Lossy compression

Lossy compression reduces file size by discarding information the algorithm decides isn’t critical to perception. Once discarded, that information is gone — re-encoding doesn’t recover it. The savings can be dramatic: a high-quality JPG is typically 10× smaller than the equivalent PNG.

How it works depends on the medium:

• JPG (images) — DCT transform + chroma subsampling + quantisation of high-frequency components. Exploits the eye’s lower sensitivity to colour vs brightness and to fine detail vs gross structure.
• MP3 (audio) — psychoacoustic model that drops frequencies the ear masks or can’t resolve.
• MP4 / H.264 / H.265 (video) — combines JPG-style intra-frame compression with motion prediction across frames. Most of a video is “same as the previous frame plus small changes,” which compresses extremely well.
• WebP / AVIF — modern lossy image formats that compress 25-50% smaller than JPG at equivalent visual quality.

The cardinal rule of lossy compression: never re-encode multiple times. Each pass introduces new artefacts on top of existing ones (“generation loss”). Always edit from the highest-quality source and save the final once at your target quality.

Lossy is the right choice when the result will be viewed/heard, not edited, and file size matters. Use lossless when fidelity is paramount or further editing is planned.