Understanding Image Compression: Lossless vs Lossy

Lossless Compression (PNG, GIF, WebP Lossless)

Lossless compression means the file size is reduced without throwing away a single piece of visual information. If you save an image losslessly, the uncompressed pixels are mathematically identical to the original file.

It does this by finding patterns. If there are 500 identical white pixels in a row, instead of saving "white, white, white..." 500 times, the encoder simply writes "500x white".

• Best for: UI design, logos, typography, sharp geometric shapes, and pixel art.
• Why? Because sharp edges and text will look blurry and terrible if you throw away pixel data.

Lossy Compression (JPEG, WebP, AVIF, JXL)

Lossy compression actively permanently deletes data from the image to achieve massively smaller file sizes. It relies on human biology: our eyes are much better at detecting differences in brightness (luma) than differences in color (chroma).

Lossy encoders group pixels into blocks and average out their colors. The lower the "Quality" slider goes, the larger these averaged blocks become, resulting in the blocky, artifact-heavy look we associate with bad JPEGs.

• Best for: Photography, complex 3D renders, and images with millions of colors or noise.
• Why? Photographs don't have large blocks of solid color for lossless compression to easily group. A photograph saved as a lossless PNG will often be 5-10x larger than a high-quality lossy WebP.

The "Generation Loss" Warning

Every time you open a lossy file (like a JPEG) and save it again as a JPEG, you lose more data. This is called "generation loss."

For this reason, you should always keep your master files as Vector (SVG) or Lossless (PNG/PSD/Figma) and only ever use lossy compression as the final export step for the web.