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Separate "drawing boards" or "canvasses" in a drawing, CAD or image editing program that are overlaid one on top of the other to complete the picture. Layers provide greater control over the illustration or image and offer different functions depending on the application. For the differences in graphics applications, see graphics.

Drawing Programs (Vector Graphics)
Computer-aided design (CAD) programs were the first software to provide layers, and advanced illustration programs followed suit. Graphic objects can be placed in separate layers on top of or below other layers, enabling related objects to be worked on as a group without affecting the rest of the drawing. See graphics and CAD.

Imaging Programs (Bitmapped Graphics)
Layers are available only in advanced imaging programs such as Photoshop, otherwise all editing is done on a single plane. In single-layer paint and image editing software, when a graphic object is added, it replaces the pixels that occupied that same space. Consequently, if the object is removed, it leaves blank space in its stead.

However, in a layered imaging program, objects are placed in separate layers and can be freely moved under and over each other. Objects can always be added, and any object can be removed or changed without affecting the others. The final result is the composite of all layers, which can be "flattened" into one layer for conversion into a JPEG, BMP, GIF or other bitmapped file format. The original image can be maintained in its layered format for future modifications, such as in Photoshop's proprietary PSD format.

Layers in Photoshop
The bitmap image above is built in 23 layers plus a background layer, enabling the individual elements to be moved independently of the others. The Photoshop layer window (below) shows the layers. The red arrow is pointing to the view option (the "eye") in the chip shadow layer. Clicking the eye toggles the layer between visible and invisible.

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References in classic literature ?
All went merry as a marriage bell until Anne's layer cake was passed.
The layer near the margin is from four to five inches thick, but towards the centre its thickness increases.
And what becomes of these worms when, during the long summer, the surface is hardened into a solid layer of salt?
The ground at the place where we stopped for the night, was incrusted with a layer of sulphate of soda, and hence, of course, was without water.
The iSCSI initiator comprises layers that are key to providing multiple data paths between servers and iSCSI storage targets, like those provided by the iSCSI intelligent storage switch.
They help photographers control perspective to imply depth, stress content and compare subjects in terms of scale, creating layers of meaning that express ideas.
Snowflakes accumulate in layers, called the snowpack, which increase in density (mass per unit of volume) as the snow settles over time (see photo, right).
First, they say, within the 50-cm section of finely laminated rocks lie three thin layers of a greenish mineral called glauconite, an iron-rich silicate that is typically a sign of slow sediment accumulation.
The architecture, comprised of three logically separate layers, separates physical control of the access network from the logical control of its services.
Using your own PC or by contracting the job out to experts, you can easily select information layers from any of 10, 15, or more choices to produce just the map you need.
The product comprises a film layer, two adhesive layers (each directly contacting one outer surface of the film layer) and at least one removable partitioning sheet layer contacting the outer surface of at least one of the adhesive layers.
The more advanced constructions covered here include odd-layer-count constructions, asymmetrical constructions, varying flex layer count designs, integrated ZIF connections, blind and buried vias, air gap flex layer constructions, multiple rigid area thicknesses, and shielded flex layers.