Quote:
Originally Posted by Quoth
I've never dithered cover image, so never seen banding.
Used covers "as is" on Kobo Touch, Original Aura H2O, Libra, Libra2, Sage and Elipsa.
Dithered would want to be PNG, as JPEG can make a mess of dithering anyway. A dithered image is a disaster to rescale, so a dithered image only suits something than can do the source to display pixels 1:1.
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It's a long time since @NiLuJe/@davidfor explained this but IIRC...
If your calibre covers are colour (almost certainly), even if you 'do nothing', dithering happens somewhere along the line between calibre send-to-device and the Kobo displaying the sleep screen cover because the Kobo device itself will dither images whenever it thinks it needs to. @NiluJe's opinion (
he was/is very picky about cover image quality) was that leaving it up to the Kobo device gave the worst end results. Hence he supplied the code for the extra cover options in KU/KoboTouch driver and @davidfor implemented them.
Enabling ALL the cover options (Keep aspect ratio, Letterbox fullscreen covers, Send B&W, Upload dithered covers, Save covers as PNG) ensures that all dithering for the sleep fullscreen image happens at the calibre end prior to sending, rather than leaving it to the tender mercies of the Kobo device. I believe using the PNG format allows dithering to the correct eink 16-colour grayscale palette, whereas JPG does not.
In summary:
If before it's sent, the fullscreen image:
- is already B&W using only the 16-colour eink grayscale palette
- already has the 2 vertical padding sidebars added in your colour of choice
- is already the exact dimensions for the Kobo model it's being sent to
then the Kobo will not need to do any further dithering.