Blockiness: mean absolute luma difference across columns/rows at each of the 8 g...
https://bild.halowerk.com/v1/jpeg-artifactsOverview
Blockiness: mean absolute luma difference across columns/rows at each of the 8 grid offsets divided by the mean elsewhere (1.0 = no blocking); analysed at native resolution on a central crop of at most 2048×2048 aligned to the 8-pixel grid, in stored orientation. A maximum at an offset other than (0,0) points to an earlier compression before a crop or shift (blocking-artifact grid, Li/Yuan/Yu 2009).