Ordered dithering

In this example, the original photograph is shown on left. The version on the right shows the effect of quantizing it to 16 colors and dithering using the 8×8 ordered dithering pattern.
The characteristic 17 patterns of the 4×4 ordered dithering matrix can be seen clearly when used with only two colors, black and white. Each pattern is shown above the corresponding undithered shade.

Ordered dithering is any image dithering algorithm which uses a pre-set threshold map tiled across an image. It is commonly used to display a continuous image on a display of smaller color depth. For example, Microsoft Windows uses it in 16-color graphics modes. With the most common "Bayer" threshold map, the algorithm is characterized by noticeable crosshatch patterns in the result.

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