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PackBits - Just Solve the File Format Problem

PackBits

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File Format
Name PackBits
Ontology

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

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