Ruud van Asseldonk —a software engineering intern at Google Research's Zurich-based compression team— has opensourced a new jpeg decoder, Knusperli. It shows a new approach that decodes JPEGs in a way that remains true to the original but creates less block error boundaries.
The boundary continuity is imposed in DCT space within the quantization boundaries, i.e., the properties of the image are not changed into something that would produce a different DCT quantization results. It doesn't solve all problems of jpeg compression, but would mitigate the artefacts in quality range 50-85 nicely.
Knusperli builds on our previous work with Guetzli and PIK.