Going retro: image dithering

Recently I have published the AmigaFFH on CRAN. This package allows you to read, write, interpret and modify Commodore Amiga file formats in R. This seems like a good opportunity to publish a small series about going retro in R. In this post I will show how to apply oldskool dithering techniques to images.

Back in the day home computers had limited memory and graphical capabilities. As a result images were usually stored and displayed using an indexed palette with only a handful of colours. When digitising photographs colour banding occurs due to this limited palette. This is why dithering was introduced. Dithering deliberately adds noise to an image to reduce colour banding when digitising photographs.

In this example I have used an image of the Amiga boing ball as an example. The technique requires two stages. First we need to a palette (first stage) to which the image needs to be dithered (second stage). The palette can be based on the original (true colour) image, or we can force our own palette to the image. In the first case we need a clustering algorithm to determine the most frequently occurring colours which can be used in a palette. In the second case you can pick any colour you like.

Below you see the result of both strategies. The top row selects a palette based on the original image, where from left to right, the number of colours are 4, 8, 16, 32 and 64. In the top row no dithering is applied. You can see how bands of different colours are formed, particularly for the low number of colours. In the middle row, you can see the same images, with the same number of colours, but in this case dither is applied. Even though the same number of colours are applied, the bands are much less apparent due to the dithering. Last, but not least, the bottom row show the same image where we force a black and white palette to the resulting image. From left to right, different dithering methods are used, each with slightly different results.

The source code for generating this image is, us usual, provided below. Note that dithering can be applied to any type of continuous information where the 'depth' is reduced (although currently not implemented in the package demonstrated here). For instance when reducing audio from 16 to 8 bit, dithering can be applied to prevent banding. In the coming month I would like to go retro some more. I will bring back pixels into your life by showing what the AmigaFFH and adfExplorer packages have to offer. And don't forget about the chiptunes which can be used in R using the ProTrackR package.