Lossless vs Lossy Compression: What's the Difference?
Lossless compression preserves every pixel. Lossy compression throws information away to save space. Here is what that means in practice for your images.
"Compress without quality loss" is a phrase you'll see on most image-compression tools. It's slightly misleading: it usually means visually lossless, not bit-exact lossless. Here's the difference and why it matters.
Lossless compression
A lossless compression algorithm produces output that decompresses back to byte-exact original. Every pixel is preserved. PNG, WebP-lossless, AVIF-lossless, and TIFF-LZW are all lossless.
The trade-off: lossless compression doesn't shrink photographs much. A typical photo has so much variation that there's little redundancy to exploit. PNG of a 4000×3000 photo is often larger than the source RAW for this reason.
Lossless shines on:
- Screenshots (lots of repeated UI pixels)
- Diagrams and graphics (flat colour regions)
- Anything you'll edit again: lossless = no compounding quality loss across edits
Lossy compression
A lossy algorithm permanently discards information to save space. JPEG, WebP-lossy, AVIF-lossy are all lossy formats. The trick is discarding the information your eyes don't notice: high-frequency colour detail, fine variations in noise, etc.
Quality settings (e.g. JPEG q85) control how aggressively to discard. q95 keeps almost everything; q50 produces obvious artefacts.
Lossy shines on:
- Photographs, where the eye doesn't care about exact pixel values
- Any image where file size matters more than bit-perfect preservation
Why "lossless" tools usually aren't
When a "lossless compression" tool processes a JPEG, it's almost always doing visually lossless compression, re-encoding the JPEG at a quality level low enough to shrink but high enough that you can't tell. The output isn't bit-identical to the input.
The only way to get bit-identical output is to use a format-native optimiser (mozjpeg in arithmetic mode, optipng, etc.) and accept much smaller savings (5–15% rather than 50%+).
For most users, "I can't tell the difference" is the right bar, not "byte-exact." That's what OmegaPix's Image Compressor defaults to: quality high enough to be visually identical, low enough to shrink usefully.
A practical comparison
A 4000×3000 photograph:
- Original JPEG (camera default): 4.2 MB
- "Lossless" optimisation (mozjpeg, q100): 3.9 MB (-7%)
- Visually lossless (q90): 1.8 MB (-57%)
- Quality 80: 1.1 MB (-74%)
- Quality 60: 580 KB (-86%)
Visually lossless gives you 57% savings with no detectable quality change at normal viewing distance. That's the sweet spot for almost any web or sharing use.
When to pick which mode
| Use case | Compression type | Recommended setting |
|---|---|---|
| Web hero image | Lossy | WebP q80 or AVIF q60 |
| Email photo to family | Lossy | JPEG q85 |
| Screenshot for documentation | Lossless | PNG or WebP-lossless |
| Photo you'll edit further | Lossless (TIFF) or original RAW | - |
| Pixel art / logos | Lossless | PNG or WebP-lossless |
| Print-ready file | Lossless | TIFF or PSD |
How OmegaPix handles this
The Image Compressor's Auto mode picks the right quality for your format: typically q85-90 for JPEG, q80 for WebP, q60-65 for AVIF. You can override in Manual mode. Everything runs client-side via WASM, so the source file never uploads anywhere.
Recommendation
Default to visually-lossless lossy compression for any photograph headed to the web. Reserve true lossless for screenshots, graphics, and source-of-truth archives.
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