COMPRESSION BASICS
Image compression quality and lossless mode are not the same thing.
Moving a quality slider very high does not automatically create true lossless output. Understand lossy encoding, lossless mode, and Skip larger outputs before expecting a specific size or quality result.
Key points
- Quality is a lossy-encoding tradeoff; lossless means reversible pixels, and the two are not interchangeable.
- Lossless re-encoding does not guarantee a smaller file.
- A lossless output cannot restore detail already lost from a JPEG source.
What a quality slider is controlling
Lossy encoding trades file size against visual fidelity. A quality setting influences that tradeoff, but formats and image contents do not respond to the same number in the same way.
Quality 80, 90, or 100 is not a cross-format standard. Treat a number as a starting point within one project, then inspect the result in the real delivery context.
True lossless is about reversibility, not a size promise
True lossless encoding aims to make decoded pixels reversible. It is useful when pixel identity matters, but it cannot promise a smaller result. A source that is already efficiently encoded can become larger after a lossless re-encode.
That is why ImgConvert offers Skip larger outputs: when a candidate is not smaller than the source, the app can avoid writing it to the output directory. It is a guardrail, not a compression-rate promise.
A lossy source cannot be repaired by a lossless container
JPEG is already lossy. Writing a JPEG to lossless AVIF or WebP can preserve the currently decoded pixels, but it cannot restore information removed by an earlier JPEG encoding.
When the goal is to reduce further loss, retaining the original, selecting quality carefully, and avoiding repeated lossy transcodes is usually more effective than switching to a lossless container afterward.
Common questions
Does quality 100 equal lossless?
No. Quality and lossless mode are different encoding choices. Use a format's lossless mode when you need reversible pixels.
Why can lossless compression be larger?
Lossless mode prioritizes reversible pixels, not minimum size. Source content and existing encoding affect the result.
Can a JPEG regain quality by converting to a lossless format?
No. A lossless output cannot recover detail removed by an earlier lossy JPEG encoding.