PackBits

PackBits is a fast, simple lossless compression scheme for run-length encoding of data.

Apple introduced the PackBits format with the release of MacPaint on the Macintosh computer. This compression scheme can be used in TIFF files. TGA files also use this RLE compression scheme, but treats data stream as pixels instead of bytes.

A PackBits data stream consists of packets with a one-byte header followed by data. The header is a signed byte; the data can be signed, unsigned, or packed (such as MacPaint pixels).

In the following table, n is the value of the header byte as a signed integer.

Header byte	Data following the header byte
0 to 127	(1 + n) literal bytes of data
−1 to −127	One byte of data, repeated (1 − n) times in the decompressed output
−128	No operation (skip and treat next byte as a header byte)

Note that interpreting 0 as positive or negative makes no difference in the output. Runs of two bytes adjacent to non-runs are typically written as literal data. There is no way based on the PackBits data to determine the end of the data stream; that is to say, one must already know the size of the compressed or uncompressed data before reading a PackBits data stream to know where it ends.

Apple Computer (see the external link) provides this short example of packed data: FE AA 02 80 00 2A FD AA 03 80 00 2A 22 F7 AA

The following code, written in Microsoft VBA, unpacks the data:

Sub UnpackBitsDemo()  Dim File As Variant  Dim MyOutput As String  Dim Count As Long  Dim i As Long, j As Long    File = "FE AA 02 80 00 2A FD AA 03 80 00 2A 22 F7 AA"  File = Split(File, " ")    For i = LBound(File) To UBound(File)  Count = Application.WorksheetFunction.Hex2Dec(File(i))  Select Case Count  Case Is >= 128  Count = 256 - Count 'Two's Complement  For j = 0 To Count 'zero-based  MyOutput = MyOutput & File(i + 1) & " "  Next j  i = i + 1 'Adjust the pointer  Case Else  For j = 0 To Count 'zero-based  MyOutput = MyOutput & File(i + j + 1) & " "  Next j  i = i + j 'Adjust the pointer  End Select  Next i  Debug.Print MyOutput  'AA AA AA 80 00 2A AA AA AA AA 80 00 2A 22 AA AA AA AA AA AA AA AA AA AA End Sub 

The same implementation in JavaScript:

/**  * Helper functions to create readable input and output  *   * Also, see this fiddle for interactive PackBits decoder:  * https://jsfiddle.net/y13xkh65/3/  */ function str2hex (str) {  return str.split('').map(function (char) {  var value = char.charCodeAt(0);  return ((value < 16 ? '0' : '') + value.toString(16)).toUpperCase();  }).join(' '); } function hex2str (hex) {  return hex.split(' ').map(function (string) {  return String.fromCharCode(parseInt(string, 16));  }).join(''); } /**  * PackBits unpack function  *   * @param {String} data  * @return {String}  */ function unpackBits (data) {  var output = '',  i = 0;  while (i < data.length) {  var hex = data.charCodeAt(i);  if (hex == 128) {  // Do nothing, nop  }  else if (hex > 128) {  // This is a repeated byte  hex = 256 - hex;  for (var j = 0; j <= hex; ++j) {  output += data.charAt(i + 1);  }  ++i;  }  else {  // These are literal bytes  for (var j = 0; j <= hex; ++j) {  output += data.charAt(i + j + 1);  }  i += j;  }  ++i;  }  return output; } var original = 'FE AA 02 80 00 2A FD AA 03 80 00 2A 22 F7 AA',  data = unpackBits(hex2str(original)); // Output is: AA AA AA 80 00 2A AA AA AA AA 80 00 2A 22 AA AA AA AA AA AA AA AA AA AA console.log(str2hex(data)); 

External links

• The TIFF PackBits Algorithm taken from the https://www.fileformat.info site with permission from Corion.net
• PACKBITS Compression or Why We Support Lossless TIFF Compression Method? the article on site https://www.universal-document-converter.com also describes the algorithm.