Done using GIMP 2.10.20 on Kubuntu Linux 18.04 LTS
So, to begin I now have updated to the current GIMP 2.10. I am not going to go back and re-do any of my previous tutorials, which are at a pretty basic level anyway, but from here on in I should be using something with the latest features. For me it opened in Single-Window Mode after installation, but that is probably something it picked up from my existing 2.8 installation. I did need to go to Edit–>Preferences–>Theme to select the System theme, which is the lightest one available. There is a notable change in the layout which we hit right away, since the Layers dialog moved from the Top Right to the Bottom Right. Of course, all of these dialogs are dockable, which means you can click-and-drag them to any position you like. I am going with the defaults for the benefit of new GIMP users.
So now we can begin to work with Layers. Of course you would start by making sure that the Layers tab is open in the bottom right in GIMP 2.10. (Reminder: I am working with GIMP in Single-Window Mode. Go to the Window menu to make this change.) If you have just opened GIMP and not done anything else it will be empty. But as soon as you create a new image you will see your first layer. Here is what happened when I created a new 640×400 image with a White background:
This layer is the only one so far, but you are only limited by your computer’s memory in how many layers can create. A GIMP expert might create dozens of layers for just one image. Next, note that there is an eye icon on the left. This tells us that the layer is currently visible. This is a switch, so you click on the eye to make it invisible, then click again to make it visible. Finally, it displays the background color, which in this case is White.
Now suppose you were instead going to work on an existing photo or image? As soon as you open the photo, a layer will be created with that photo:
In this case again the layer is visible as seen by the eye icon, but instead of the background we see a thumbnail of the photo along with the name of the photo.
Adding a New Layer
At the bottom of the Layers dialog box there are icons for a number of layer tasks. The first of these is the Create a New Layer button:
If you click this button, a new dialog opens for creating this new layer:
First, you can give your new layer a name in the top space. Then if you like you can give it a color tag. This only affects how the layer will appear in Layers dialog. For example, if I pick the Yellow color, I will see this:
It only puts in a background behind the Eye icon on the left. This could be useful if you were working with a number of layers and wanted to group them by type or category.
For the moment, we will skip over Modes, because there 38 of them and they are a more advanced topic, and we cannot do anything to Blend space Since it is grayed out.
Composite space has to do with the way the three color channels (RGB) are combined. Leaving this to Auto is probably fine for most purposes.
Composite mode has to do with how layers combine. If you look at the options and see something that looks vaguely Boolean you are on the right track. If you want two layers to add their contents together pick Union, or just where they intersect, choose Intersection. The other two options are to keep all of one image and add in anything that overlaps form the other image.
Opacity comes in when you remember that we are thinking of layers like they were a stack of transparencies. If something is 100% opaque, nothing below it can be seen. But as you reduce the opacity more and more of the layer(s) beneath it will come through.
Width and height are by default pulled from the existing layer when you create a new one. That is good because 99% of the time that is what you want. You just want a stack of identically-sized layers. But you could change the dimensions if you wanted.
The fill is by default Transparency, but you could also fill it with a color or a Pattern. This is again a good default because more often than not you would want a new layer to have a transparent background.
As to the Switches, by default your new layer will be visible.
The thing you should keep in mind is that you can change these settings for an existing layer simply by right-clicking on the layer. That will pop up a dialog that lets you change the properties of the Layer:
Opacity and Transparency
We have seen that we have settings for two things that seem related, Opacity and Transparency. And what does it mean to create a layer that is Transparent and also 100% Opaque? And when you understand it there is no contradiction here. Transparency is the background setting of the layer. That means that if draw something on my layer, and then stack it on top of another layer, my drawing will be on top, but the rest of the image will come through from the bottom layer because of the transparent background. Same thing if I drop some text onto the transparent layer. This is something that is very often done in creating images. You add some text on a transparent layer, and you can play around with the text all you want and have no effect on the underlying image. Opacity, on the other hand, would affect the text. If the opacity is less than 100%, some of the underlying image from the beneath layer would come through (technically, it would be mixed in by an algorithm). So they are related, but they have distinct effects.
When working with layers, it is important to remember that we are thinking of them like they are sheets of transparent film stacked one on another. And that means that the order matters a lot. If you have a layer with a background that is not transparent, that has to be the bottom layer. Otherwise, it might block all of the other layers from being seen. That is why most layers have transparent backgrounds. But even so, all of the layers have content on them that might overlay anything beneath them. So to keep the order straight, layers will display in top-to-bottom order in exactly the order they appear in the Layers Dialog
If the layers are not in the order you need them to be in, you can use the up and down arrows on the bottom to move a layer up or down in the stack. Remember that the first layer in the Layers dialog will be the Top layer, so it may obscure everything underneath it. And the bottom layer in the Layers dialog will be the bottom of the stack, so if you need an opaque background of some kind this is where it should go..
If you are creating an image, such as a logo, you could open a new image, give it a background, then draw images on it, then add text boxes, and so on, until you had a completed image. And while this would work, I would counsel you to not do it this way. The better way would be to create a series of layers. Make the background a layer of its own. Make each image element a layer of its own. Then add a text layer, or maybe even several. For example, if you were doing a logo for a company that had several different locations, you could put the address text on its own layer. In fact, you could create all of the address layers at once, and use the “eye” icon to select which one is visible at any moment. That way you can keep everything in one *.xcf file, and then export different *.jpg files as needed for each location.
You should probably try to play with this by creating a new image in GIMP, and then creating layers for it. Put different things on each layer, and see how the result changes when you move layers up and down. Then turn some layers off by clicking the “eye” icon, and then turn them on again. These are all basic skills for working with Layers, and getting comfortable with them will help a lot.