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Reflowable vs. fixed layout
Why is the Mobipocket file format adapted for eBooks
The OPF source format
The source format and the binary format
More information about the Mobipocket file format


The Mobipocket file format is a binary format for the distribution of eBooks. It is one implementation of the Open eBook Publication format with a number of proprietary extensions. The Open eBook Publication format is developed and specified by the IDPF (International Digital Publishing Forum) an independent organization formerly known as the Open eBook Consortium. As a consequence, the Mobipocket format is based on HTML and is reflowable. Mobipocket supports most features of standard HTML to format and layout text and images.

Reflowable vs. fixed layout

Publication file formats can be separated into two categories, reflowable formats and fixed layout formats (but the distinction is not always that simple).

Fixed layout formats are typically designed for printing, they include PostScript and Adobe PDF. A vast majority of PDF files have a fixed layout, but occasionally some PDFs can contain extensions to make them reflowable. The content of fixed layout files is displayed the same way things will appear once printed. Images, words, paragraphs, columns are positioned at fixed coordinates within a page. The size of the page is imposed.

Reflowable formats are typically designed to be displayed on a screens for which the size is not known or imposed. HTML and plain text are reflowable formats.

Figure 1: a typical page of text.
Figure 2: the same page of text using a fixed layout on a different screen size.
Only a fraction of the page can be displayed at a time.
The user has to scroll right and left to read each line of text.
Figure 3: the same page of text reflowed on a different screen size.
The width of a line of text always matches the width of the screen.
This is the best reading experience for this text and this screen size.

Mobipocket is a reflowable file format. There is no way to create Mobipocket eBooks with a fixed page layout.

Why is the Mobipocket file format adapted for eBooks

Mobipocket eBooks are designed to be read on a variety of devices and not just desktop PCs. The file format was design to solve a number of problems that arise when you try to display content on devices that have very differing capabilities. Specifically, the file format solves the following issues:

  • Content is reflowed to optimally adapt to the screen size.
  • The user (rather than the publisher) chooses the font family, font size, margin size, line spacing, etc. to make the reading more comfortable. All these parameters are highly dependent on the device and the user. It is therefore logical to adjust them at reading time.
  • The file format and its compression guarantee a fixed time to display any given page of the book. In order to display page 1000 of a book, you do not need to decompress or repaginate the 999 previous pages. We consider it essentiel to have a very responsive reading application even on devices with low CPU power.
  • Images can be embedded in different sizes. Depending on the screen resolution, the most adapted image is displayed (more information).
  • Different frames can be used depending on the type of the reading device. This is useful to avoid cluttering the limited screen space on portable devices (more information).
  • Mobipocket files are cross-platform. You do not need a different file for the different platforms, the same binary file will work on PC, Pocket PC, Palm, Symbian smartphones, Blackberry, etc...
  • Mobipocket is compliant with open standards such as IDPF 1.0, 2.0 and XHTML
  • The file format is secure because it implements a DRM for distribution through the eBookBase DRM servers.
  • Mobipocket eBooks are designed for reference and complex content: advanced navigation and indexing support, frames, superior compression, etc...

The IDPF source format

The OPF (Open Publication Format) is a standard of the IDPF that defines how to make a publication for electronic reading devices. The .OPF file is an XML data file which describes two things:

  • The list of HTML content files in the publication (these files contain the actual text of the book along with images and formatting)
  • The metadata of the eBook (Title, Author, Publisher, Cover, ...)

Figure 4: an OPF publication: one master XML file, multiple HTML content files

The Mobipocket Creator, which is the tool to create and build Mobipocket eBooks, manipulates OPF files natively.

The source format and the binary format

An OPF publication is meant to be used when exchanging data between publishers or between a service company and a publisher, or between a publisher and a retailer. It is not meant to be delivered to the reading device of the customer. The OPF publication must first be compiled into a binary eBook. Once compiled, an eBook is fit to be viewed on reading devices but cannot be modified anymore. To modify the eBook, the source OPF publication must be modified and then recompiled. The compilation of an eBook can be performed with Mobipocket Creator and includes the following steps:

  • Packaging of all contents into a single file (for easy delivery)
  • Optimization of the packaged file (for faster performance on reading device)
  • Compression (to decrease file size)
  • Indexing (to enable various search features)
  • Encryption (Digital Rights Management (DRM))

Figure 5: the generation of an eBook from an OPF source publication

More information about the Mobipocket file format

How do I create a Mobipocket eBook file
Mobipocket file format reference documentation
Open eBook format specifications

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