[iggldiggl]

And nowadays, for subsampled images libjpeg post classic version 6 insists on doing the chroma upscaling using DCT where possible, so for classic 4:2:0 subsampled images (i.e. chroma resolution is half the luma resolution both horizontally and vertically) each subsampled 8x8 chroma block is now upscaled individually to 16x16 for the final image, which can and does introduce additional artefacts at the boundaries between each 16x16 px block in the final image. But the current libjpeg maintainer insists on that new algorithm because it is mathematically more beautiful…

Granted, the introduced artefacts aren't massive, but under certain circumstances they are noticeable, which is how I stumbled across that topic in the first place.

Thankfully, most software that isn't still stuck on libjpeg 6 has switched to libjpeg-turbo or some other library instead which continues using a more sensible algorithm for chroma upscaling.