Three strokes painted with the same round fuzzy brush (outline shown
in upper left), using the Pencil (left), Paintbrush (middle), and
Airbrush (right).
The tools in this group are GIMP's basic painting tools, and they
have enough features in common to be worth discussing together in
this section. Features common to all brush tools are described in
the Common Features
section. Features specific to an individual tool are described in
the section devoted to that tool.
The Pencil is the crudest of the tools in this group: it makes
hard, non-anti-aliased
brushstrokes. The Paintbrush is intermediate: it is probably the
most commonly used of the group. The Airbrush is the most
flexible and controllable: it is the only one for which the
amount of paint applied depends on the speed of brush movement.
This flexibility also makes it a bit more difficult to use than
the Paintbrush, however.
All of these tools share the same brushes, and the same options
for choosing colors, either from the basic palette or from a
gradient. All are capable of painting in a wide variety of modes.
3.4.1. Key modifiers
Ctrl
Holding down the Ctrl key changes each of these
tools to a Color
Picker: clicking on any pixel of any layer sets the
foreground color (as displayed in the Toolbox Color Area) to the
color of the pixel.
3.4.2. Options
Mode
The Mode dropdown list provides a selection of paint application
modes. This setting appears in the Tool Options for all
brush tools, but it is grayed out for all except the tools
in this group, the Ink tool, and the Clone tool. A list of
possible modes can be found in the Glossary. For the most part
these modes are the same as the layer combination modes
available in the Layers dialog, and you can understand their
effects by imagining that the paint is applied to a separate
layer above the target layer, with the mode for the layer
set as specified. Three of the modes are special, though:
Color Erase.
This mode erases the foreground color, replacing it with
partial transparency. It acts like the Color to Alpha
filter, applied to the area under the brushstroke. Note
that this only works on layers that possess an alpha
channel; otherwise, this mode is identical to Normal.
Behind.
This mode applies paint only to non-opaque areas of the
layer: the lower the opacity, the more paint is applied.
Thus, painting opaque areas has no effect; painting
transparent areas has the same effect as normal mode. The
result is always an increase in opacity. Of course none of
this is meaningful for layers that lack an alpha channel.
Figure 8.21.
Dissolve mode example
Two brushstrokes made with the Airbrush, using the same
fuzzy circular brush. Left: Normal mode. Right:
Dissolve mode.
Dissolve.
In this very useful mode, for fuzzy brushes the gray level
of the brush determines not the paint density, but rather
the probability of applying paint. This gives a nice way of
creating rough-looking paintstrokes.
Gradient
Figure 8.22.
Gradient options for painting tools.
Instead of using the foreground color (as shown in the Color
Area of the Toolbox), by checking the "Use color from
gradient" option you can choose to paint with a gradient,
giving colors that change gradually along the brush
trajectory. For basic information on gradients, see the
Gradients section.
You have several options to control what gradient is used
and how it is laid out:
Gradient.
Here you see a display of the current gradient.
Clicking on it brings up a Gradient Selector, which will
allow you to choose a different gradient.
Reverse.
Normally a brushstroke starts with colors from the left
side of the gradient, and progresses rightward. If
"Reverse" is checked, the stroke starts with colors from
the right side, and progresses leftward.
Length.
This option sets the distance corresponding to one
complete cycle through the gradient colors. The default
units are pixels, but you can choose a different unit
from the adjoining Units menu.
Table 8.1.
Illustration of the effects of the three gradient-repeat
options, for the “Abstract 2” gradient.
Abstract2 Gradient
None
Sawtooth
Triangular
Repeat.
This option determines what happens if a brushstroke
extends farther than the Length specified above. There
are three possibilities: "None" means that the color
from the end of the gradient will be used throughout the
remainder of the stroke; "Sawtooth wave" means that the
gradient will be restarted from the beginning, which
will often produce a color discontinuity; "Triangular
wave" means that the gradient will be traversed in
reverse, afterwards bouncing back and forth until the
end of the brushstroke.
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