The Foreground Select Tool in GIMP is one of the selection tools that attempts to make complex selections in an automatic way.
When it works well, it can be like magic, however the success or otherwise is largely dependent on the image that is being worked upon. Some pictures are more suitable for the Foreground Select Tool than others as the tool requires there to be specific differences between the object that you wish to select and the surrounding areas of the image. It can, however, produce a good starting point that can be further edited by hand, perhaps to produce a layer mask.
The tool is used in two steps. First, you outline the object that you wish to select, as if using the Free Select Tool, which places a colored preview over the rest of the image. Then you paint roughly on the object to select all the colors that you want included, and the tool attempts to extract it. You can further refine the selection after this first attempt by painting in more of the object or painting out the background.
While the tool is named the Foreground Select Tool, the part of the image doesn't need to be in the foreground and you can use it to select any distinct and contrasting part of an image.
There are a few settings that can be adjusted within the Tool Options, to change how the Foreground Select Tool operates.
Mode
As with other selection tools, the default is Replace the current selection, but there are also alternative modes that allow multiple selections to be combined in different ways. These are Add to the current selection, Subtract from the current selection and Intersect with the current selection.
Feather Edges
This can be used to produce a selection with softly graduated edges that will blend into surrounding pixels. When selected, the Radius slider is displayed to allow you to adjust the amount of feathering.
Contiguous
When checked, the selection will only include pixels of the correct color values that are adjacent to the selected pixels.
Interactive Refinement
After the Foreground Select Tool has made it's initial selection, you can refine it by using a paint brush to specify areas that should be included either in the foreground (Mark foreground) or background (Mark background). You can use the slider to adjust the size of the brush to make your selections more accurate.
Smoothing
If your selection includes small holes, rather than using the Mark foreground option, you could adjust the Smoothing slider to automatically include holes of a specific pixel diameter in the selection. The default is three pixels, but it can be increased to eight pixels, though at higher settings the selection edge may become less accurate.
Preview Color
This allows you to choose from Red, Green and Blue for the preview color of the image areas that are not part of the selection. You may want to change this to select a Preview color that contrasts with the image to make it more clearly visible.
Color Sensitivity
This option operates on the LAB color principle and allows you to further refine the selection by adjusting the level of sensitivity for the elements that make up specific colors. Sue Chastain, our Graphics Software Guide, has written a Color Primer Lesson for Photoshop users, but much of the information and links on that page will also help GIMP users acquire a better understanding of basic color theory, including LAB color.
This GIMP feature can be very effective with some images with very little work, but when trying to make complex and detailed selections, it can be worth spending some time refining with the Foreground Select Tool to produce the best possible result.