dithering vs ordered dithering

I understand how dithering works etc, but what is the differance be开发者_Go百科tween dithering vs ordered dithering?

Also can anyone point me to some good resources?

Taken from here:

Random dither

Random dithering could be termed the "bubblesort" of digital halftoning algorithms. It was the first attempt (documented as far back as 1951) to correct the contouring produced by fixed thresholding, and it has traditionally been referenced for comparison in most studies of digital halftoning. In fact, the name "ordered dither" (which will be discussed later) was chosen to contrast random dither.

Ordered dither

While patterning was an important step toward the digital reproduction of the classic halftone, its main shortcoming was the spatial enlargement (and corresponding reduction in resolution) of the image. Ordered dither represents a major improvement in digital halftoning where this spatial distortion was eliminated and the image could then be rendered in its original size

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The main difference between dither vs ordered dither is the way of the quantisation error spreading.

1. Dither - the quantisation error is spread (in Floyd-Steinberg) from the current pixel, to the right, bottom and right-bottom pixels. Thus, every pixel quantisation affects the neighbour pixels. In results, the dithering has more smooth vision (like drawing with strokes)

2. Ordered dither - the quantisation error is used from a pattern (matrix) with specified size. While processing pixels, a corresponding value for threashold is obtained from the pattern and applied to the pixel. The type of thresholds distribution in the pattern, determines the visual effect that will be produced.

Usually, the thresholds are evenly distributed and the result image is as smooth as possible.

For example, if the high-value thresholds are concentrated around the center of the pattern, the effect is "halftoning"

In conclusion it is worth to mention that Ordered dither is far simple and much faster. It was used back in the '90s in Windows 95/98 when the monitors' resolution were 256 colors or 16bit colors.

You can get source code and demo project from here