ThunderScan compression
From Just Solve the File Format Problem
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Each coded byte represents one of four things, as indicated by the two highest bits: | Each coded byte represents one of four things, as indicated by the two highest bits: | ||
| − | + | {| class="wikitable" | |
| − | + | ! Byte value | |
| − | + | ! Interpretation | |
| − | + | |- | |
| + | |<code>00xxxxxx</code> || A run of pixels whose colors are the same as that of the previous pixel | ||
| + | |- | ||
| + | |<code>01xxxxxx</code> || Three pixels whose colors are very similar to that of the previous pixel | ||
| + | |- | ||
| + | |<code>10xxxxxx</code> || Two pixels whose colors are very similar to that of the previous pixel | ||
| + | |- | ||
| + | |<code>11xxxxxx</code> || A single pixel of arbitrary color | ||
| + | |} | ||
== Software == | == Software == | ||
Revision as of 17:59, 9 May 2013
Contents |
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:
| 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.
