The Fuzzy Select Tool is basically the equivalent of the Magic Wand in Adobe Photoshop and it is used to make potentially complex selections based on similarities of color. This can be very useful if you want to select an area of an image of similar color values and apply some form of transformation to these pixels only, such as adjusting their color.
If you're familiar with GIMP, you may be aware of the similarity between the Fuzzy Select Tool and the Select By Color Tool, but the difference between them is that Fuzzy Select makes a single contiguous selection. The Select By Color Tool, on the other hand, may result in multiple unconnected selections, selecting every pixel in an image with a similar color value.
To make a selection with the Fuzzy Select Tool, you only have to click on an image at a point that is the color you want to select and all the pixels immediately surrounding the pixel that you clicked that fall within the threshold of similarity will be included in the selection. As you make the selection, you can adjust the size by holding the mouse button down and dragging downwards or to the right to increase the size or upwards or to the left to decrease it.
There are several settings available in the Tool Options that can change how the Fuzzy Select Tool operates.
The default setting is Replace the current selection which will only allow you to ever have a single selection active. By switching to Add to the current selection, it is possible to use the Fuzzy Select Tool to make unconnected selections within an image simultaneously, but these selections will always be contiguous to themselves. The other two options, Subtract from the current selection and Intersect with the current selection, can be used to modify a selection to produce more complex results.
This is checked by default and produces a selection edge that appears to be smoother.
When Feather Edges is checked, a Radius slider is displayed that allows you to edit the amount of feathering that is applied to the edge of the selection. The result is an edge that will gently blend out to nothing if the selection is filled with a solid color.
Select Transparent Pixels
If this is checked, the selection can include completely transparent pixels if they are within the threshold that is set.
This is worth considering if you're targeting a layer that has other layers below it and the target layer has its Opacity set lower than 100 or its Layer Mode is anything other than Normal. If Sample merged is checked, the Fuzzy Select Tool will produce a selection based on the composite result of all the relevant layers. If not checked, then only the target layer will influence the result of the selection.
This affects the sensitivity of the Fuzzy Select Tool, determining how similar colors must be to the selected pixel in order to be selected. The further to the right this slider is set, the larger the selection will be.
This defaults to Composite and for most users this will be the best setting as the results will most closely match what your eye sees. It is however possible to make a selection based on a specific aspect of a color, these being Red, Green, Blue, Hue, Saturation and Value.
The Fuzzy Select Tool can be a very easy way to produce quite complex selections that would not be possible with some of the other selection tools available in GIMP, though it can require a few attempts to achieve the desired result with some images.