ThunderScan compression

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:

Software

 * (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.

Possibly-related things named ThunderScan include:
 * An image file format supported by old versions of Photoshop (including v1.0.1 - see the source code)
 * A scanner device for Apple II computers
 * ThunderScan User's Guide
 * ThunderScan GS Software Guide