ThunderScan compression

Overview
ThunderScan compression, or ThunderScan RLE, is a compressed image format for 4-bit grayscale images. It can be used in TIFF files, and is identified by TIFF compression code 32809.

Format
The compression scheme is pixel-oriented, and uses run-length encoding and delta encoding.

Each coded byte represents one of four things, as indicated by the two highest bits:

00xxxxxx = A run of pixels whose colors are the same as that of the previous pixel 01xxxxxx = Three pixels whose colors are very similar to that of the previous pixel 10xxxxxx = Two pixels whose colors are very similar to that of the previous pixel 11xxxxxx = A single pixel of arbitrary color

Software

 * libtiff (read-only)

Sample files

 * pics-3.8.0.tar.gz → libtiffpic/text.tif

Editors notes
Libtiff calls this format "the 4-bit compression algorithm used by the ThunderScan program", but we have not been able to confirm its origin. Nearly all the information about it comes from libtiff.

ThunderScan is a scanner device for Apple II computers.
 * ThunderScan User's Guide
 * ThunderScan GS Software Guide