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Antialiasing is a technique used in digital imaging to reduce the visual defects that occur when high-resolution images are presented in a lower resolution. Aliasing manifests itself as jagged or stair-stepped lines (otherwise known as jaggies) on edges and objects that should otherwise be smooth.
Antialiasing makes these curved or slanting lines smooth again by adding a slight discoloration to the edges of the line or object, causing the jagged edges to blur and melt together. If the image is zoomed out a bit, the human eye can no longer notice the slight discoloration that antialiasing creates.
Jaggies appear when an output device does not have a high enough resolution to represent a smooth line correctly. This is also an inherent problem on a computer monitor. The pixels that make up the screen of the monitor are all shaped in rectangles or squares. Because lighting up only half of one of these square pixels is not possible, the result is a jagged line.
The jagged line effect can be minimized by increasing the resolution of the monitor, making the pixels small enough that the human eye cannot distinguish them individually. This is not a good solution, however, because images are displayed based on their resolution. A single image pixel may take up many monitor pixels, making it impossible for a higher resolution monitor to mask the jagged edges. This is where antialiasing is required.
Antialiasing removes jagged edges by adding subtle color changes around the lines, tricking the human eye into thinking that the lines are not jagged. The slight changes in color around the edges of an image help the line blend around curves, giving the impression that the line is true. These color changes are made on a very small scale that the human eye cannot detect under normal circumstances. In order to be able to see that an image has been antialiased, it would have to be magnified.
Antialiasing is often implemented by graphics cards and computer games. Depending on the application, different methods of antialiasing may be applied. This technique is also used in digital photography and digital audio.