Genozip compression is lossless relative to the underlying data, which means that the data reconstructed during decompression is exactly identical to the original data compressed.
Verification of Losslessness
When uncompressing with genounzip (and genocat with some exceptions), genounzip verifies that the reconstructed data is exactly identical to the source data, using an MD5 or Adler32 digest. More details: Verifying file integrity.
Exceptions to Losslessness
1. There are some cases in which you may request genozip to change the source data before compressing it. In these cases, the digest is not calculated. These cases are:
- Using --match-chrom-to-reference
- VCF: Compressing a Luft file (a lifted-over dual-coordinates file)
- SAM/BAM: adding taxonomy information with --kraken
- KRAKEN: removing the SEQLEN and KMER fields with --no-kmers
2. Compressing already-compressed files (BGZF): Many tools that generate genomic files use BGZF to compress them. These typically have a .gz extension (although not all files a .gz extension are compressed with BGZF), and BGZF compression is also used internally in .bam and .bcf files. When genozip compresses a BGZF-compressed file, it first decompresses it to recover the original underlying data, and then compresses the data with Genozip. Likewise, when genounzip uncompresses a .genozip file, it recompresses the data back to BGZF if the original file was compressed with BGZF. genozip records the parameters of the BGZF compression in the .genozip file (such as estimated compression level), and genounzip attempts to recompress to BGZF format using the same parameters. However, since there are many BGZF compression libraries, each with dozens of parameter combinations, it is possible that genounzip BGZF compression will achieve a slightly different compression level than the BGZF compression of the original file, resulting in the final BGZF-compressed file differing from the original BGZF file. However, the underlying data is still exactly identical and verified with the digest.
3. Compressing already-compressed files (.bz2 .xz .zip and non-BGZF .gz): genozip is capable of compressing files that are already compressed with these methods. It does so by first uncompressing the file, and then recompressing with Genozip. However, in these cases, genounzip does not recompress the files back to .bz2 .xz .zip or .gz.
4. CRAM and BCF: Genozip does not compress CRAM and BCF files natively - it first converts their data to BAM or VCF format respectively, and when uncompressing, it outputs the data as a BAM or VCF file. Other than this file format change, the compression is lossless.