[danielheath]

JXL offers re-encoding of existing jpeg at smaller size with no quality loss, and has an efficient progressive mode.

As the head of tech for a site with a few tb of user-supplied jpeg files (which are thumbnailed), I can see the appeal…

[jbverschoor]

Not just no quality loss. Byte-for-byte identical

[Eisenstein]

If something is 'byte-for-byte identical' wouldn't it be the same size?

[adgjlsfhk1]

It's byte for byte identical once you expand to an array of pixels. The storage format uses more advanced (lossless) compression, but the pixels that come out are identical.

[edflsafoiewq]

When you transcode back to JPEG, the JPEG is byte-for-byte identical to the original JPEG. But if you decode to pixels you do not always get identical results as the JPEG because the JPEG standard does not specify bit-exact decoding and different decoders may differ slightly.