There is not currently a setting in the OMERO.web viewers (iviewer or the older default viewer) to adjust the compression level.
However, OMERO.web rendering API does provide the ability to adjust compression.
If you use your browser dev tools and look under the Network tab, filtering for Images, you’ll see the Images being loaded by the viewer. You can click on these and open in a new browser Tab.
The URL can be quite long, but it will contain
q=0.9 for the iviewer.
If you edit this to
q=1 you should be able to see an improvement (I saved each jpeg and opened in Photoshop to compare).
This doesn’t entirely remove all compression artefacts: The OMERO rendering engine still returns a jpeg. But it would be helpful to know how much of an improvement this is for you?
To fix this entirely, the rendering engine would need a new API which is a much bigger task.
I’ve opened an Issue for iviewer, to allow users to choose the lower compression level: https://github.com/ome/omero-iviewer/issues/338