NiLuJe did some testing, comparing one of the dithering algorithm available with ImageMagic and the one used by his devices, and in his opinion, even with the most recent device at the time I asked him (the Elipsa), pre-rendering the pictures with ImageMagick provided better picture quality.
He spoke a bit about the command used, in 2018 (since then he replaced Riemersma by FloydSteinberg because the former had a problem with the most recent ImageMagick version when we spoke, last year).
https://www.mobileread.com/forums/sh...1&postcount=17
Another benefit of converting JPG to PNG using only the "colors" allowed by an e-ink screen is the size. Using the Ubuntchu example from the KCC website, with an Elipsa as the targeted device, KCC pictures (excluding cover.jpg, a duplicate of another file) weight 6.2 MB, while using ImageMagick, they weight 4.4 MB. The first two attached picture are examples from the Ubuntchu manga.
As far as I can see, KCC doesn't dither the picture produced, so the device will need to do it. The third attached file is a comparison between 16 gray-level dithered PNG on the left and the JPG file produced by KCC on the right, using the color cover of one of my manga (dithering is invisible on device, except maybe with a magnifying glass
).
From my limited testing, KCC is faster: it took ~20 secondes to convert a 200 pages long manga on my Mac M1 Pro (despite KCC not being a native Apple Silicon app and using Rosetta) while with ImageMagick, it took 137 secondes (and it was done using the Apple Silicon binary). The same manga would take around 5 minutes on a mini-PC using a Core i5-8500T inside a VM using the Windows Linux Subsystem and maybe 2-3 minutes more with my previous Macbook Pro (using a Intel i7 with four cores running at 2.3 GHz).
Using that manga, the original EPUB file weights 124.3 MB, the JPEG pictures produced by KCC weight 95.8 MB and the PNG ones produced by ImageMagick weight 75.7 MB.
Another difference is KCC custom palettes, tailored for each device: pictures produced by KCC tend to be darker than the one produced by ImageMagick, using the CMAP file provided by NiLuJe. It can be useful, especially with some mangas with light drawing, but it can crush some details. It should be possible to use similar palettes with ImageMagic by replacing NiLuJe's CMAP file.
KCC will crop pictures (as seen on the first attached file), which can be useful with the smaller device, to reduce the wasted screen, less so with a bigger one (especially one like the Elipsa).