ThunderScan compression

File Format
Name	ThunderScan compression
Ontology	Electronic File Formats Compression 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:

Byte value	Interpretation
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.

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