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A > Aliasing and Anti-aliasing - Pg. 24

A Aliasing and Anti-aliasing 24 Digital systems store information about the original image as a code. When a display or printer interprets that code to create an image we are seeing an alias. If the sampling process is not sufficiently good ­ if it does not produce enough samples ­ we see a distortion of the original. In the world of signal processing, this is called aliasing. Techniques to overcome poor digitisation are known as anti-aliasing. This can involve adding false data; using grey pixels of varying shades around the image edges to smooth out the steps (jaggies) produced by the sampling process. Anti-aliasing is used to achieve a smooth selection edge when selecting a range of colour pixels in Photoshop. Vector Smooth line and detail of original vector (mathematically described) image. Alias Aliasing in sampled image. The staircase lines are referred to as `jaggies' and are the inevitable outcome of producing a raster image (sampled across a regular grid) from a vector image. Anti-alias Anti-aliasing minimises the effect of sampling and smoothes out the jaggies using shaded pixels to smudge the outline. see Artefacts 31, Moiré Pattern 165