BEAR image compression

BEAR image compression was used on the Amiga and Acorn Archimedes computers. It was designed for digitized images with 16 levels of grey. It gives a better compression ratio than GIF on continious tone images and is very simple to implement. The small differences between pixels is coded with short symbols, large differences are coded with 7 bits and loads the absolute value of the pixel.

   Code   Operation
      0 - No change
     10 - Decrease brightness
    110 - Increase brightness
111xxxx - Set pixel to xxxx

(Codes are transmitted from left to right)

Any code that could be replaced by a shorter code or would go outside the range of 0-15 is reserved. These codes were supposed to be used for run-length encoding but this was never implemented.

This algorithm ran efficiently on the ARM CPU in the Archimedes but it was fairly slow on the MC68000 CPU in the Amiga.