ThunderScan compression
From Just Solve the File Format Problem
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.
Contents |
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.
ThunderScan is a scanner device for Apple II computers.
