Quick Answer
Lossless compression preserves 100% of the original data. You can decompress the file and get back exactly what you started with. The trade-off is modest file size savings (typically 5-15% for images).
Lossy compression permanently removes data that humans are unlikely to notice. In exchange, you get dramatically smaller files (50-80% reduction for images). The quality loss is usually imperceptible, but the process is irreversible.
| Attribute | Lossless | Lossy |
|---|---|---|
| Data Integrity | 100% preserved | Some data removed |
| Typical Savings | 5-15% | 50-80% |
| Reversible? | Yes | No |
| Best For | Logos, icons, archival | Photos, web images |
How Compress.FAST Implements Both
PNG Compressor
Our PNG compressor offers both modes:
- Lossy mode (default): Uses libimagequant, the same technology behind TinyPNG. Achieves 50-80% reduction by intelligently reducing to a 256-color palette.
- Lossless mode: Uses oxipng, a Rust-based PNG optimizer. Achieves 5-15% reduction with pixel-perfect output.
Toggle between modes using the "Lossless" switch in the options panel.
JPG Compressor
JPG is inherently a lossy format. Our JPG compressor uses MozJPEG with trellis quantization for optimal quality-to-size ratio.
Which Should You Use?
- Logos, icons, screenshots: Start with lossy mode (it auto-detects graphics and keeps edges crisp). If you see artifacts, switch to lossless.
- Photos for web: Lossy is almost always the right choice. The file size difference is significant.
- Master files for archival: Use lossless, or keep the original and only export lossy versions.
Want the full technical deep-dive?
We've written a comprehensive guide covering the algorithms (DCT, chroma subsampling, libimagequant, oxipng), generational loss, format comparisons, and when to choose each approach.
Read the full guide: Lossless and Lossy Compression