Holding down the Ctrl key has a special effect on every paint tool. For the Pencil, Paintbrush, Airbrush, Ink, and Eraser, 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 Blur/Sharpen 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 most paint tools: it places the tool into straight line mode. To create a straight line with any of the paint 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 paint tools, the option appears for the sake of consistency but is always grayed out. A list of modes can be found in 2 – „Režim“.

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 paint 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 paint 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 paint 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 ±1.00. 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

This determines the ratio between the height and the width of the brush. The slider is scaled from -20.00 to 20.00 with the default value set to 0.00. A negative value from 0.00 to -20 will narrow the height of the brush while a positive value between 0.00 and 20.00 indicates the narrowing rate of the width of the brush.

This option makes the brush turn round its center. This is visible if the brush is not circular or made from a rotated figure.

This option sets the distance between the brush marks in a stroke.

Obrázek 14.40. Spacing option

Different spacings

Modifies the size of the brush hard center.

Obrázek 14.41. Hardness option

On the left: hardness=50 On the right: hardness=100.

Modifies gain.

Obrázek 14.42. Force option

Force is 10%, 20%, 40%, 80%.

Brush dynamics let you map different brush parameters to several input dynamics. They are mostly used with graphic tablets, but some of them are also usable with a mouse.

You can read more about dynamics in Dynamics

When stroking paths and selections using a paint tool there is an option to select „Emulate brush dynamics“. That means that when you stroke, 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.

These options are described in Dynamics Options

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 Amount slider.

Obrázek 14.43. „Jitter“ example

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

Jitter is also available in the Paint Dynamic Editor where you can connect jitter to the behavior of the brush.

This option doesn't affect the rendering of the brush stroke but its „shape“. It takes away the wobbles of the line you are drawing. It makes drawing with a mouse easier.

When this option is checked, two setting areas appear, Quality and Weight. You can change the default values to adapt them to your skill.

High weight values rigidifies the brush stroke.

Obrázek 14.44. „Smooth Stroke“ example

Trying to draw a straight line and a sine curve with the mouse. 1 : option unchecked 2 : default values 3 : maximum values

When you are working on an image that is bigger (in pixels) than your screen, you have to zoom in and out a lot. This option allows a very natural "iterative refinement" process with no need to repeatedly ask the application to change brush size as you go between the broad strokes and the detailing.

If the brush size is relative to the canvas (option unchecked), zooming in makes the brush zoomed also and it appears larger (takes up more pixels on the screen). If you're working with a 300 pixels radius brush and you zoom in from 12% to 100%, the brush is now half the size of your screen! So you have to shrink the brush back down.

If the brush size is relative to the screen (option checked), then when you zoom in, the size of the displayed brush doesn't change, it looks smaller and so you can work on tiny details.

Obrázek 14.45. Lock brush example

This image is the screenshot of a small part of a TIFF image bigger than my screen. Here, GIMP displays this TIFF image at 50% zoom, so the whole image is visible. The option is not checked, brush size in toolbox is 100 pixels high.

Screenshot: zoom 200. The option is not checked. We paint with pencil and the pepper brush that is 100 pixels high in toolbox: displayed brush stroke is 200 pixels high.

Screenshot: zoom 200. The option is checked. We paint with pencil and the pepper brush that is 100 pixels high in toolbox: displayed brush stroke is 100 pixels high.

"Lock brush to view" can also be used to lock brush to view rotation:

Obrázek 14.46. Lock brush to view rotation example

Screenshot. Here, the option is unchecked and we use View → Flip and Rotate → Rotate 15° clockwise: brush stroke is rotated.

Same thing but the option is checked: the brush is locked to the original, not rotated view and brush stroke is not rotated.

Applies the effect incrementally as the mouse pointer moves.

The incremental checkbox does not seems to work as everyone expect. If it is deactivated (the default value) the maximum effect of a single stroke is determined by the opacity set in the opacity slider. If the opacity is set to less than 100, moving the brush over the same spot will increase the opacity if the brush is lifted in the meantime. Painting over with the same stroke has no such effect. If Incremental is active the brush will paint with full opacity independent of the slider's setting. This option is available for all paint tools except those which have a „rate“ control, which automatically implies an incremental effect. See also 2 – „Režim“.

Obrázek 14.47. Dissolve mode example

Dva tahy Rozprašovačem vytvořené pomocí stejné kruhové stopy. Vlevo: normální režim. Vpravo: režim rozpouštění.

V tomto velmi zajímavém režimu šedé části stop neaplikují barvu s nižším krytím, nýbrž určují pravděpodobnost, že na pixel bude aplikována barva s plným krytím. Je to zajímavý způsob tvorby pěkných hrubě vypadajících tahů štětcem.

Obrázek 14.48. 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.

Obrázek 14.49. Example for layer mode „Behind“

Wilber over a blue background layer

Layers dialog

Filled with pattern

Tento režim aplikuje barvu pouze na průsvitné a průhledné oblasti - čím průhlednější oblast, tím více barvy se aplikuje. Malování na zcela neprůhledné oblasti tak nemá vůbec žádný efekt; malování na zcela průhledné oblasti má stejný efekt jako v normálním režimu. Výsledkem je vždy snížení průhlednosti (zvýšení krytí). V případě vrstev bez alfa kanálu tento režim samozřejmě postrádá smysl.

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 brush strokes with pencil, red color at 100%, 50%, 25%: only transparent or semi-transparent pixels of the layer are painted.

Obrázek 14.50. Painting in „Behind“ mode

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

Obrázek 14.51. 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 brush strokes with pencil:

Obrázek 14.52. Painting in „Color Erase“ mode

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