How Image Compression Works: Lossy vs Lossless
Compression reduces the bytes needed to store an image. Lossless methods preserve the encoded pixel information exactly, while lossy methods trade some information for a much smaller file.
Lossless compression finds repetition
Lossless encoders represent repeated patterns efficiently without changing the decoded pixels. PNG is a familiar example. It works well for flat colors, sharp edges, text, interface screenshots, and transparency.
Lossless does not mean small in every situation. Natural photographs contain complex variation and noise, so a lossless file can be much larger than a carefully compressed JPG or WebP.
Lossy compression removes less noticeable detail
JPG and lossy WebP approximate image information so fewer bytes are required. At moderate settings the change can be difficult to see at normal size. At aggressive settings, blocks, ringing, smearing, and banding can become visible.
Every source behaves differently. Fine hair, leaves, fabric, noise, small text, and high-contrast boundaries often need more care than a softly focused photo.
Repeated lossy saves can accumulate damage
Opening and re-saving a JPEG applies another encoding decision. Repeated generations can gradually add artifacts. Keep a high-quality original and create delivery copies from it instead of repeatedly editing the smallest compressed result.
Converting a damaged JPG to PNG stops new JPEG loss in that output, but it does not restore information that was already discarded.
Choose by purpose
Use lossless storage when exact pixels, transparency, or future editing matters. Use sensible lossy compression for photographs delivered over the web or under a size limit. The practical goal is not to avoid every change—it is to make a deliberate trade-off that suits the destination.
- Lossless: exact decoded pixels, often larger.
- Lossy: smaller files, possible artifacts.
- Keep a high-quality original.
- Inspect the delivery copy at its real display size.