Lossy vs Lossless Compression — Key Differences
Compare lossy and lossless image compression. Understand quality tradeoffs, use cases, and which compression type to use for different types of visual content.
| Feature | Lossy Compression | Lossless Compression |
|---|---|---|
| Quality Loss | Yes, permanent | None |
| File Size (photos) | Very small | Large |
| File Size (graphics) | Can be large (artifacts) | Smaller |
| Re-save Safety | No — degrades each time | Safe — no degradation |
| Algorithms | JPEG DCT, WebP lossy, AVIF | PNG DEFLATE, WebP lossless |
| Artifacts | Yes (blocking, ringing) | Never |
| Best For | Photos, web delivery | Screenshots, logos, archives |
| Medical/Legal Use | Prohibited in many contexts | Required |
Verdict
Use lossy compression (JPG/WebP) for photographs and any image where small quality differences are acceptable — this is most web delivery. Use lossless compression (PNG/WebP lossless) for source files, screenshots, logos, and any image that must be pixel-perfect. Always archive originals in lossless format.
When Lossy Compression Is Right
The human eye cannot distinguish a well-compressed JPG from a lossless image in normal viewing conditions. The JPEG algorithm exploits known limitations of human vision: we're less sensitive to color (chrominance) than to brightness (luminance), so JPEG allocates more bits to luminance data. We're also less sensitive to high-frequency detail in areas of uniform color, so smooth gradients in sky or skin tones compress extremely well. For a news website serving thousands of photos per day, the bandwidth difference between lossless and 80% JPG is enormous. The quality difference to readers is negligible. Using lossy compression wisely is responsible web development.
Archival and Professional Workflows
Professional photographers, archivists, and medical imaging systems always work with lossless source files. Camera RAW files (CR2, NEF, ARW) are lossless, capturing the full sensor data. Professional workflows process RAW files, export lossless TIFF or PSD for editing, and produce optimized JPGs only for final delivery. This 'lossless source, lossy delivery' workflow is the correct approach: always keep the original lossless file, produce compressed versions for specific delivery needs. For website workflows, this means keeping lossless PNG/TIFF originals in asset management and generating compressed WebP/JPG versions during the build process.
Frequently Asked Questions
In double-blind studies, most people cannot distinguish a JPG saved at 80-85% quality from the original photograph when viewed at normal size. At 70%, some degradation becomes visible under careful examination. Below 60%, artifacts (blocking, color banding) become clearly visible. For web images, 75-85% is the sweet spot.
Converting a JPG to PNG saves it losslessly, preventing future quality loss. But it cannot recover detail lost in the original JPG compression. The PNG will be larger and higher quality than a re-saved JPG, but won't match the original pre-compression image. Always keep originals before converting.
Perceptual compression (used in modern codecs like AVIF and WebP) exploits human visual system limitations — we're less sensitive to color detail than brightness, and less sensitive to high-frequency changes in peripheral vision. Modern lossy codecs allocate more bits to perceptually important areas and fewer to less-noticed regions, achieving better quality at the same file size than older algorithms like JPEG.