Holding down the Ctrl key has a special effect on every brush tool . For the Pencil, Paintbrush, Airbrush, Ink Tool, and Eraser 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; for the Dodge/Burn tool, it switches between dodging and burning.

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.

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.

The Mode drop-down list provides a selection of paint application modes. 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. A list of modes can be found in Section 2, “ Layer Modes ”.

In this list, some modes are particular and are described below.

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.

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.

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. You can obtain the same result if you have correctly set your mouse-wheel in the Preferences. See How to vary the size of a brush

Figure 13.35. The Brush Dynamics check box.

Brush dynamics let you map different brush parameters, commonly at least size and opacity, to one or more of three input dynamics: pressure, velocity and random. They are mostly used with tablets, but Velocity and Random are also usable with a mouse. The Ink tool, that supported velocity before, has been overhauled and now handles velocity-dependent painting much better.

A new option has been added in stroking paths. Stoke Path and Stroke Selection have now a check box for emulating brush dynamics when you stroke using a paint tool. That means that when you stoke, brush pressure and velocity are varying along the length of the stroke. Pressure starts with zero, ramps up to full pressure and then ramps down again to no pressure. Velocity starts from zero and ramps up to full speed by the end of the stroke.

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:

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.

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.

Figure 13.36. “Jitter” example

From top to bottom: without jitter, jitter = 1, jitter = 4.

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 ”.

Figure 13.37. 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:

Figure 13.39. Dissolve mode example

Two brush-strokes made with the Airbrush, using the same fuzzy circular brush. Left: Normal mode. Right: Dissolve mode.

For any paint tool with opacity less than 100%, this very useful mode doesn't draw transparency but determines the probability of applying paint. This gives nice patterns of dots to paint-strokes or filling.

Figure 13.40. Painting in Dissolve mode

This image has only the background layer and no Alpha channel. The background color is sky blue. Three strokes with Pencil and various opacities: 100%, 50%, 25%. Foreground color pixels are scattered along brushstroke.

Figure 13.41. Example for layer mode “Behind”

Wilber over a blue background layer

Filled with pattern

This mode applies paint only to transparent 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.

In the above example image, Wilber is on the top layer, surrounded by transparency. The lower layer is solid light blue. The Bucket Fill tool was used, with the Fill Whole Selection option checked and the entire layer was selected. A pattern was used to paint with the Bucket Fill tool.

The next image (below) has two layers. The upper layer is active. Three brushtrokes with pencil, red color at 100%, 50%, 25%: only transparent or semi-transparent pixels of the layer are painted.

Figure 13.42. Painting in “Behind” mode

Painting with 100%, 50%, 25% transparency (from left to right)

Figure 13.43. Example for layer mode “Color erase”

Wilber over a blue background layer

White foreground color erased

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.

In the above example image, the color of the Bucket Fill tool was white, so white parts of Wilber were erased and the blue background shows through.

This image below has only one layer, the background layer. Background color is sky blue. Three brushtrokes with pencil:

Figure 13.44. Painting in “Color Erase” mode

Painted with 1. blue; 2. red; 3. background color