Subsections
2.1 Layers and the Role They Play in Images
Figure
2.1 will be used to describe some of
Figure 2.1:
A Three-Layer Image
 |
the fundamental features of layers. Although this image is
constructed of three layers, this is not evident from looking at it in
the image window. To understand and work with the layer structure of
an image you must use the Layers dialog.
However, before describing this
important tool, let's look at Figure
2.2,
which is a
Figure 2.2:
Visualizing the Layer Stack in 3D
 |
useful illustration of how layers are used to make up images.
Figure
2.2 shows a 3D break-out of how the
three-layer stack relates to the 2D image shown in
Figure
2.1. The bottom layer is a picture of
a gopher couple; this layer is fully opaque. The middle layer has the
same dimensions as the bottom layer and is completely transparent
except for a small white rectangular region. The white rectangle
itself is partially transparent (thus, the bottom layer can be
perceived through it). Finally, the top layer's dimensions are
smaller than either the bottom or middle layers. The top layer is
also transparent except for the text Prairie Pranks. The vertical red
dashed lines in Figure
2.2 show that the text
in the top layer is positioned directly over the white rectangle in
the middle layer. The black frames shown in the figure have been
drawn in to show the positions of the text and the white rectangle but
are not actually part of the image (refer to
Figure
2.1).
The 3D layer stack shown in Figure
2.2 is a
useful mental model for understanding how layers work in an image. It
also underscores the utility of having image components on layers.
Because the text is on a layer, it can be positioned independently of
the lowest layer. Because the white rectangle is on a layer, its
partial transparency can be adjusted without affecting the text.
2.1.1 The Layers Dialog
It is impossible to work with images without knowing their layer
structure. This information is provided by the Layers dialog, which
is part of the Layers & Channels window. The Layers & Channels window can be found in
the Image:Layers menu, the Image:Dialogs menu, or can be
invoked by simply typing C-l in the image window.
Figure
2.3(a)
Figure 2.3:
Layers Dialog
 |
shows the multilayer image from Figure
2.1,
and Figure
2.3(b) shows the associated Layers
dialog.
The Layers dialog has many components. Let's
take a moment to briefly describe each one.
Figure
2.4 illustrates the dialog's
Figure 2.4:
Layers Dialog Detail
 |
various components. The most important feature of the Layers dialog
is the Layers Palette
. This region of the dialog consists of horizontal strips,
each representing a single layer in the image. It can be seen from
Figure
2.4 that this is the Layers
dialog for an image with three layers. Note that each strip displays
a thumbnail image of its layer's contents. The strips are organized
vertically in a stack, and this stack shows the 3D relationship of the
layers in a 2D manner. The top, middle, and bottom layers of the
stack represent the layers analogously to the conceptual 3D view shown
in Figure
2.2.
In addition to showing the layer ordering structure of an image, the
Layers Palette provides useful information and tools for working with
the image. As shown in Figure
2.4,
the area to the far left of the Layer Thumbnail contains the Eye
icon. If this icon is visible, the
corresponding layer is visible in the image window. The Eye icon can
be toggled off by clicking on it. This makes the corresponding layer
invisible in the image window. Toggling it again brings the layer
back.
The region of the strip just to the left of the thumbnail and to the
right of the Eye icon is the position of the Layer Link icon.
Clicking in this area makes a four-way arrow appear, as shown in two
of Figure
2.4's layer strips. When
this icon is activated for several layers at once, they are linked
together with respect to the Move tool. These
layers will now move together as a single unit (the Move tool is
covered in Section
2.6.1).
To the right of the thumbnail is the Layer Title
Area, which, for a new image, is named
Background by default. The title can be changed by double-clicking in
the Layer Title Area. This brings up a dialog box where the new title
can be entered.
The middle layer strip in Figure
2.4
is highlighted in blue, indicating that it is the
active. layer GIMP tools
applied to the image window are applied to this layer. This is very
important, so read these last two sentences again! The GIMP tools and
filters are applied to the active layer. Not knowing which
layer is active is a formula for confusion and frustration. The
identity of the active layer can only be reliably determined by having
the Layers dialog open. Any layer can be made active by clicking on
its thumbnail or Title Area. Only one layer can be active at a time.
Outside of the Layers Palette area are several other important
features of the Layers dialog. First, there are two pull-down menus
labeled Image and Mode. The Image
menu is used to specify the context of the Layers dialog. The GIMP
can have many image windows open simultaneously. When there is more
than one image window open, this menu is used to select the one whose
layers are displayed in the Layers Palette area. If the Auto Context
button is toggled on (which it is by default), any keystroke in an
image window will change the Layers Palette context auto-magically to
that window. I like to use the space bar for this.
The Mode menu selects how the pixels of the active layer are visually
blended with those beneath it. Each layer has a blending mode context
that is specified using this menu. Blending modes are described in
more detail in Chapter
5.
The remaining features of the Layers dialog consist of the Opacity
slider, the Keep Trans. toggle button, and, at the bottom of the
dialog, a row of function buttons that I call the button bar. These
are all discussed in more detail later. However, of these features,
the Opacity slider is of immediate interest
because it is used in the example shown in
Figure
2.3.
Figure
2.3(b) shows that the the middle layer
strip is highlighted, indicating that this layer, containing the small
white rectangle, is active. Careful examination of the Layers dialog
shows that the Opacity slider is set to 60.0% for this layer. This
means that the layer below can be seen through the white rectangle
because the rectangle's layer is only partially opaque. Like for the
blending modes, opacity can be independently set for each layer.
