How to Use GIMP’s Bucket Fill Tool

The Bucket Fill tool in GIMP seems like one of the simplest tools available in this powerful image editing app, but it has some surprising tricks up its sleeve that are great for specific situations.

It can handle everything from background changes to quick color patches to complex line art detection that prevents your fills from spreading out of control. 

Let’s take a closer look at how it works! 

How to Use the Bucket Fill Tool in GIMP

In its most basic form as a color fill tool, GIMP’s Bucket Fill is very simple to use. Select the Bucket Fill tool using the bucket icon in the Toolbox or by pressing the keyboard shortcut Shift + B. 

By default, the Bucket Fill tool is set to fill your selection or active layer with the current foreground color. 

To fill an entire layer with your current foreground color, place your cursor anywhere within your document and click once. 

If you want to fill a specific area, make a selection around it first, and then click anywhere inside the selection. 

To change the current foreground and background colors, click the swatches at the bottom of the Toolbox. GIMP will open a color selection dialog window that allows you to choose your desired color using a number of different options and formats. 

Your foreground color is displayed in the upper left swatch, and the background color is displayed in the bottom right swatch. If you forget which is which, remember that the foreground swatch slightly overlaps the background swatch. 

You can quickly swap the two selected colors from foreground to background by pressing the X key, or you can reset them to the defaults of black foreground color and white background color by pressing the D key. 

Simple enough, right? But the Bucket Fill tool also has a number of other settings that can transform it from a simple color fill tool into a more complex version of itself.

Configuring the Bucket Fill Tool 

For such a simple tool, Bucket Fill actually has a remarkable number of customization options that can dramatically affect how it operates. 

If you’re not getting the results you wanted or expected from the default settings, then take a closer look at the Tool Options panel to see if any of the following options will do the trick.

Mode

Blending modes are one of the most powerful but underused features in GIMP – and they’re also one of the most frequently misunderstood. 

Blending modes change the way that the pixels in each layer are composited with other layers, which is why they have such dramatic effects, but the reasons behind the results are buried in complex mathematics that we don’t have time to go into here. There are 38 different blending modes, after all, and each one is extremely technical.  

Suffice it to say that if you need to use a specific layer mode, it’s usually a better idea to apply it to the layer as a whole using the Mode setting in the Layers panel rather than applying it just to your Bucket Fill tool results, since this approach gives you some additional flexibility later on in the editing process. 

Opacity

The Opacity setting determines the overall transparency of your fill contents. 

Like the Mode option, I believe that this setting is best applied at the layer level rather than on your fill area specifically.

Once you’ve applied your fill at a specific opacity, you can’t change that setting, but if you apply the fill at 100% on a new layer and then adjust the opacity setting for the whole layer, you can tweak it and change it at any time to get the perfect result. 

Fill Type

This setting is fairly self-explanatory: you can apply your fill using the currently active foreground color or the currently active background color, or you can fill using a pattern. 

GIMP’s default pattern library is a bit basic, but a pattern fill feature is a powerful tool when used with your own custom patterns. As long as the edges of your pattern image tile together properly, you can seamlessly fill any size space. 

Affected Area

Since the Bucket Fill tool is arguably a semi-automatic tool, there are a couple of key options that control how the fill is applied to your document. Depending on the Affected Area that you select, you’ll get different options that allow you to fine-tune the result. 

Fill whole selection is the default option, and it does more or less exactly what it says on the tin: it will fill your entire selection area. If you don’t have an active selection, it will fill the entire active layer instead. 

Fill similar colors uses the same principles as the Fuzzy Select tool to automatically decide which areas of your image should be filled and which shouldn’t. 

You can include or exclude transparent areas, sample every visible layer or only the active layer, and specify whether diagonally-connected pixels should be checked for matching colors or not. 

The most important setting in this section is the Threshold setting. Threshold is a universal concept in GIMP, so it’s a good idea to learn how it works. Ranging from 0-255, Threshold specifies how closely a pixel has to match with your selected pixel in order to be filled. 

If you click on a pure white pixel with a Threshold set to 0, only other pure white pixels that are directly connected to your selected pixel will get filled. If you set to do the same with the threshold set to 127, all pixels that are 50% gray or lighter will be filled. 

Fill by line art detection

This is probably the most unusual setting that I can think of in a GIMP tool, but it’s also a very useful option for certain types of projects. 

If you have to fill line art with color using the Fill Similar Colors option, you may notice that your bucket fills won’t stay within the intended shapes, because the lines of the artwork don’t always fully enclose a space. This allows the fill to expand outside its intended boundaries, and basically ruins your coloration project.

If you select Fill by line art detection, GIMP uses some fancy math wizardry (and some custom settings) to ‘guess’ where object edges should be, even in the absence of a fully-enclosed shape. This is a bit hard to explain, so check out the basic example below. 

Normally, if you tried to fill this circle using the Bucket Fill tool, the fill would not be contained by the almost-complete circle, and the entire white area would get filled. But with the Fill by line art detection setting enabled, look at what happens:

GIMP uses the Bucket Fill settings to determine that this circle was probably supposed to be closed, and it doesn’t allow the fill to extend far beyond the boundaries of the circle. 

A Final Word

That covers everything you need to know to use GIMP’s bucket fill tool successfully in any project! Bucket fills are a handy feature that you will find yourself returning to regularly, so it’s a good idea to practice your keyboard shortcuts and get familiar with the tool and all of its options. 

Happy filling!

I’ve been working with digital images since the year 2000 or so, when I got my first digital camera. I've tried many image editing programs. GIMP is a free and powerful software, but not exactly user-friendly until you get comfortable with it, and I wanted to make the learning process easier for you here.