right-clicking in the Layers dialog and selecting
Add Alpha Channel
from the popup menu that appears. To remove an alpha channel,
activate the bottom layer by clicking on it in the Layers dialog,
and then select
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 "open eye" 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
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 Layer menu.
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 glossary for
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.
“Keep transparency” setting
In the upper right corner of the Layers dialog appears a small
checkbox that controls the “keep transparency”
setting for the
layer. 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