When preparing a lecture on image fraud in scientific papers, I frequently find the PubPeer excuse of compression artifacts. So lets have a look at that issue using jpegoptim (manpage) and also jpeginfo (manpage).
German Wikipedia explains the usual effects
Ringing – high contrast areas with waves
Contouring, Banding, Pposterisation – extreme brightness changes
Staircase noise – along curving edges
Blockiness in “busy” regions – may mimick duplications
Gibbs phenomenon – small highlighted area of large contrast
Alias, Moiré, frequent color/brightness change
Now let’s have a look at the compression artifacts in descending quality (BTW this also good exercise for the upcoming 125kB Content Upload Filter, I am therefore adding also filesize here)
Maybe the most frequent issue that I see in published PDFs is the 4% or 8% blockiness in the shadow area.
Issues like sharp vertical or horizontal lines in images of gel blots are no compression artifacts but due to to rectangle selections from copied image areas.