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Layers are used in digital image editing to separate different elements of an image. A layer can be compared to a transparency on which imaging effects or images are applied and placed over or under an image. Today they are an integral feature of image editors.
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Layers were first commercially available in Fauve Matisse (later Macromedia xRes), and then available in Adobe Photoshop 3.0, in 1994, but today a wide range of other programs, such as Photo-Paint, Paint Shop Pro, GIMP, Paint.NET, StylePix, and even batch processing tools also include this feature. In vector images editors which support animation, layers are used to further enable manipulation along a common timeline for the animation; in SVG images, the equivalent to layers are "groups".
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.
In graphics software, a layer is the term used to describe the different levels at which you can place an object or image file. In the program you can stack, merge or define layers when creating a digital image. Layers can be partially obscured allowing portions of images within a layer to be hidden or shown in a translucent manner within another image, or you can use layers to combine two or more images into a single digital image. For the purpose of editing, working with layers allows you to go back and make changes within a layer as you work.
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 colour 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 layer mask 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.
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 changing 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.