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KevinH · 04-15-2011, 12:14 PM

Hi,

I chose an epub I had and loaded it into calibre and converted it to .mobi, and then used mobiunpack.py to simply look at the structure of the resulting .mobi.

I took the same .epub and converted it using kindlegen (for Mac) to a .mobi and then used mobiunpack.py to examine the structure of the resulting mobi.

Here is what I found (any unknown metadata keys are shown as numbers and their associated values as hex strings)

Kindlegen Generated .mobi

Unpacking Book ...
number of sections 467
0 3816
1 12484
2 13577
...

length of this header 232
book title offset 456
offset to start of extended header 248
extended header length 208
extended header num_items 9

MetaData
ISBN -> 978-0-385-53313-3
Creator -> Dan Brown
Publisher -> Doubleday
Rights -> Copyright 2009
300 -> 03000000000000000000000000000080002000000000000000 00000000000000ecbef4ed01e001fc01a901b5409440934099 4098409c409d
204 -> 000000ca
205 -> 00000001
206 -> 00000002
207 -> 0000821b

Here is the same information for the Calibre generated .mobi

Unpacking Book ...
number of sections 354
0 2912
1 3472
2 4656
...

length of this header 232
book title offset 544
offset to start of extended header 248
extended header length 292
extended header num_items 12

MetaData
Creator -> Dan Brown
Publisher -> Doubleday
ISBN -> 978-0-385-53313-3
Published -> 2011-04-15 15:25:24+00:00
Contributor -> calibre (0.7.54) [http://calibre-ebook.com]
Rights -> Copyright 2009
ASIN -> 0f7dd9a0-003a-45d8-9c87-c2adcef46ca1
CoverOffset -> 11
202 -> 0000001d
203 -> 00000000
501 -> 45424f4b
Updated Title -> The Lost Symbol

So it appears that the used up space of the extended headers are similar, although KindleGen seems to create some unknown MetaData keys/values: 204, 205, 206, 207, and 300. I will look online to see what these keys are actually for.

The biggest difference seems to be the size of section 0. For the Kindlegen generated mobi, section 0 is 12484 - 3816 = 8668 bytes long, whereas for the Calibre generated mobi, section 0 is 3472 - 2912 = 560 bytes long.

I am not sure what else is found in section 0 besides the extended header but the size difference is quite large.

So perhaps, what Amazon is referring to is the size of section 0 or the fact that certain metadata key/values are missing?

Hard to say without more information.

04-15-2011, 12:14 PM	#11
KevinH Sigil Developer Posts: 8,806 Karma: 6000000 Join Date: Nov 2009 Device: many	Hi, I chose an epub I had and loaded it into calibre and converted it to .mobi, and then used mobiunpack.py to simply look at the structure of the resulting .mobi. I took the same .epub and converted it using kindlegen (for Mac) to a .mobi and then used mobiunpack.py to examine the structure of the resulting mobi. Here is what I found (any unknown metadata keys are shown as numbers and their associated values as hex strings) Kindlegen Generated .mobi Unpacking Book ... number of sections 467 0 3816 1 12484 2 13577 ... length of this header 232 book title offset 456 offset to start of extended header 248 extended header length 208 extended header num_items 9 MetaData ISBN -> 978-0-385-53313-3 Creator -> Dan Brown Publisher -> Doubleday Rights -> Copyright 2009 300 -> 03000000000000000000000000000080002000000000000000 00000000000000ecbef4ed01e001fc01a901b5409440934099 4098409c409d 204 -> 000000ca 205 -> 00000001 206 -> 00000002 207 -> 0000821b Here is the same information for the Calibre generated .mobi Unpacking Book ... number of sections 354 0 2912 1 3472 2 4656 ... length of this header 232 book title offset 544 offset to start of extended header 248 extended header length 292 extended header num_items 12 MetaData Creator -> Dan Brown Publisher -> Doubleday ISBN -> 978-0-385-53313-3 Published -> 2011-04-15 15:25:24+00:00 Contributor -> calibre (0.7.54) [http://calibre-ebook.com] Rights -> Copyright 2009 ASIN -> 0f7dd9a0-003a-45d8-9c87-c2adcef46ca1 CoverOffset -> 11 202 -> 0000001d 203 -> 00000000 501 -> 45424f4b Updated Title -> The Lost Symbol So it appears that the used up space of the extended headers are similar, although KindleGen seems to create some unknown MetaData keys/values: 204, 205, 206, 207, and 300. I will look online to see what these keys are actually for. The biggest difference seems to be the size of section 0. For the Kindlegen generated mobi, section 0 is 12484 - 3816 = 8668 bytes long, whereas for the Calibre generated mobi, section 0 is 3472 - 2912 = 560 bytes long. I am not sure what else is found in section 0 besides the extended header but the size difference is quite large. So perhaps, what Amazon is referring to is the size of section 0 or the fact that certain metadata key/values are missing? Hard to say without more information.