The Tool Behavior dialog

The Tool Behavior dialog box is essential to configure most of the tools. If you do not see this window, you can still bring it back using View | Tool Behavior. You can also make it compact and revert it by clicking the small triangle in the topleft corner. It looks like the following:

The first two buttons configure whether a drawn rectangle or ellipse should be outlined or filled. You can set the line width of outlined shapes and straight lines using the edit box in this row. The last tool button indicates whether the Eyedropper (Y) tool switches back to the previously selected tool after finishing its work.

The controls in the second row can be used to set the brush size and shape. You can draw with rounded, sharp, slash, backslash, horizontal line and vertical line-shaped brushes, or use the Brush tool as a paint spray. Note that the mentioned controls affect not only the Brush (B), but also the Retouch (U), Eraser (E) and Recolor (Q) tools.

The edit box in the third row help you set the color tolerance of some tools, for instance, the Wand (W) tool. The higher the value set, the more pixels will be affected by the tool. Set the value to 0 to select only pixels of exactly the same color. The four tool buttons on the right can be used to configure the kind of gradient drawn by the Gradient (G) tool.

You can find an edit box in the fourth row which can be used to set the strength of the Eraser (E) tool. 100 means that the eraser changes all affected pixels to fully transparent. If a smaller value is set, the eraser only decreases the opacity by a certain level. The scrolldown list can be used to set the action the Retouch (U) tool should perform.

The remaining controls affect the Gradient tool. You can configure the gradient repetition and mode with them. There is a picture below which helps you understand the three repetition types, which are: none, symmetric and asymmetric, respectively:

There are two gradient modes: color and transparency. The color mode means that the gradient is simply drawn using the foreground and background colors. The transparency mode alters the alpha channel, it can be used to create fading parts of the image.