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3. Brush Tools

3.1. Common Features

Figure 8.16.  The Brush tools

The Brush tools

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

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.


 
 
  Published under the terms of the GNU General Public License Design by Interspire