How to Fill an Area with Transparency in GIMP

One of the most common image editing tasks is to place an object on a transparent background so that it can be placed into new images in a realistic manner, and there are plenty of other tasks you can accomplish in GIMP that also require working with transparent areas. 

But when you’re deleting pixels, GIMP doesn’t assume that you want to create a transparent area, and depending on your image settings, it will often just fill the selected area with your background color instead. 

Let’s take a look at how you can fix it and fill any area you want with transparency. 

The Quick Way to Fill an Area with Transparency in GIMP

If you want to cut right to the chase, here’s how to create a transparent area in GIMP:

  • Step 1: Open the Layer menu, select the Transparency submenu, and click Add Alpha Channel.
  • Step 2: Select the area you want to make transparent using any of GIMP’s selection tools.
  • Step 3: Press the Delete key.

That’s all there is to it! All the magic happens in that first step, and that’s where most people get confused because most image editing apps take care of the process for you automatically. 

If you want to peer behind the curtain and learn more about channels and transparency in digital images, or if you just want to explore the steps in a bit more detail, then read on – I’ll even include a bonus method at the end for automatically converting an entire color to transparency across your whole image.

What is an Alpha Channel?

All digital images are made up of separate color channels, which are combined together to create the colors you see on the screen. 

Most digital images use the RGB color space, which means they have three channels: a Red channel, a Green channel, and a Blue channel – RGB. 

Each color channel is just a grayscale image, but the white pixels in the channel correspond to the pure color that the channel represents. Grayscale pixels are used to represent partial color values, while black pixels in a color channel indicate that none of the corresponding color should be visible.  

In order to display a transparent section instead of colored pixels, digital images need an additional type of channel known as an alpha channel. Instead of storing color information, the alpha channel stores transparency information. 

White pixels in the alpha channel indicate that the corresponding area is fully visible, while black pixels indicate that the corresponding area is fully transparent. Grayscale pixels can be used to indicate partially transparent areas. 

GIMP is one of the only image editors I’ve ever worked with that forces you to add an alpha channel manually instead of just adding it for you automatically. 

Arguably, the extra step that GIMP requires makes everything more precise, which may be of help to people who use GIMP for technical imagery, but for casual users who just want to edit their photos, it feels unnecessarily confusing.   

Making a Selected Area Transparent in GIMP

Now that you have a better understanding of the technical side of digital images and how they can create transparency, let’s take another look at the steps to fill an area with transparency in GIMP. I’m going to assume that you have already opened the image you want to work on, but if not, start with that. 

Step 1: Add an Alpha Channel

With your image loaded in GIMP, open the Layer menu, select the Transparency submenu, and click Add Alpha Channel

GIMP doesn’t provide any feedback that the step has been completed, but you can verify the existence of an alpha channel by checking the Channels panel. By default, the Channels panel is located in a tabbed layout next to the Layers panel, in the bottom right corner of the interface. 

If it’s not visible, you can bring it back by opening the Windows menu, selecting the Dockable Dialogs submenu, and clicking Channels

Step 2: Make a Selection

This is usually the hardest stage of the process, but it all depends on what you’re trying to accomplish. 

If you’re removing the background from a product image or something similar, you’ll need to use a combination of tools to carefully select all of the background pixels, but if you just want to cut a round hole in your picture, that’s simple enough to do with the Elliptical Selection tool. 

You can use any of GIMP’s selection tools to make the selection, so feel free to experiment with the different options to find the one that works best for your project. 

Step 3: Delete!

Once you’re happy with your selection, simply press the Delete key, and GIMP will remove all the pixels in the selected area, turning them transparent. 

Transparent pixels are represented by a repeating checkerboard pattern in GIMP. This is fairly standard across image editing apps, although I think it is another design legacy from Adobe that every other developer has simply adopted. 

Now that you’ve got a working alpha channel, you can delete as many areas of your image as you need for your project. 

Bonus Method: Color to Alpha

As I promised back at the beginning, there’s one quick tool in GIMP that can quickly and easily convert an entire color into transparency across your whole image. This is perfect for product shots on solid-color backgrounds and many other projects, and it can save a huge amount of time when compared to selecting the entire background by hand.

Open the Colors menu and select Color to Alpha.

GIMP will open the Color to Alpha dialog window, which gives you a few customization options, such as which color should be converted to transparency. 

By default, the Color value is set to white, but you can use the eyedropper icon next to the Color setting in order to select the background color of your image (or whichever area you want to remove). 

I didn’t even have to click, and it removed the background for me! 

This method has the unusual benefit of making it relatively simple to transfer an object’s shadow onto a new background since the shadow pixels aren’t technically the same color as the rest of the background that you’re removing. 

When using other techniques, shadows often need to be modified or entirely faked, which can destroy the composite visual illusion you’re creating unless it’s done properly. 

Color to Alpha won’t always make this process so easy, but when it does, it’s a real timesaver. 

Unfortunately, it can also have the negative result of converting white specular highlights from your object when removing a white background, although you can quickly paint with pure white underneath these areas on another layer to restore them. 

A Final Word

Congratulations, you’ve just learned how to fill an area with transparency in GIMP using a couple of different methods! While it can be a bit tedious to have to create the alpha channel manually, it’s not very difficult once you remember that it’s the missing piece of the puzzle. 

Enjoy the emptiness! 😉

About Thomas Boldt
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.

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  • Stephen

    Thanks Thomas. I’ve been working as a developer for many years (> 40) but never really understood how transparent backgrounds worked until now.