Layers are used in digital image editing to separate different elements of an image. A wide range of image editing programs include this feature. Adobe Photoshop has had the functionality since 1994.
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Layer types
There are different kinds of layers, and not all of them exist in all programs. They represent a part of a picture, either as pixels or as modification instructions. They are stacked on top of each other, and depending on the order, determine the appearance of the final picture.
Layer (basic)
The standard kind of layer is called simply "Layer" in most programs. It contains just a picture which can be superimposed on another one. The picture can cover the same area as the resulting picture, just a part of it, or, in some cases, a bigger part than the final picture.
A Layer can have a certain transparency/opacity and a number of other properties. In a high end program like Adobe Photoshop, a basic layer may have more than a hundred different possible settings. Even though some of them overlap and give the same result, they give a skilled user a lot of flexibility. A free program like the GIMP may not have as many settings, but well used they can often provide a satisfactory result. Two Layers can blend using one of several modes which result in different light and color combinations.
Layer Mask
A Layer Mask is linked to a layer and hides part of the layer from the picture. What is painted black on the adjustment layer will not be visible in the final picture. What is grey will be more or less transparent depending on the shade of grey. As the layer mask can be both edited and moved around independently of both the background layer and the layer it applies to, it gives the user the ability to test a lot of different combinations of overlay.
The blue sky used as background layer.
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The greenery used as top layer.
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The layer mask that was applied to the top layer.
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Adjustment layer
An adjustment layer typically applies a common effect like brightness or saturation to other layers. However, as the effect is stored in a separate layer, it is easy to try it out and switch between different alternatives, without destroying the original layer. In addition, an adjustment layer can easily be edited, just like a layer mask, so an effect can be applied to just part of the image.
