Opacity is a basic layer control that every editor should be familiar with, and changing layer opacity in GIMP is very simple – so this is going to be a very short guide. There’s only one step to the process, you just have to know where to look!
If you want to learn about more advanced opacity control using layer masks, I’ll include a bonus tip at the end of the post.
The Opacity slider can be found in the Layers panel, which is located in the bottom right corner of the GIMP interface by default (see below for a close-up view).
If your Layers panel is missing, it may have been closed by accident. You can bring it back by pressing the keyboard shortcut Ctrl + L (use Command + L if you’re using GIMP on a Mac), or you can open the Windows menu, select Dockable Dialogs, and choose Layers.
Select the layer you want to make transparent, and then simply adjust the Opacity slider until you’re happy with the result. You can also use the keyboard to enter specific values for more precise control.
You can also edit the layer opacity in the Layer Attributes window, which is accessible by double-clicking on any layer thumbnail or right-clicking the layer and selecting Layer Attributes (see below).
This gives you access to every possible setting you might need for the layer, but it doesn’t really make sense to open the full panel every time you want to change the opacity of your layer.
Using Layer Masks
The other way to change your layer opacity is with a layer mask. Instead of changing the opacity of the entire layer all at once, you can pick and choose which areas you want to be transparent or opaque based on the color painted in the layer mask.
A layer mask acts as an invisible pixel layer. If a pixel is white in the mask, the corresponding pixel in the image layer will display as 100% opaque, and if a pixel is black, the corresponding pixel will be 100% transparent.
Adding a mask to your layer is almost as simple as changing the opacity. Along the bottom of the Layers panel, you’ll find a series of buttons. In the screenshot below, the highlighted one will add a layer mask.
If you don’t want to go hunting for the right button, you can also right-click on the layer in the Layers panel and select Add Layer Mask.
Select the default state you want your layer mask to be in from the available options, and click Add. You’ll notice a new layer mask visible in the Layers panel, and you can switch between the mask and the image content by clicking the appropriate thumbnail.
Using the standard Paintbrush tool, simply paint with black, white, or any gray you desire to adjust the opacity of your layer. Best of all, if you want to go back and edit it later, you can simply edit the layer mask! This is another great example of the value of non-destructive editing.
If you want to see your layer mask in the image window instead of just seeing the results, you can open the Layers menu, select Mask, and choose Show Layer Mask. You can also hold down the Ctrl + Alt keys (Command + Option on Mac) and click on the layer mask thumbnail in the Layers panel.
I’m not sure why GIMP doesn’t include the option to visualize the layer mask as a translucent overlay, but maybe there’s a trick to it that I’m not aware of. I know it can do Quick Masks, so it should be capable of it, but I can’t figure out how to enable it.
That’s just about everything there is to know about how to change opacity in GIMP! As with most things in GIMP, there’s a quick way and a complex but powerful way. I think it’s always a good idea to know them both, and now you do too =)About Thomas Boldt