Thank you for your helpful answers and comments!
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Originally Posted by DaleDe
One very simple way to do an invisible watermark is to place a comment in the jpg image. This won't show and most people wouldn't even notice it. It can also be used to identify the source and indicate copyright. Irfanview is a program that can add a comment to an image. Of course it is not as obscure as modifying the image itself.
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I guess you mean adding metadata for a JPEG image. That is one good solution because it's invisible unless the metadata is being checked. Too bad that the metadata can be easily erased.
Quote:
Originally Posted by Jellby
... and you can add something like a PGP signature of the whole book, created with your private key.
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I don't know much about PGP digital signature yet but I googled about it. I'll try it out and see how it works.
Quote:
Originally Posted by kovidgoyal
It's not possible to make a watermark that cannot be deleted easily.
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I agree but I personally believe that piracy people just want to share the files. If I have enough complicated watermark system, it's not worth to solve it because it doesn't show up anywhere and it doesn't prevent the file for being spread around internet. Everyone is still able to read my e-books even though there's a hidden watermark.
Quote:
Originally Posted by michaelhughes
While this wouldn't be a visible watermark, if you wanted to be able to determine who the source of a violation was, you could always drop an extra file with ownership data into the content directory before you zip the files up.
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It's a good idea but extra files are easily detected and easily deleted as well.