Generation loss: what repeated saving does to a JPEG

Opening a JPEG decodes it into pixels. Saving it runs the encoder again: the image is transformed, the coefficients are quantised, and detail is discarded.

The critical point is that the second encoder cannot tell the difference between real detail and the first encoder's artefacts. The block edges and ringing left by the first pass are treated as genuine image content, and they get quantised in their turn — producing new errors on top of the old ones.

Do this repeatedly and you accumulate layers of approximation, each built on the last. That is generation loss, and it is why the damage compounds rather than plateauing.

It depends almost entirely on the quality setting.

At quality 90 or above, several re-saves produce differences most people cannot see, because so little is being discarded each time. At quality 70, damage is noticeable after three or four generations. At quality 50, two saves are enough to see it clearly.

There is also a stabilising effect worth knowing about: if you re-save repeatedly at the same quality with no editing in between, the image partially converges — much of the detail that would be discarded has already gone. It is the combination of re-saving and editing, or re-saving at varying qualities, that degrades fastest.

Causes it

Opening in an editor and saving. Chat apps re-compressing on send. Some CMS platforms re-encoding on upload. Cropping and re-saving. Screenshotting an image.

Does not cause it

Copying or moving a file. Renaming it. Emailing it as an attachment. Uploading and downloading unchanged. Opening it and closing without saving.

Special case

Lossless JPEG operations — rotating by 90°, or cropping on block boundaries, with a tool such as jpegtran — rearrange the existing data without re-encoding, so they cause no loss at all.

The distinction is simply whether the pixels were decoded and re-encoded. If the bytes were only moved around, nothing is lost.

The characteristic smeared, blotchy, washed-out look of an image that has been round several group chats is generation loss at its most visible.

Each app re-compresses on send, often aggressively to save mobile data. Forward it again and it is compressed again — from the already-degraded copy, not the original. Ten forwards is ten lossy encodes stacked on top of each other, and no setting recovers any of it.

Sending the original file rather than forwarding a received copy avoids the entire chain.

• Keep a master. The camera original, or a lossless export, untouched.
• Edit from the master every time, not from your last export.
• Export once, at the settings you actually want.
• Testing several settings? Run each against the original, never step one file down repeatedly.
• Intermediate files in a workflow should be lossless — PNG or your editor's own format.
• Send as a document where chat apps offer it, which bypasses their re-compression.