BasicsApril 4, 20265 min read

Lossy vs lossless – understanding image compression

Compression always means making data smaller. But there are two fundamentally different paths — one throws information away, the other doesn't. Knowing which is used when helps with every format choice.

Diagram of lossy and lossless compression

Lossless: every pixel survives

Lossless compression uses computer-science tricks: encode repetitions more compactly (think ZIP), detect patterns and reference them. The result decompresses to a pixel-perfect copy of the original.

Examples: PNG, GIF, WebP lossless, BMP RLE, FLAC for audio. Typical size reduction: 20–60% depending on the image. Text and diagrams compress more than photos because they contain more repetition.

Lossy: information gets thrown away

Lossy compression exploits the fact that the human eye doesn't catch everything. JPG breaks the image into blocks and reduces details we barely notice — fine luminance gradients survive, subtle colour shifts get smoothed.

Result: dramatically smaller files (often 90%), but the original is mathematically no longer recoverable. At low quality you'll see block artefacts, blur and colour fringing. At high quality you see nothing — and that's the point.

When to use which

Lossless is mandatory for: logos, screenshots, diagrams, pixel art, medical imaging, archiving originals. Pixel accuracy matters; any loss is data loss.

Lossy is king for: web photos, streaming video, everyday audio. Perceived quality at minimal size is what counts. Tip: always store masters losslessly and export the final version lossy. That way you can re-export anytime without stacking generation loss.

Where lossy lies — and you can see it

Lossy codecs use frequency transforms (DCT for JPG, asymmetric codecs for AVIF). They throw away high-frequency detail the eye supposedly can't see. Smooth gradients survive fine; hard edges and text don't.

The classic giveaway: red text on a white background at JPG quality 70. The letter edges develop a pinkish halo — that's chroma subsampling. Fix: pick quality 90+ or jump straight to PNG/WebP-lossless for text-heavy images.