I frequently find the PubPeer excuse of compression artifacts in doctored images.
So lets have a look at that issue using the jpegoptim (manpage) and also jpeginfo (manpage). These are 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 filesize also)
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.