Advanced GIMP Layer: the Alpha Channel {Complete Guide}

alpha channel layers

In this series of (7) GIMP layer guides, we’ve covered all the basics. This final series will explore a more advanced methodology for using layers in GIMP, the Alpha Channel.

If you’re just starting out with GIMP or would like to learn all there is to know about layers, I recommend the other 6 articles in this series (in this order);

Table of Contents

What Is An Alpha Channel In GIMP?

alpha channel

Let’s first explore what a “channel” is. In GIMP and other editing software like Photoshop, there are three primary colors that, when mixed together, create the millions of colors you see in an image. This includes Red, Green, and Blue (RGB).

When you capture an image with a camera or smartphone, it’s recording your scene in RGB values! Each of the three colors is separated into its own channel. So, a channel is used to hold these three-pixel color values. If they were all on one channel, it would be difficult to edit an image.

You can see these RGB channels by going to; Windows > Dockable Dialogs > Channels. The one thing missing is the Alpha channel.

When you open an image for the first time, it doesn’t include an Alpha Channel. If you add another image, create a new layer, or duplicate the existing layer. It will then create an Alpha Channel.

You can also add an Alpha Channel by right-clicking on the layer and choosing “Add Alpha Channel.”

What is “Alpha”?

An “Alpha” is a channel too. Although it doesn’t represent any colors. Instead, it’s blank or transparent. Turning it off in the channels does… nothing. But it can affect the layer if it doesn’t have this channel. More on that coming up.

What Is The Purpose of An Alpha Channel?

The purpose of an Alpha Channel is to give a layer or graphic the ability to render transparency. Layers by themselves do not have this channel built-in like the RGB channels. Instead, we have to create one. But the question is why. Let’s find out…

For this next image, I made a (quick) selection of the sky and deleted the pixels. Although the pixels are gone, I’m left with a pink color. Not what you’d expect when removing pixels.

no alpha channel

The reason for the pink color is that it is what I have chosen in the Background Color swatch. If I change the background color to something else, it will render that color when I clear the pixels.

Odds are, you don’t want this solid color. At least I don’t. To fix it, we need to give the layers the magical powers of transparency.

This is done by right-clicking on the layer and choosing “Add Alpha Channel.”

Nothing visibly has changed until you begin erasing the pixels. In this next image, you can see the same square before. But this time, instead of a solid color, I’m left with a grey checkerboard.

alpha channel added

The checkerboard in this image is a visual that represents transparency. Now, when I place another layer below it, I’ll be able to see through the top layer.

new background layer

The bottom layer now shows through thanks to the transparent parts of the top layer.  Note: this is not the perfect background swap! It’s only for demonstration purposes.

How To Adjust the Layer Transparency In GIMP

ghost layer

This image is from another GIMP tutorial on how to remove anything from a photo. After removing the subject, I was able to use the original photo to create the ghost (by lowering the Opacity).

It’s possible to make a layer transparent without an Alpha Channel. All you have to do is adjust the Opacity lower to make this happen. The problem is the entire image is transparent and harder to see.

How To Use the Alpha Channel For Images

The best example of the need for an Alpha Channel is when you want to replace a background or even a sky (as shown in the images above). You first make a selection of what it is that needs to be replaced, and then you clear (or delete) those pixels.

This way, when you replace the background or sky with another image layer below, it will show through the transparent section of the top layer!

A more advanced and recommended method for making a part of an image transparent is through using a Layer Mask. Check out this tutorial on how to use a Layer Mask to replace a background.

How To Use the Alpha Channel For Graphics

Let’s look at an example of adding an Alpha Channel for text effects. The design below comes from this GIMP text tutorial.

waves

Adding an image to text can be done in several ways. I prefer selecting the text, cutting it out, and placing the picture below the text layer. Once cut out, you will then see the image show through the text.

This is only possible when you use the “Add Alpha Channel” tool. Otherwise, as before, you’ll end up with a solid color and no transparency.

Now What?

Have you completed all the layer guides in this series? If not, I’d recommend going back here to check out all the tutorials on layers.

Like this article? If so, please share!

2 Responses

  1. might want to mention that when you decide to save a file as a JPG (by exporting it), the multiplication process is used to figure out the final color values for each X,Y- and the alpha channel is lost.

    similarly, if you save a layered composition as a PNG file, you lose the alpha values associated with each layer. your result, then, is reduced to a color value for each X,Y and also a single alpha value.

    For the math inclined, you might think that one should be able to represent color with a single scalar value (eg wavelength, or frequency) and then how much of that frequency is present with another scalar value- lets call it “gain” or “intensity”. That is a silly idea, though, and even representing light as a linear combination of 3 frequencies with their respective intensities (such as RGB) or 4 dimensional systems (like uhm CYAN, MAGENTA, YELLOW , and BLACK) loses a good deal of possible information. A better representation would be https://en.wikipedia.org/wiki/Radiant_flux , which is sort of the linear light version of the more perceptual https://en.wikipedia.org/wiki/Luminous_flux. But those still don’t do light justice. Neither does https://en.wikipedia.org/wiki/Spectral_density – although thats getting closer because then we have a full frequency versus power distribution. To model light more accurately, one needs to think of every “voxel” as having a set of rays cast through it, each ray having its own individual power spectrum. Its a good way to reduce the complexity of the actual situation, which involves billions of photons of all sorts of frequencies moving in all sorts of directions through that voxel. Okay /rant sorry hehe.

  2. Alpha channel, I was wondering. Thanks for the clarity. I am trying to escape Photoshop/Lightroom completely and trying GIMP now. So far WIMP printing is trying. Internet searching about GIMP printing show many comments such as GIMP printing broken, don’t use GIMP to print. Any thoughts about using GIMP to print and if not, what about alternatives which omit PS/LR? Regards, H. Mark Macha

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