Optimizing EPUBs before transfer option adds Koreader progress sync offset

[o-schwartz]

Hello,
using the "Optimize EPUBs before transfer" option breaks the Koreader progress sync by introducing a 8% offset.

My setup:
Jailbroken Kindle with Koreader 2026.07.1
Xteink X3 with crossink 1.5.0 (tried current crosspoint as well)
Calibre 9.14
Crossreader Plugin 0.2.7.

How to reproduce:
Have the unaltered epub on Kindle, set progress to 20%
Push the progress to sync server (I'm using kosync.eu)
Sync servers page displays progress at 20%
Send book to x3 via calibre wireless and your plugin, having the optimize option enabled and set to x3 device.
Open the book on x3, execute "sync progress" action
epubs progress is set to 12%

If I set the progress on my kindle to 30%, I end up at 22% on my x3.
If I don't use the optimize option, syncing works great and lands me where it's supposed to. (minus some minor offset because of page size discrepancy)

If I can provide any more information please let me know.

Best regards

[itsthisjustin]

That's a pretty big difference so I'm not sure what the optimizer is finding that removes that much junk from the epub? Can you compare the two files with Ai or something?

[lukestein]

Thanks for reporting @o-schwartz . Would you mind trying the optimizer but turning off its "split chapters" option? It seems to be introducing some problems with chapter progress on device (see #54) and I'm wondering if this might be related.

[o-schwartz]

Hello, sorry for not getting back to you earlier.
Disabling the "spit chapters" option fixes the problem, thank you very much!
Now I can still compress the jpgs and the epub loads a lot faster, but progress sync also works.

Thank you for developing this plugin!