Lossy Compression
A compression method that discards imperceptible data for high compression ratios. Cannot restore originals.
Lossy compression irreversibly removes data to significantly reduce file size. The original cannot be reconstructed, but discards information at levels imperceptible to human senses, maintaining practical quality with dramatic size reduction.
Key techniques for images:
- Transform coding: DCT or DWT converts spatial data to frequency domain, quantizing less perceptible high-frequency components. JPEG uses DCT; JPEG 2000 uses DWT
- Predictive coding: Encodes only residuals between predicted and actual values. Used in WebP and AVIF to remove spatial correlation
- Chroma subsampling: Exploits lower sensitivity to color vs luminance. At 4:2:0, chroma resolution is 1/4 of luminance
The advantage is compression ratio - JPEG at quality 80 typically reduces to 10-20% of original size. For web delivery, this improves page speed and saves bandwidth, benefiting UX and SEO.
The trade-off is generation loss - re-compressing causes cumulative degradation. Save in lossless formats during editing; apply lossy only for final output.