@jackie_w: I just spent the evening re-testing my
cover processing patches on FW 4.12 on both the H2O and the Forma, and, err, stuff got more confusing
.
On the H2O, I definitely get what you meant by "it got better": indeed, Nickel will grayscale & dither both thumbails & sleep cover itself, and it does a pretty good job of it!
Grayscaling is done in sRGB space, and not linear light, but that's minor, and the dithering handles corner-cases much better than Calibre's approach.
On the flip side, I get an extra pass of... something on my pre-dithered PNG files, which means I no longer get bit-exact results. Thankfully, that processing pass is minor.
But on the Forma, oh, my...
First of all: sleep cover handling is just plain broken: I figure it's expecting Aura One sized covers (which is what it both generates for sideloaded content & downloads for kePubs), so you get a fantastic upscaling pass that's generally not great...
Second: it doesn't actually follow the same pattern as to *when* it's dithering and grayscaling?! It appears to do that for thumbnails (which don't suffer from such an egregious size issue, although I still suspect there's no dedicated Forma codepath there either), but NOT for the full-screen sleep cover?! (Both issues may actually be linked: i.e., that extra upscaling codepath doesn't do dithering)
Which generally yields worse results, no matter what you do.
TL;DR:
Without IM, the best settings I could come up with:
H2O: JPG (or PNG), BW & NO Dither
Forma: Well, nothing, really. PNG, BW & DITHER, but ideally, with IM doing the dithering
. And even then, the upscaling pass pretty much wrecks it.
All my tests & results are available
here.