KF8

KF8 is Amazons new Kindle Format 8 used on the Kindle Fire. It is also supported by fourth-generation Kindle devices running firmware version 4.1.0 or later, by Kindle for PC and by Kindle Reader for the Mac.

Overview

KF8 (also called AZW3) is basically a compiled ePub that has been compiled using a Palm database and Amazon's DRM scheme. It is targeting ePub 3 version support but existing ePub can also be used. The latest KindleGen software will create KF8. KF8 replaces MOBI but actually includes both a MOBI database and a KF8 database in the same file for backwards compatibility using older Amazon Kindle readers.

Features include:

• HTML5 support - An ePub 3 feature.
• Drop caps - Can be done in ePub 2.1 using CSS
• CSS3 support - An ePub 3 feature.
• Kindle Text Pop Up
• Fixed layouts - but not the same as anyone else is using. See Fixed layout ePub#Kindle KF8
• Kindle Panel View - a variation on Fixed layouts for images.
• Embedded fonts - An ePub feature.

The Format

The internal format has been decoded using mobi_unpack. It reveals that the basic format remains as a PDB file similar in structure to MOBI. Generally an KF8 file will contain both a MOBI file at the beginning and the newer KF8 version of ePub later. This of course increased the size of the book file although there is some attempt to share resources such as images between the two objects. Mobi_unpack can be used to separate the two structures by building a traditional MOBI file and a KF8 file without the MOBI part (actually a small dummy structure remains) to make the file smaller. There are flags near the start of the database that can be used to identify the type of file.

In spite of the fact that KF8 is targeted at ePub 3 source it still carries forward some HTML constructions that have been de-standardized for years and left over from MOBI's use. Amazon does publish the HTML statements and CSS3 statements that it will recognize. In addition KF8 has extended the ePub 3 format in incompatible ways to support its fixed layout option.

Compilation

Both KindleGen and the Kindle Preview program can be used to create KF8 by compiling a source ePub file. An ePub file is a zip file containing several standard human readable files. KF8 compiles this into a binary form by creating an internal structure. When this is done all of the original filenames disappear and any links to these are replaced by direct branches in the final database. Image files are retained as a binary structure with knowledge of the original image structure but file names and metadata are gone. Metadata that MOBI supports is still present in the results.

For more information

• Amazon's KF8 description
• mobi_unpack can take apart KF8 files.
• A list of supported HTML tags and CSS3 elements. - Note that some of these tags are de-standardized in HTML documentation but are recognized in MOBI.