Lossless Compression
A compression method restoring original data bit-for-bit. Reduces file size with zero quality loss.
Lossless compression produces output that decompresses to bit-for-bit identical data. No information is lost, enabling size reduction without quality sacrifice - foundational wherever data integrity is required.
Major algorithms for images:
- Deflate: LZ77 sliding window + Huffman coding. Used in PNG, implemented via zlib
- LZW: Dictionary-based, used in GIF and TIFF. Builds dictionary dynamically, replacing patterns with short codes
- Run-Length Encoding: Simplest technique - "value x count" for consecutive identical values. Used in BMP
- Entropy coding: Variable-length codes by frequency. Arithmetic coding achieves near-theoretical limits, used in JPEG 2000 and AVIF lossless
Ratios depend on content. Uniform areas and patterns (screenshots, logos) achieve 50-80% reduction; complex photographs only 10-30%. Significantly lower than lossy compression.
Essential for medical imaging (DICOM), scientific data, legal evidence, and print masters. In editing workflows, lossless intermediates prevent generation loss from repeated saves.