Quote:
Originally Posted by jackm8
Test 1:
Ebook that includes plenty of low resolution illustrations:
Three Men in a Boat: EPUB3 version: "pg308-images-3.epub" https://www.gutenberg.org/ebooks/308
I converted this epub into kfx, then back into epub using CLI:
I opened both epub files, and extracted images. Cover images appear to be identical. Same file sizes. Under extreme pixel peeping, I could not spot a single difference. Illustration images are changed slightly. File sizes in "pg308-images-3 back to kfx.epub" can be slightly smaller or larger compared to files in "pg308-images-3.epub". Image quality isn't worse, but it's possible to spot differences in this case.
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I repeated your test and observed basically the same result. When converting to KFX and back to EPUB the cover image (1704462598893215816_cover.jpg) is identical but the first interior image (1704462598893215816_p0s.jpg) is re-encoded, changing the file size and pixel values slightly.
I then changed the original file so that the contents of the first interior image and cover image were swapped. In that case the interior image converts unchanged and the cover is modified. That shows there is something about the content of some images and not others that triggers the re-encoding process.
I opened the original EPUB using the Kindle Previewer, saved the converted KPF file and examined its contents. The images there were identical to the KFX file that this plugin produces, showing that all of the image manipulations in this case are due to the processing by the Previewer and not this plugin.
It is unclear why this image manipulation is being done in some cases and not others. The Kindle Previewer's conversion process is completely undocumented.