PEF 1.0 - Portable Embosser Format

Working draft, revised 21 October 2008

This version:
http://www.daisy.org/projects/braille/braille_workarea/pef/pef-specification.html
Author:
Joel Håkansson
Contributors:
Sean Brooks, Linus Ericson, Markus Gylling, Stefan Hofstetter, Robert Jaquiss, Piotr Kiernicki, Fredrik Larsson, Rob Longstaff, Björn Nyqvist, Stephen Phippen, Matthias Ragaz, Per Sennels, Richard Stones, Jennifer Sutton, Björn Westling, Markus Wildi

Abstract

This document specifies the Portable Embosser Format (PEF), a data format for representing braille books. PEF is an XML 1.0 application.

Status of this Document

This document is a working draft.

Table of Contents

Introduction

This section is informative.

What is PEF?

The Portable Embosser Format (PEF) is a document type that represents braille pages in digital form, accurately and unambiguously; regardless of language, location, embosser settings, braille code and computer environment. It can be used for braille embossing and archiving anywhere in the world, no matter where nor how it was produced. PEF allows new features to be added, such as ink overlay or tactile graphics, thus enabling a completely automated production of advanced braille books.

Why is PEF Necessary?

PEF brings a number of things to braille production:

What is Unicode?

Unicode provides a unique number for every character, regardless of platform, software and language.

Computers store letters and other characters by assigning a number for each one. Before Unicode was invented, there were hundreds of different encoding systems for assigning these numbers. No single encoding could contain enough characters: for example, the European Union alone requires several different encodings to cover all its languages. Even for a single language like English no single encoding was adequate for all the letters, punctuation, and technical symbols in common use.

These encoding systems also conflict with one another. That is, two encodings can use the same number for two different characters, or use different numbers for the same character. Any given computer (especially servers) needs to support many different encodings; yet whenever data is passed between different encodings or platforms, that data always runs the risk of corruption. [Unicode]

Why Use Unicode Braille Patterns?

PEF uses Unicode braille patterns rather than ASCII characters for several reasons:

  1. Unicode and the Unicode braille patterns are locale independent. The translation between ASCII characters and braille patterns isn’t. For example, common characters like .?()+$% and # render different braille patterns depending on locale. It is impossible to send an ASCII file for embossing on an embosser configured for another locale. This is a major issue for the non English speaking part of the world and it is the main reason for using Unicode braille patterns instead of ASCII characters.
  2. Unicode braille patterns are unambiguous. Even if there were only one braille pattern for each ASCII character, the interpretation would still be ambiguous since ASCII braille - though depending on an alternate interpretation of ASCII characters - provides no means of differentiating ASCII braille from regular ASCII.
  3. The Unicode braille patterns are part of the Unicode standard, a global and widely accepted standard. ASCII braille does not follow any known standard.
  4. Translating Unicode braille patterns into any ASCII based braille format is a simple one-to-one-mapping. However, the opposite is not true, unless you know beforehand which locale it was produced for.

What is XML?

The Extensible Markup Language (XML) describes a class of data objects called XML documents. XML was conceived as a means of retaining the power and flexibility of SGML, while reducing its complexity [XML].

Why Use XML?

PEF uses XML because:

  1. It allows metadata. A PEF file can contain information about its contents.
  2. It is extensible. New features can easily be introduced in future versions, such as ink overlay or tactile graphics. Third party custom features such as heading or paragraph markers are also possible via namespace additions.
  3. XML tools are widely available and easy to use. No parsing or validating tools need to be developed to implement PEF.
  4. It allows structural markup. Following XML convention, PEF uses markup rather than control characters for structural elements such as rows and pages. Structural markup simplifies processing in an XML environment and ensures compatibility with future extensions.

On Hardware Limitations

Typically, embossers have very limited RAM and processing power, and might not be able to read and interpret PEF files natively. However, the PEF specification does not require that PEF support is implemented in the embosser's hardware. Alternatively, support for PEF can be added to the embosser print driver or other bundled software.

Scope Limitations

PEF is an delivery and archive format. There are many issues in braille production that PEF neither can nor should solve. For example, issues involving:

Further Development

Currently, there is very little common ground between embossers. Many aspects of page layout were impossible to include as requirements in this version of PEF because it would have limited the range of potentially compatible embossers too much. However, future versions will be more demanding since page layout is an important part of PEF.

Definitions

This section is normative.

The following terms and definitions are used within this document.

Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

General Terms

6-dot braille
6-dot braille is a writing system that only utilizes braille patterns consisting of a combination of the uppermost six dots. See also Unicode character range 2800-283F.
8-dot braille
8-dot braille is a writing system that utilizes all possible braille patterns. See also Unicode character range 2800-28FF.
Braille cell
A braille cell is a single braille unit or character. At least one braille cell is needed to form a single print character.
Braille pattern
A braille pattern is a combination of dots that make up a braille cell.
Document
A document is a stream of data that, after being combined with any other streams it references, is structured such that it holds information contained within elements, that are organized as defined in an associated schema. See Document Conformance for more information.
Facilities
Facilities are elements, attributes, and the semantics associated with those elements and attributes.
Implementation
See User agent.
Parsing
Parsing is the act whereby a document is read, and the information contained within the document is filtered into the context of the elements in which the information is structured.
Rendering
See definition in Device Independence Glossary document [DI Glossary].
Schema
A schema is a collection of markup rules that, as a collection, define the legal structure, elements, and attributes available for use in an XML document that complies to the defined schema.
Unicode character range 2800-283F
The range of 6-dot braille patterns in Unicode. See also 6-dot braille.
Unicode character range 2800-28FF
The complete range of braille patterns in Unicode; the range of 8-dot braille patterns in Unicode. See also 8-dot braille.
Unicode character range 2840-28FF
The range of braille patterns in Unicode that are unique to 8-dot braille, i.e. any 8-dot braille pattern that isn't in the 6-dot braille pattern subset.
User agent
See definition in Device Independence Glossary document [DI Glossary].
Validation
Validation is a process whereby documents are verified against the associated schema, ensuring that the structure, use of elements, and use of attributes are consistent with the definitions in the schema.
Well-formed
A document is well-formed when it is structured according to the rules defined in Section 2.1 of the XML 1.0 Recommendation [XML].

Axes and Node Types

The use of axes in this document (such as "child", "parent", "ancestor", "descendant") is consistent with Section 2.2 of the XPath 1.0 Recommendation [XPath].

The use of node types in this document (such as "root", "element", "text", "attribute") is consistent with Section 5 of the XPath 1.0 Recommendation [XPath].

Relationship to Other Specifications

This specification is based on the specific versions of the standards and specifications referenced herein, which are used as defined except as noted in this document. Any refinement or replacement of a referenced specification by a newer or different version is not directly applicable to this standard. Conformance to this standard is based on the versions of the standards and specifications in effect at the time of writing.

Definition of PEF 1.0

This section is normative.

Document Conformance

A conforming PEF document is an XML document that requires only the facilities described as mandatory in this specification. Such a document must meet all of the following criteria:

  1. It must conform to the constraints associated with the namespaces used in it.
  2. The local part of the root element of the document must be pef.
  3. The root element of the document must be in the PEF namespace.
  4. The start tag of the root element of the document must contain an xmlns declaration for the PEF namespace [XMLNS]. The namespace URI for PEF 1.0 is defined to be http://www.daisy.org/ns/2008/pef. An example root element might look like: <pef version="2008-1" xmlns="http://www.daisy.org/ns/2008/pef">
  5. It must be encoded with UTF-8 or UTF-16 encoding as per [Unicode].

User Agent Conformance

A conforming user agent must meet all of the following criteria:

  1. The user agent must support UTF-8 encoding as per [Unicode].
  2. The user agent must support Unicode character range 2800-283F.
  3. When the user agent claims to support facilities defined within this specification or required by this specification through normative reference, it must do so in ways consistent with the facilities' definition.
  4. If a user agent encounters an element it does not recognize, it must process the element's child nodes, except its text node children. For example, a user agent should read

    ...
    <pef:page xmlns:pef="http://www.daisy.org/ns/2008/pef">
    <ext:group xmlns:ext="http://www.example.org">
    <ext:annotation>This is a row group annotation</ext:annotation>
    <pef:row>&#x2800;&#x2801;</pef:row>
    <pef:row>&#x2800;&#x2802;</pef:row>
    <pef:row>&#x2800;&#x2803;</pef:row>
    </ext:group>
    </pef:page>
    ...

    as

    ...
    <pef:page xmlns:pef="http://www.daisy.org/ns/2008/pef">
    <pef:row>&#x2800;&#x2801;</pef:row>
    <pef:row>&#x2800;&#x2802;</pef:row>
    <pef:row>&#x2800;&#x2803;</pef:row>
    </pef:page>
    ...

    if the namespace associated with the ext prefix is unknown to the user agent. Note that the annotation text was not read.

