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. Packbit compression was also used in ILBM files.

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.

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

This corresponds to the unpacked bitstring AA AA AA 80 00 2A AA AA AA AA 80 00 2A 22 AA AA AA AA AA AA AA AA AA AA, as shown in the following table:

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

SubUnpackBitsDemo()DimFileAsVariantDimMyOutputAsStringDimCountAsLongDimiAsLong,jAsLongFile="FE AA 02 80 00 2A FD AA 03 80 00 2A 22 F7 AA"File=Split(File," ")Fori=LBound(File)ToUBound(File)Count=Application.WorksheetFunction.Hex2Dec(File(i))SelectCaseCountCaseIs>=128Count=256-Count'Two's ComplementForj=0ToCount'zero-basedMyOutput=MyOutput&File(i+1)&" "Nextji=i+1'Adjust the pointerCaseElseForj=0ToCount'zero-basedMyOutput=MyOutput&File(i+j+1)&" "Nextji=i+j'Adjust the pointerEndSelectNextiDebug.PrintMyOutput'AA AA AA 80 00 2A AA AA AA AA 80 00 2A 22 AA AA AA AA AA AA AA AA AA AAEndSub

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/ */functionstr2hex(str){returnstr.split('').map(function(char){varvalue=char.charCodeAt(0);return((value<16?'0':'')+value.toString(16)).toUpperCase();}).join(' ');}functionhex2str(hex){returnhex.split(' ').map(function(string){returnString.fromCharCode(parseInt(string,16));}).join('');}/** * PackBits unpack function *  * @param {String} data * @return {String} */functionunpackBits(data){varoutput='',i=0;while(i<data.length){varhex=data.charCodeAt(i);if(hex==128){// Do nothing, nop}elseif(hex>128){// This is a repeated bytehex=256-hex;for(varj=0;j<=hex;++j){output+=data.charAt(i+1);}++i;}else{// These are literal bytesfor(varj=0;j<=hex;++j){output+=data.charAt(i+j+1);}i+=j;}++i;}returnoutput;}varoriginal='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 AAconsole.log(str2hex(data));