[tomsem]

We don't know if it is obfuscation yet, in the strict sense (as in 'rendering it obscure, unintelligible, or unclear'). It is as plausible to be the logical outcome of the technical implementation chosen, exclusive of any intent to obscure. For example, they may have come up with something that is smaller to send over the internet, or which loads from storage into memory more quickly than KF8.

We actually have a pretty good idea of what is in the file: text plus 'markup', and maybe a cover image (we should pick things without many images in them). Moreover the text is known to us (we can read it, or in some cases, go to gutenberg.org and download it), and we can compare the source size to the mobi-ized size to the kfx size and at least learn something more about what is going on.

For example, I just downloaded an .azw3 file from my account page and zipped it. the .zip file is about 10% smaller (694KB vs original 768KB). When downloaded to my Paperwhite (2012, no typography update) the .azw3 is about 3% smaller than the .zip file. So it looks like there might be some sort of compression applied just as part of sending it to the device.

On my Fire HD6, I downloaded the same book, and took a peek with ES File Explorer. Its Book folder has 3.2 MB. Inside that is a .prc file which I presume is the main file. It is 750KB, more or less the same size as the .azw3 file downloaded from account page.

But this is just a 'regular' book.

I have another book that is legitimately 'kfx': "meditations" by marcus aurelius (DRM free). On my ancient Paperwhite it is 274KB (azw3). On Fire, the corresponding Books folder has 134KB. There is no .prc. Instead, there's a folder in there with 'kfx' in the name, and there is 100KB of stuff in it. There are 3 files about 30KB each, and the other 8 make up the balance of 9KB. But there is a huge difference between 274 and 134. Now this particular book has images in it, which might somehow account for this (I'm guessing the images are getting especially compressed as part of KFX processing). But is this the explanation? I need to find a text-only book to eliminate this variable, compare the kfx folder contents, and learn a bit more hopefully.

The book is I think derived from gutenberg source, and divided into part 1 and part 2. The gutenburg text file is 415KB, zipped it is 152KB. So, half of that is 75KB, and so the 134KB seems reasonable if you add in heavily compressed images, and 274KB with less heavily compressed images.

But I need to find better experimental subject: a 'real', text-only kfx. On the surface it would appear that in some cases, KFX uses half of the storage of AZW3. That alone would be reason enough for Amazon to choose to adopt a new approach. It would save them some money they now spend on bandwidth.

Interestingly, downloading it from my account page yields a azw3 file that is 846KB, quite a bit larger. Okay, so I ran Kindle Unpack and compared the image folder sizes. Yup, 123KB (off of the Kindle device) vs 696KB (from account download). So at least that accounts for size differences between device and account page downloads: image compression, vs no image compression.

Naturally I'm curious to know if someone could do a similar comparisons involving Kindle (as opposed to Fire) KFX.