The GIMP Toolbox includes nine "brush tools", all grouped together
at the bottom (in the default arrangement). The feature they all
have in common is that all of them are used by moving the pointer
across the image display, creating brushstrokes. Four of them – the
Pencil, Paintbrush, Airbrush, and Ink tools – behave like the
intuitive notion of "painting" with a brush. The others use a brush
to modify an image in some way rather than paint on it: the Eraser
erases; the Clone tool copies from a pattern or image; the Convolve
tool blurs or sharpens; the Dodge/Burn tool lightens or darkens; and
the Smudge tool smears.
The advantages of using GIMP with a tablet instead of a mouse
probably show up more clearly for brush tools than anywhere else:
the gain is fine control is invaluable. These tools also have
special "Pressure sensitivity" options that are only usable with a
tablet.
In addition to the more common "hands-on" method, it is
possible to apply brush tools in an automated way, by creating a
selection or path and then "stroking" it. You can choose to
stroke with any of the brush tools, including nonstandard ones
such as the Eraser, Smudge tool, etc., and any options you set for
the tool will be applied. See the section on Stroking for more information.
Brush tools work not only on image layers, but on other types of
drawable objects as well: layer masks, channels, and the
selection. To apply a brush tool to a layer mask or channel,
simply make it the image's active drawable by clicking on it in
the Layers dialog or Channels dialog. To apply a brush tool to
the selection, switch to QuickMask mode. "Painting the
selection" in this way is a very
powerful method for efficiently creating precise selections.
3.1.1. Key modifiers
Ctrl:
Holding down the Ctrl key has a special effect on every
brush tool except the ink tool. For the Pencil, Paintbrush,
Airbrush, Eraser, and Smudge tools, it switches them
into "color picker" mode, so that clicking on an image pixel
causes GIMP's foreground to be set to the active layer's
color at that point (or, for the Eraser, GIMP's background
color). For the Clone tool, the Ctrl key switches it into a
mode where clicking sets the reference point for copying.
For the Convolve tool, the Ctrl key switches between
blur and sharpen modes; the the Dodge/Burn tool, it switches
between dodging and burning.
Shift:
Holding down the Shift key has the same effect on all brush
tools: it places the tool into straight
line mode. To create a straight line with any of
the brush tools, first click on the starting point,
then press the Shift key. As long as
you hold it down, you will see a thin line connecting the
previously clicked point with the current pointer location.
If you click again, while continuing to hold down the Shift
key, a straight line will be rendered. You can continue
this process to create a series of connected line segments.
Ctrl-Shift:
Holding down both keys puts the tool into
constrained straight line mode. This
is similar to the effect of the Shift key alone, except
that the orientation of the line is constrained to the
nearest multiple of 15 degrees. Use this if you want to
create perfect horizontal, vertical, or diagonal lines.
3.1.2. Tool Options
Figure 8.17.
Tool Options shared by all brush tools
Many tool options are shared by several brush tools: these are
described here. Options that apply only to one specific tool,
or to a small number of tools, are described in the sections
devoted to those tools.
Opacity
The Opacity slider sets the transparency level for the brush
operation. To understand how it works, imagine that instead
of altering the active layer, the tool creates a transparent
layer above the active layer and acts on that layer.
Changing Opacity in the Tool Options has the same effect
that changing opacity in the Layers dialog would have in the
latter situation. It controls the "strength" of all brush
tools, not just those that paint on the active layer. In
the case of the Eraser, this can come across as a bit
confusing: it works out that the higher the "opacity" is,
the more transparency you get.
Mode
The Mode dropdown list provides a selection of paint application
modes; a list of modes can be found in the
glossary. As with the opacity, the easiest way to understand
what the Mode setting does is to imagine that the paint is
actually applied to a layer above the layer you are working on, with the layer
combination mode in the Layers dialog set to the selected
mode. You can obtain a great variety of special effects in
this way. The Mode option is only usable for tools that can
be thought of as adding color to the image: the Pencil,
Paintbrush, Airbrush, Ink, and Clone tools. For the other
brush tools, the option appears for the sake of consistency
but is always grayed out.
Brush
The brush determines how much of the image is affected by
the tool, and how it is affected, when you trace out a
brushstroke with the pointer. GIMP allows you to use
several different types of brushes, which are described in
the Brushes section. The
same brush choices are available for all brush tools except
the Ink tool, which uses a unique type of procedurally
generated brush. The colors of a brush only come into play
for tools where they are meaningful: the Pencil,
Paintbrush, and Airbrush tools. For the other brush tools,
only the intensity distribution of a brush is relevant.
Pressure Sensitivity
The Pressure Sensitivity section is only meaningful if you
are using a tablet: it allows you to decide which aspects
of the tool's action should be affected by how hard you
press the stylus against the tablet. The possibilities are
"opacity", "hardness", "rate", "size", and "color". They
work together: you can enable as many of them as you like.
For each tool, only the ones that are meaningful are listed.
Here is what they do:
Opacity
The effect of this option is described above.
Hardness
This option applies to brushes with fuzzy edges. If it
is enabled, the harder you press, the darker the fuzzy
parts of the brush will appear.
Rate
This option applies to the Airbrush, Convolve tool, and
Smudge tool, all of which have time-based effects.
Pressing harder makes these tools act more rapidly.
Size
This option applies to all of the pressure sensitive
brush tools, but only if you are using a parametric
brush, that is, a brush created using the Brush Editor.
If the option is checked, and the brush is parametric,
then pressing harder will increase the size of the area
affected by the brush.
Color
This option only applies to the painting tools: the
Pencil, Paintbrush, and Airbrush; and only if you are
using colors from a gradient. If these conditions are
met, then pressing harder causes colors to be taken from
higher in the gradient.
Fade Out
This option causes each stroke to fade out over the specified
distance. It is easiest to visual for painting tools, but
applies to all of the brush tools. It is equivalent to
gradually reducing the opacity along the trajectory of the
stroke. Note that, if you are using a tablet, this option
does not change the effects of brush pressure.
Incremental
The Incremental checkbox activates incremental mode for the
tool. If it is deactivated, the maximum effect of a single
stroke is determined by the opacity, and moving the brush
repeatedly over the same spot will not increase the effect
beyond this limit. If Incremental is active, each
additional pass with the brush will increase the effect, but
the opacity cann't exceed the opacity set for the tool.
This option is available for all brush tools except those
that have a "rate" control, which automatically implies an
incremental effect.
Hard Edge
Activating this option causes fuzzy brushes to be treated as
though they were black-and-white, and inactivates sub-pixel
anti-aliasing. The consequence is that all pixels affected
by the tool are affected to the same degree. This is often
useful if you work at a very high zoom level, and want to
have precise control of every single pixel.
"Hard edge" is available for all brush tools except
the painting tools (Pencil, Paintbrush, and Airbrush),
where it would be redundant, because giving a hard edge to
the Paintbrush or Airbrush would simply make them behave
like the Pencil tool.
3.1.3. Further Information
Advanced users may be interested to know that brush tools
actually operate at a sub-pixel level, in order to avoid
producing jagged-looking results. One consequence of this is
that even if you work with a hard-edged brush, such as one of
the Circle brushes, pixels on the edge of the brushstroke will
only be partially affected. If you need to have all-or-nothing
effects (which may be necessary for getting a good selection, or
for cutting and pasting, or for operating pixel-by-pixel at a
high zoom level), there are two things you can do: (1)
for painting, use the Pencil tool, which makes all brushes
perfectly hard and disables sub-pixel anti-aliasing, or (2) for
other types of brush tools, check the "Hard edge"
box in the Tool Options.
