a dither algorithm; small, spatially stable, based on magic numbers and arithmetic.
GIF animations and eink benefit from the spatial stableness; since fewer pixels change during partial changes, The tiny code size; and not LUT makes it suited for small footprint code/logic size implementations.
|bayer 1.2M||a dither 1.3M (addition based)||a dither 1.3M (xor based)||error diffusion 2.9M|
The above videos shows how the dither applies to a bitdepth reduction task of RGB 8:8:8 to RGB 3:3:2 with a total of 256 colors - below is a 1bit dither with various methods on a gray ramp.
as of 2014-03-25 ffmpeg master has these dithering methods; the way the addition based GIF above was created is the following commandline:
ffmpeg -i input.mp4 -vf scale=320x180:sws_dither=a_dither -r 10 -ss 40 -frames 100 -pix_fmt rgb8 output.gif -y
The main novelty of a dither is its simplicity/compactness and ease of implementation. For dithering to RGB 565 or 24bit RGB it is sufficient, it is however not faster than using a LUT with dither array, either of a dither's or a much higher quality dither mask like void and cluster. The way a dither is employed is exactly like other dither masks would be - but it is procedural/shader based. If someone comes up with better magic numbers to use on variations of the above formulas, please tell me :), I consider a-dither public domain.
The part of the big-buck bunny clip chosen is chosen to highlight problems of 'a dither', if I wanted to make a-dither look good I would instead hunt out examples making bayer and error diffusion look bad ;)Øyvind Kolås, firstname.lastname@example.org , august 2013