The Core Difference
Lossless compression reduces file size without discarding any image data. The decompressed image is pixel-for-pixel identical to the original. Lossy compression achieves smaller file sizes by selectively removing image data that is statistically unlikely to be noticed by the human eye.
How Lossy Compression Works
JPEG compression divides an image into 8×8 pixel blocks and applies a mathematical transform (Discrete Cosine Transform) to each block. High-frequency detail — fine textures, sharp edges in non-critical areas — is discarded. The lower the quality setting, the more aggressively this data is removed.
WebP's lossy mode uses a more sophisticated prediction algorithm that typically achieves better quality at the same file size compared to JPEG.
How Lossless Compression Works
PNG uses DEFLATE compression — the same algorithm used in ZIP files. It finds repeating patterns in the image data and encodes them more efficiently. No information is ever discarded. This is why PNG is ideal for screenshots, logos, and images with large areas of flat color — these have many repeating patterns that compress extremely well losslessly.
When to Use Lossy Compression
- Photographs and camera images
- Website images where page speed matters
- Social media posts
- Any situation where file size is more important than pixel-perfect accuracy
When to Use Lossless Compression
- Logos and brand assets
- UI screenshots and mockups
- Images that will be edited further (avoid re-compression artifacts)
- Medical or scientific imaging where accuracy is critical
- Images with transparency requirements
The Quality Setting Explained
The "quality" percentage in lossy compression controls how aggressively data is discarded. At 100%: nearly lossless, very large file. At 75%: imperceptible quality loss for photos, 70–80% smaller file. At 50%: visible artifacts in some images, very small file. At 30%: significant quality degradation, extremely small file.
Which Does Noxoro Use?
Noxoro gives you the choice. Select JPG or WebP with the quality slider for lossy compression. Select PNG for lossless compression. For most web images, lossy at 75–80% quality delivers the best results — small files that look identical to the originals in normal viewing.