PreComp utility

What is Precomp?

Precomp is a command line precompressor. You can use it to achieve better compression on some filetypes (works on files that are compressed with zLib or the Deflate compression method, and on GIF files). Precomp tries to decompress the streams in those files, and if they can be decompressed and "re"-compressed so that they are bit-to-bit-identical to the original stream, the decompressed stream can be used instead of the compressed one.

The result is a .pcf file (PCF = PreCompressedFile) that contains more decompressed data than the original file. Note that this file is larger than the original file, but if you compress it with a compression method stronger than Deflate, the compression is better than before (or use lprepaq to get it precompressed and compressed in one step).

What is Precomp Comfort?

Precomp Comfort is a variation of Precomp. It supports drag and drop of single files and uses an INI file for the parameters.

It is included in the ZIP file. Precomp.exe is the original version, Precomf.exe is the Comfort version.

What is lprepaq?

lprepaq combines lpaq6 by Matt Mahoney and Precomp. It first precompresses the input file, then compresses it using the powerful PAQ compression method. lprepaq is a complete compressor/decompressor, so use this if you just want to compress your files.

What is prepaq?

prepaq v2 (aka paq8o8pre v2) by Jan Ondrus combines paq8o8 by Matt Mahoney and Precomp. It first precompresses the input file, then compresses it using the powerful PAQ compression method. Just like lprepaq, prepaq is a complete compressor/decompressor, but is it much slower than lprepaq and compression is better.

Filetypes

Here is a list of filetypes that can eventually achieve better compression with Precomp and how you can check if they can.

Note that this list is not complete, and that other filetypes can contain Deflate or zLib streams, too, but you should use the slow mode parameter (-slow) for them.

PDF

Adobe's PDF files often use zLib compression to compress their documents.

Check: "FlateDecode" appears in the file.

JPG

Precomp uses packJPG by Matthias Stirner to losslessy compress JPG images.

MJPEG

MJPEG is a video format that consists of JPG images without huffman tables. Precomp inserts them so that packJPG is able to compress the images.

ZIP/JAR

Most ZIP files use Deflate for compression. JAR files basically are ZIP files with an additional manifest for use with Java.

PNG

PNG uses Deflate to compress its filtered image data.

GIF

The GIF format uses LZW to compress its image data.

GZ

GZip files use Deflate for compression.

BZ2

bZip2 is a format often used in Linux environments.

SWF

Macromedia's Shockwave Flash files can use zLib compression since Version 6.

Check: First three bytes of file are CWS (instead of FWS for uncompressed files).

MIME Base64

This encoding is used to attach binary files to e-mails.

SVGZ

These files contain SVG files compressed with GZip.

ODT

OpenOffice Document files consist of zipped XML data.

SIS (slow mode only)

These files contain informations about software installation on Symbian OS for mobile phones. They use zLib compression.

3DM (slow mode only)

This is a file format for 3D geometry used by Rhino3D that contains zLib streams.

zeno (slow mode only)

Zeno is a file format used by the german Wikipedia DVD.

Download

Precomp (and Precomp Comfort) v0.4: precomp.zip (392 KB)

lprepaq v1.3 (including source): lprepaq.zip (259 KB)

Note: Perhaps you'll be asked for MSVCR80.DLL. Download it here.

prepaq v2 (aka paq8o8pre v2, including source): paq8o8pre.zip (311 KB)

Old versions

Precomp is not backwards compatible. If you want to recompress some PCF file made with a different version of Precomp, you'll have to download it here:

Precomp v0.3 Precomp v0.31 Precomp v0.32 Precomp v0.33 Precomp v0.34

Precomp v0.35 Precomp v0.36 Precomp v0.37 Precomp v0.38

How to use it

Easiest way (lprepaq):

"lprepaq 5 input_filename output_filename" to compress a file.

"lprepaq d input_filename output_filename" to decompress a file.

5 selects 99 MB memory. Options range from 0 (6 MB) to 9 (1539 MB).

In general, option N uses 3 + 3*2N MB.

Easy way (Precomp Comfort):

Drag and drop a file on precomf.exe to precompress the file into a .pcf file with the same name.

To get back the original file, do the same with the .pcf file.

Using the command line: (Precomp)

"precomp input_filename" to precompress a file into a .pcf file with the same name

"precomp -rpcf_filename" to restore the original file (-d is still valid, too)

Errorlevels

For batch jobs, you'll find these errorlevels useful that are returned: Error level Description

0 No error

1 Various errors (f.e. file access errors)

2 Nothing could be decompressed (PCF output is the same except PCF header)

3 Disk full

4 Temporary file disappeared

5 Parameter error: Ignore position too big

6 Parameter error: Identical byte size too big

7 Parameter error: Recursion level too big

8 Parameter error: Recursion level set more than once

9 Parameter error: Minimal identical byte size set more than once

10 Parameter error: Don't use a space after -o

11 Parameter error: More than one output file

12 Parameter error: More than one input file

Additional switches: (Precomp / Precomp Comfort)

-o[filename]:

Specifies the output file name. For precompression, default is the original file name with extension .pcf, for "decompression", it is the original file name. If the output file exists, you will be asked if you want to overwrite it. Nevertheless, you can specify a different output file name with this option.

-c and -m: (Comfort: Compression_Levels, Memory_Levels)

After precompressing a file with Precomp, it tells you how to use these both parameters to speed up the precompression the next time you precompress this file. These are the compression level and memory settings which are tried on this file. If you would use this on a different file, it could be that Precomp misses some compressed parts of it.

-t: (Comfort: Compression_Types)

Enables or disables detecting of certain compression types. For command-line use, there are two variants:

t+ enables certain types and disables the others, while t- disables certain types and enables the others.

Using -t-j for example disables JPEG recompression and leaves all other types as before, using -t+pf enables only PDF and GIF precompression, disabling everything else.

-l: (Comfort: Maximal_Recursion_Level)

Sets the maximal recursion level. Several streams can contain additional streams inside, for example ZIP or MIME Base64 streams. This switch specifies the maximal "depth" where Precomp will look for streams. Setting this to 0 disables recursion, the default is 10 which should be enough for most filetypes.

-f: (Comfort: Fast_Mode)

Fast mode to speed up Precomp. This uses the first found compression for all streams instead of trying all 81 combinations when not sure. This will work fine on files that use only a few compression methods, but will result in worse compression for files with many compression methods used. Good candidates are PDF and ZIP/JAR/GZ files. Bad candidates are archives containing many files.

In non-fast mode, there is a message when only one level combination is used. This means that fast mode will do absolutely the same on this file, but faster.

-slow: (Comfort: Slow_Mode)

Slow mode will slow down Precomp much. It looks for raw zLib headers, and recognizes more file formats like SIS and SWF or special formats used only for one single program. However, the zLib header consists of only 2 bytes, so there can be many false-detected streams that aren't zLib streams but are handled like them, which results in a slower and more instable behaviour.

Slow mode can be combined with fast mode, but it could happen that a false-detected stream is the first stream and prevents further real streams to be detected, so combine them with caution. Use this mode if you have files that use zLib compression but are not supported (SIS, SWF, game ISO files...).

-brute: (Comfort: Brute_Mode)

Brute mode will slow down Precomp very much. It assumes that there could be zLib streams without headers everywhere. This even recognizes most exotic file formats that don't include zLib headers but will take very much time (more than a minute even for filesizes around 10 KB). If you should have data that has to be processed with this mode, better try to add zLib headers on your own.