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Definition
Lossless compression reduces file size without discarding any data — the original file can be perfectly reconstructed from the compressed version. PNG, FLAC, and ZIP all use lossless compression. Files are larger than lossy equivalents but maintain perfect quality.
Lossless compression works by finding and encoding patterns in data more efficiently. Instead of discarding information like lossy compression, it uses algorithms like DEFLATE (used in PNG and ZIP), LZ77, Huffman coding, or arithmetic coding to represent the same data in fewer bytes. The original data can always be reconstructed exactly.
In images, PNG uses lossless compression and produces files that are pixel-identical to the original. This is critical for screenshots, text-heavy graphics, and images that will be edited further. In audio, FLAC compresses music to about 50-60% of the original WAV size while preserving every sample perfectly.
The trade-off is file size. Lossless compression typically achieves 2-3x size reduction for images, compared to 10-20x for lossy compression. For web delivery, this means PNG photos are much larger than JPEG versions. The choice between lossy and lossless depends on the use case: lossless for archival, editing, and precision work; lossy for web delivery and streaming.