  5. If a user agent encounters an attribute it does not recognize, it must ignore the entire attribute specification (i.e., the attribute and its value).
  6. If a user agent encounters an attribute value it does not recognize, it must ignore the entire attribute specification (i.e., the attribute and its value).

A conforming user agent is recommended to meet the following criteria:

  1. The user agent is recommended to support Unicode character range 2840-28FF.
  2. The user agent is recommended to support UTF-16 encoding.

Rendering Definitions

Dot-to-dot height
The vertical distance between dot centers within a Braille cell.
Page alignment
Pages align to the top-left corner of the embosseble area of a sheet.
Row height
Row height, excluding row gap, is four times the dot-to-dot height, regardless of whether the user agent supports Unicode character range 2840-28FF or not.
Row gap
Row gap is rendered below the row it is associated with. Row gap is expressed as an integer multiple of the dot-to-dot height.
Sections
A section always starts on a blank sheet of paper. The reason for this is that a change from duplex to simplex or vice-versa can't be accurately effectuated unless the sheet is blank.

Attributes

The value specified by the attributes cols, rows, rowgap and duplex applies to the element where it is specified, and to its descendants unless overridden by another instance of these attributes.

If the definition below should conflict with the Relax NG rule set, the latter takes precedence.

cols
Description: Defines the maximum number of braille cells contained on a row within scope.
Occurence: Mandatory on volume elements and optional on section elements.
Value: A positive integer.
duplex
Description: Defines whether or not both sides of the sheet should be embossed.
Occurence: Mandatory on volume elements and optional on section elements.
Value: The string "true" or "false".
rowgap
Description: Defines the size of the gap following a row.
Occurence: Mandatory on volume elements and optional on section, page and row elements.
Value: A non negative integer.
rows
Description: Defines the maximum number of rows contained on a page within scope.
Occurence: Mandatory on volume elements and optional on section elements.
Value: A positive integer.
version
Description: Defines the pef version.
Occurence: Mandatory on pef elements.
Value: The string "2008-1".

Using PEF with Other Namespaces

The PEF namespace may be used with other XML namespaces as per [XMLNS].

Internet Media Type and File Extension

The Internet media type for PEF is "application/x-pef+xml".

It is recommended that PEF files have the extension ".pef" (all lowercase) on all platforms.

Elements

This section is informative.

Unprefixed elements are in the PEF namespace. The prefix dc is used in this section on elements in the [Dublin Core] namespace.

Elements are listed in document order.

pef

Description
The pef element is the root element of a PEF document.
Child elements
head (mandatory)
body (mandatory)
Description
The head element contains header information, such as metadata.
Parent element
pef
Child elements
meta (mandatory)

meta

Description
The meta element contains metadata information.
Parent element
head
Child elements
dc:format (mandatory)
dc:identifier (mandatory)
dc:title (optional)
dc:creator (optional)
dc:subject (optional)
dc:description (optional)
dc:publisher (optional)
dc:contributor (optional)
dc:date (optional)
dc:type (optional)
dc:source (optional)
dc:language (optional)
dc:relation (optional)
dc:coverage (optional)
dc:rights (optional)

dc:format

Description
The file format, physical medium, or dimensions of the resource.
Parent element
meta
Value
The string "application/x-pef+xml".

body

Description
The body element contains the body of the document, its contents.
Parent element
pef
Child elements
volume (at least one)

volume

Description
The volume element defines a volume of braille, i.e. a range of pages that, once embossed, are to be joined together to form a physical unit.
Parent element
body
Child elements
section (at least one)

section

Description
The section element defines a range of pages in a volume that share some common property, such as duplex or dimensions settings.
Parent element
volume
Child elements
page (at least one)

page

Description
The page element defines a braille page.
Parent element
section
Child elements
row (zero or more)

row

Description
The row element defines a row of braille.
Parent element
page
Value
A string containing Unicode braille patterns.

Metadata Extension

This section is informative.

PEF includes Dublin Core metadata elements for convenience, but the metadata section can easily be extended to include other elements. Here's an example of how to extend the metadata section.

