Precompressor
Encyclopedia
A precompressor is a computer program
, which alternates file content so that a real lossless compression program will achieve better results than without precompressing. This is possible by creating such patterns that compression programs will recognize.
) is poor because the data seems random to the compressor. However, compression can usually be improved by decompressing the ZIP file and recompressing it with a better compressor (e.g. one of the PAQ
family). But to achieve lossless
compression, the original ZIP file has to be restored using exactly the same version of the ZIP compressor and the same compression settings.
Using this strategy, certain filetypes that so far achieved poor compression rates can be processed.
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
, which alternates file content so that a real lossless compression program will achieve better results than without precompressing. This is possible by creating such patterns that compression programs will recognize.
Compressing already compressed files
Usually, the compression rate on files that are already compressed (e.g. using ZIPZIP (file format)
Zip is a file format used for data compression and archiving. A zip file contains one or more files that have been compressed, to reduce file size, or stored as is...
) is poor because the data seems random to the compressor. However, compression can usually be improved by decompressing the ZIP file and recompressing it with a better compressor (e.g. one of the PAQ
PAQ
PAQ is a series of lossless data compression archivers that have evolved through collaborative development to top rankings on several benchmarks measuring compression ratio . Specialized versions of PAQ have won the Hutter Prize and the Calgary Challenge...
family). But to achieve lossless
Lossless data compression
Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. The term lossless is in contrast to lossy data compression, which only allows an approximation of the original data to be reconstructed, in exchange...
compression, the original ZIP file has to be restored using exactly the same version of the ZIP compressor and the same compression settings.
Using this strategy, certain filetypes that so far achieved poor compression rates can be processed.