Lossy vs Lossless Image Compression — A Visual Guide
“Should I use lossy or lossless compression?” is one of the most common questions in image processing. The answer depends entirely on what you’re compressing and what you plan to do with it.
What’s the Difference?
Lossless compression reduces file size without discarding any data. Decompress the file and you get back exactly what you started with, bit for bit.
Lossy compression achieves smaller files by permanently removing data that’s considered less important to human perception. You can’t get the original back from a lossy-compressed file.
Format Map
| Format | Compression Type | Typical Reduction |
|---|---|---|
| PNG | Lossless | 10–50% (depends on content) |
| GIF | Lossless (256 colors max) | Varies |
| TIFF | Lossless (LZW) | 20–40% |
| BMP | None | 0% |
| JPEG | Lossy | 60–90% |
| WebP | Both (lossy default) | 70–90% lossy, 30–50% lossless |
| AVIF | Both (lossy default) | 80–95% lossy, 40–60% lossless |
WebP and AVIF are notable for supporting both modes. Use lossless WebP when you need PNG-like quality with smaller files.
When to Use Lossy
Photos and complex images: Human eyes are remarkably bad at noticing lossy compression in photographs. At quality 85, most people cannot distinguish a JPEG from the uncompressed original — even in a side-by-side comparison.
Lossy is the right choice when: - File size matters (web, email, mobile) - The image is a photograph or has complex gradients - You’re serving the image to end users, not archiving it - You need the smallest possible file
Use KoalaPic’s compressor to experiment with quality levels and see the difference yourself.
When to Use Lossless
Screenshots, text, logos, and graphics: These have sharp edges, solid colors, and fine detail that lossy compression handles poorly. A screenshot compressed as JPEG often shows visible blurring around text — a problem called “compression artifacts.”
Lossless is the right choice when: - The image contains text or sharp lines - You need to edit the file further (each lossy save degrades quality) - You’re archiving originals for future use - Color accuracy is critical (medical imaging, design proofs)
The Quality Slider
For JPEG and WebP, quality 1–100 controls the compression aggressiveness:
- 100: Minimal compression, largest file, best quality
- 85: Excellent quality, 60–70% smaller
- 75: Very good quality, 70–80% smaller
- 50: Acceptable for thumbnails, noticeable artifacts up close
- 25: Severely degraded, very small files
The relationship isn’t linear — the biggest size savings come between quality 100 and 85. Below 70, artifacts become noticeable.
Smart Quality: Let the Algorithm Decide
Instead of guessing a quality number, smart quality mode uses SSIM (Structural Similarity Index) to find the lowest quality setting that’s still visually identical to the original:
- High (0.98 SSIM): Indistinguishable from original
- Medium (0.95 SSIM): Excellent, minor differences only visible zoomed in
- Low (0.92 SSIM): Good, some detail softening
Try it: KoalaPic Smart Compress
Generation Loss
Lossy compression gets worse with each re-save. Converting JPEG → edit → JPEG → edit → JPEG compounds artifacts. Always keep a lossless original (PNG, TIFF, or RAW) and produce lossy versions from it.
Next Steps
- Experiment with compression: KoalaPic Compress
- Read the format comparison guide
- Learn about common conversion mistakes