3.  Brush Tools

The GIMP Toolbox includes thirteen “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 brush-strokes. Four of them

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 advantages of using GIMP with a tablet instead of a mouse probably show up more clearly for brush tools than anywhere else: the gain in 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.

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.

Paint Modes

Mode

The Mode drop-down 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.

In this list, some modes are particular:

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.

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.

Scale

This option lets you to modify precisely the size of the brush. You can use the arrow keys to vary by ±0.01 or the Page-Up and Page-Down keys to vary by ±0.05.

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. If the option is checked, 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.

Apply Jitter

You know “spacing” in brush strokes: strokes are made of successive brush marks which, when they are very near, seem to draw a continuous line. Here, instead of being aligned brush marks are scattered over a distance you can set with the Quantity slider.

Incremental

The Incremental check-box 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 can't exceed the opacity set for the tool. This option is available for all brush tools except those which have a “rate” control, which automatically implies an incremental effect. See also Section 2, “ Layer Modes ”.

Color from Gradient

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.

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.