@NiLuJe,
Thanks, I've downloaded your image pack. My brain is beginning to hurt again in anticipation of revisiting this image stuff, so I'll need to wait until I've got enough time to concentrate on it properly.
To cut a long story short:
Since the 4.11.xxxxx upgrades I've re-simplified my hacked KoboTouchExtended by removing the quantize/dither steps. I also reverted back to calibre sending all full/thumbnail covers in B&W in JPG format rather than PNG. I'm still doing the design-to-match grey filler bars, for the time being. I use the same driver processing for all of H2O/GloHD/ClaraHD/AuraOne/Forma.
I wasn't aware of the Forma full-size screensaver dimensions anomaly. Now you mention it I can see that my standard test cover, with lots of smooth gradient changes, does look slightly better on the KA1 than the Forma. ATMO I'm not sure the Forma is sufficiently worse to bother me. I am still interested in anything you discover, though.
P.S. Glad to hear that Santa brought you a Forma
Can we hope for good things on the Forma buttons/fbink front?