PackBits
PackBits (or Apple Macintosh PackBits, or Macintosh RLE) is a form of run-length encoding often used to compress image data.
Contents |
Format details
The compressed data stream consists of a code byte, followed by zero or more data bytes, followed by another code byte, and so on.
| Code byte (N) | Instructions |
|---|---|
| 0 ≤ N ≤ 127 | Interpret the next N+1 bytes literally. |
| N = 128 | Reserved |
| 129 ≤ N ≤ 255 | Repeat the next byte 257−N times. |
For whatever reason, PackBits is usually described in terms of signed bytes, which leads to the equivalent definition:
| N = −128 | Reserved |
| −127 ≤ N ≤ −1 | Repeat the next byte 1−N times. |
| 0 ≤ N ≤ 127 | Interpret the next N+1 bytes literally. |
The reserved code byte (128 or −128) should be ignored, unless a format specification says otherwise. A few formats use it to mark the end of the compressed data.
Variants
PackBits compresses a sequence of bytes, but variants exist in which a unit of decompressed data is more than one byte (e.g. some PICT, Palm bitmap, and IFF-DEEP compression schemes), or even less than one byte (e.g. GlowIcons).
Related formats
Formats that may use PackBits compression include MacPaint, PICT, TIFF, ILBM, and DEGAS.
Specifications
- TIFF 6.0 Specification, Section 9: PackBits Compression
- Technical Note TN1023: Understanding PackBits
- AtariForumWiki: PackBits Compression Algorithm