<head>
<meta xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:org="http://www.example.org">
<dc:title>Butterfly Test Pattern</dc:title>
<dc:creator>Joel Håkansson</dc:creator>
<dc:date>2008-09-26</dc:date>
<dc:format>application/x-pef+xml</dc:format>
<dc:identifier>org.pef.00001</dc:identifier>
<dc:description>
A PEF 1.0 example. The file contains a test pattern that is symmetric along the horizontal and vertical axis and uses all 6-dot patterns. In most cases, a faulty conversion will render an asymmetrical pattern.
</dc:description>
<org:paper-size value="A4"/>
<org:grade value="1"/>
<org:margin-top value="5cm"/>
<org:margin-left value="2cm"/>
<org:binding content="coil"/>
</meta>
</head>

Error Handling

This section is normative.

If a user agent encounters a document that it is unable to process must inform the user and abort. However, because of embosser limitations, fallback behaviour is acceptable under the following circumstances:

The user agent cannot set row gap to the specified value
The user agent may choose the closest possible smaller or larger value that it supports, as long as no exception is raised as a consequence. The user agent may prompt the user before continuing.
The user agent cannot set specified duplex value
The user agent may choose a value that it supports and continue processing. The user agent may prompt the user before continuing.
The user agent cannot produce volumes
The user agent may disregard volume information and continue processing. The user agent may prompt the user before continuing.
The user agent cannot process braille patterns in range 2840-28FF
The user agent may prompt the user for one of the following actions:
  1. ignore unsupported braille patterns (rows will be shortened)
  2. replace unsupported braille patterns with a fixed replacement, e.g. 2800 (empty cell) or 283F (full 6-dot cell)
  3. trim unsupported braille patterns by ignoring dots 7 and 8, e.g. by bit masking the code point with 283F.

A user agent that implements fallback behaviour must inform the user of the rendering implications.

Rule Sets

The rule sets mentioned below are included in the zip file for this specification. Users looking for local copies of the rule sets to work with should download and use this archive rather than using the specific references below.

Relax NG

This section is normative.

The Relax NG rule set "validation/pef-2008-1.rng" forms a normative part of this specification. It defines the data model for the PEF and Dublin Core namespace. Other namespaces must be filtered out before evaluating against this rule set.

XML Schemas and DTD Rule Sets

This section is informative.

The Dublin Core XML Schema "validation/dc.xsd", the PEF XML Schema "validation/pef-2008-1.xsd" and the PEF DTD "validation/pef-2008-1.dtd" rule sets form an informative part of this specification. They are provided for compatibility reasons but they do not include all the normative restrictions and features enforced by the Relax NG rule set.

The Dublin Core specification is maintained by [Dublin Core].

References

Normative References

This section is normative.

[DI Glossary]
"Glossary of Terms for Device Independence", Rhys Lewis, 18 January 2005.
Latest version available at: http://www.w3.org/TR/di-gloss/
[Dublin Core]
"Dublin Core Metadata Element Set, Version 1.1"
Latest version available at: http://dublincore.org/documents/dces/
[RFC2119]
"RFC2119: Key words for use in RFCs to Indicate Requirement Levels", S. Bradner, March 1997.
Available at: http://www.ietf.org/rfc/rfc2119.txt
[Unicode]
"Unicode Standard, Version 5.1.0", The Unicode Consortium. The Unicode Standard, Version 5.1.0, defined by: The Unicode Standard, Version 5.0 (Boston, MA, Addison-Wesley, 2007. ISBN 0-321-48091-0), as amended by Unicode 5.1.0.
Latest version available at: http://www.unicode.org/versions/latest/
[XML]
"Extensible Markup Language (XML) 1.0 (Fourth Edition)", T. Bray, J. Paoli, C. M. Sperberg-McQueen, E. Maler, F. Yergeau, 29 September 2006.
Latest version available at: http://www.w3.org/TR/xml
[XMLNS]
"Namespaces in XML 1.0 (Second Edition)", T. Bray, D. Hollander, A. Layman, R. Tobin, 16 August 2006.
Latest version available at: http://www.w3.org/TR/xml-names
[XPath]
"XML Path Language (XPath) Version (1.0)", J. Clark, S. DeRose, 16 November 1999.
Latest version available at: http://www.w3.org/TR/xpath

Informative References

This section is informative.

[Example PEF document: Om våren]
"examples/poem.pef". A poem by the Swedish poet Nils Ferlin called "Om våren" (the text is in Swedish). Accessibility note: The Unicode braille patterns in this file may not display with JAWS or braille display.
[Example PEF document: Butterfly Test Pattern]
"examples/butterfly.pef". A test pattern that uses all 6-dot braille patterns, rendering a butterfly shape.
Accessibility note: The Unicode braille patterns in this file may not display with JAWS or braille display.