posterisation

Posterisation is a digital image effect that reduces the number of distinct colors in an image. It is also known as color quantization or color reduction. This process works by mapping a wide range of colors to a smaller, predefined palette of colors. The result is a simplified, graphic appearance that resembles a poster.

The effect is achieved by limiting the number of bits used to represent each color channel (e.g., red, green, blue). For example, a typical 24-bit color image can represent over 16 million colors. Posterisation might reduce this to, say, 256 or even just 16 colors. Algorithms are used to determine which colors from the original image should be replaced by the closest available color in the limited palette. Dithering techniques can sometimes be employed to simulate intermediate colors by strategically placing pixels of the available palette colors.

This effect can be used for artistic purposes, to create a stylized or retro look. It is often seen in graphic design, illustration, and certain types of photography. In some cases, posterisation can also be used to reduce file size for images where exact color fidelity is not critical. However, excessive posterisation can lead to noticeable color banding and a loss of detail. Most image editing software offers a posterise filter or adjustment that allows users to control the number of colors in the output image.