Yes JPEG has a lossless mode.
Re:Compressing Images Using Google’s Guetzli
by klodolph on Tuesday May 23, 2017 @08:26AM
(#14400598)
The answer is "no", because lossless compression is still compression. But there's no chance that a website was using lossless JPEG, since it's very poorly supported.
There are no standard losless jpegs. Even a quality of 100% will introduce loss. Only JPEG2000 is lossless, but unsupported for the web because of patent claims.
I was refering at https://en.wikipedia.org/wiki/Lossless_JPEG which is not compatible with regular JPEG.
> Should be noted that there was not any form of image compression before.
I consider that unlikely, given that they were starting out with JPEG files, which are usually compressed. (Does JPEG even have a non-compressed mode?)
> Being a JPEG encoder, it cannot output PNGs (so no transparency). But it can convert and compress your PNGs.
How does Guetzli compare against PNG compression for the things that PNG is good at, i.e. diagrams and sketches with a small palette? I'd like to see a comparison where the same PNG file is compressed with optipng on one hand, and converted to JPEG and compressed with Guetzli on the other hand, and then they look at file size and the amount of artifacts.
optipng can really squeeze PNGs if the palettes are small. For example, this decorational image from a website I did is 2400x160 pixels large, but fits in barely over 1 KiB: https://fsfw-dresden.de/theme/img/banner.png