Layers. Layers are a key concept of computer graphics that make use of the computer's ability to hold various properties of an image in memory. Even properties that you can't see. It won't just be different variations of the same image options that you can hold in memory, like contrast or brightness, but parts of the image within itself. Let's try to explain with a few visual examples.

Examine the image below, examine it closely, very closely.

Now at first it would seem that what you are looking at is a single image of a smiling face. But it's actually a combination of 3 different images. Within a painting program for instance you might see a "layer browser" that will show you all of your available layers (most programs have no limit on how many layers you can use) but the main canvas area will show the final, combined version.

Here are what the different layers may look like if they were extracted and separated from one another.

As said there may be a "layer browser" within your program that shows you these layers. Within Adobe Photoshop for instance the layer browser for the above image will look like the image below...

The important thing to take note of here is this: The higher layers in the layer browser will appear on top of the of the lower layers when seen on the final product. In this particular example the hair layer is *above* the other layers within the layer browser and so it appears *on top* of all the other images within the combined image.

The beauty of this system is that layers are a continuous aspect of your project. Imagine the 3 images above as if each was printed on transparent plastic or painted upon a pane of glass. If those 3 painted panes of glass were stacked one on top of the other you would see the combined image of all 3. And just like those panes of glass you can pull one individual layer out to work on without touching the other remaining layers.

Be aware that there are only a few particular file types that support the use of layers.

File Extension Layer Support
.bmp no
.gif no
.jpg no
.psd yes
.png no
.raw no
.xcf yes

Yes, when I said a few, I meant a few. It's important to keep in mind which file types you are saving to because if you do save to a file type that does not support layers, a .jpg for instance, then you will lose the layer information of the project you are working on. The different layers will be combined into a single image and you will not be able to separate them again.

So no matter what kind of project you are working on always remember to keep your original project file saved somewhere so that you can access the raw form of the project with the layer information (and any other non-destructive edits) again if you need to.

While actually editing an image you may or may not see a light grey checkerboard pattern in the background of your image. This is pretty much the univeral indication of transparency. This means that if, for instance, you were editing an image in Adobe Photoshop and exported an image with this type of background then you will have parts of the image that are transparent if you are exporting a file format that supports transparency.

We'll talk about transparencies in computer graphics more in depth in the next section.

If you are studying 3D modeling and animation then you may still also encounter a similar idea of "layers". In that case it can have multiple uses however. In some cases it can refer to the hiding of different sets of objects to more easily work on a different set of objects. Other times, specifically within 3D sculpting, it may refer to keeping layers of detail on the same surface so that you can toggle on and off different changes to that model.

All of these uses will be covered more in depth in their particular pages.

Here is an amended version of the file format table that indicates which formats can contain transparency.

File Extension Layer Support Transparency Support
.bmp no no
.gif no yes
.jpg no no
.psd yes no
.png no yes
.raw no no
.xcf yes no