Content Model

What is the DIAGRAM Content Model?

The DIAGRAM Content Model defines a data model for image description metadata. It provides a structured, standard way for image description data to be modeled, including multiple different types of descriptions for each image, teacher annotations, and pointers to tactile graphic files. The flexibility of the DIAGRAM Content Model to hold information about a variety of image alternatives is a critical and revolutionary acknowledgment of the breadth of alternatives that new technology has made possible.

The first section of this summary article just below (“Why Standards Are Important”) is intended for a non-technical audience and explains the concept of standards and how they apply to creating accessible image representations in digital books.  The second section (The DIAGRAM Content Model in More Detail) explains the development process and provides an example of how the model makes it possible to create several accessible representations for a typical graphic you would find in a science text book.  If you wish to see the full details of the model, the final section (Content Model Draft Candidate) contains links to the DAISY ZedAI web site where profile, schema and vocabulary information can be found.

Why Standards Are Important

One of the challenges involved in creating accessible image alternatives for students with print disabilities is having a way to identify and utilize the data needed to create, read, or interpret the image alternative.  The accessible representation might be in the form of a text description which can be voiced by DTB software, or a tactile graphic that can be embossed on paper or displayed on a refreshable tactile display.  The ability to present these types of content requires a set of specifications, called ‘metadata,’ and a set of rules about how to use the metadata, called ‘standards.’

We see the benefits of standards almost everywhere in our world, for example: time zones, drinking water quality, automotive safety, electrical power delivery, etc.  Having a country-wide standardized electrical system which defines the current type, voltage, and plug design, makes it a common task to plug in an appliance and have it work properly instead of causing a house fire.  Standardization improves efficiency, enhances safety, and reduces the cost of products and services.

You might not be aware of them, but in order for your computer to display or voice a page from the Internet, which seems an intuitive task to us, there are underlying standards for the networks which transmit the data, the language which contains the formatting commands which govern the appearance of the page, and the browsers which interpret and display the content in its intended format.  Without standards, pages on the Internet would not display consistently.

The same concept applies to creating accessible image alternatives.  Right now there are many ways to create alternatives for graphical content in a digital book.  Images can be described in the text of a book, or the descriptions can be coded so they can be voiced by reading devices or software.  The image can be separately produced as a tactile graphic using a variety of techniques which place a raised image on paper or a tactile display.  Three-dimensional models can be made for the student to touch; and haptic systems are under development which can give sensory feed back as a person “touches” a virtual object.

At present, all of these techniques require significant human effort, specialized equipment, or both; which raises the cost and amount of time required for creating the image alternatives.  However, if a set of standards existed for incorporating accessible alternatives into digital books, then authoring and reading tools could be developed to take advantage of them, thereby improving the efficiency and reducing the cost of creating the alternatives.  Ultimately, students with print disabilities would have more timely access to a greater variety of accessible image representations.

The future electronic “book” could include not only the text, but selectively accessible alternatives for each image based on the learning needs of the user.  These might include: a summary description, a long description, a simplified language description, or tactile representations of different types.  Instead of having to create these image alternatives independently using separate tools and methods, through the acceptance of standards image alternatives can be integrated into, or referenced from, electronic books and be readily available to the student.

The DIAGRAM Content Model in More Detail

In order to reach this goal, you need to have a standard that tells you how to create and present the image alternatives.  The DAISY (Digital Accessible Information SYstem) Authoring and Interchange Framework is defining modules and profiles for the representation of books, journals, etc. Using this modular approach, the DIAGRAM project is defining a content model for alternatives to the original graphical content found in the publications. HTML and EPUB documents would have graphical elements linked to specific instances of descriptions that use the content model. These specific instances are expected to be stored outside of the page that contains the graphical content. The goal is that graphical alternatives can be presented to persons with print disabilities in a way that makes sense to them.

To build the content model, members of the working group began by defining uses cases in three areas: producers, facilitators, and consumers.  From the use cases, information requirements were developed and from those requirements, the metadata elements to support them.  These are explained in more detail in the sections below.

Content Model Use Cases

Production Use Cases are targeted towards all users that will participate in creation of (or facilitating the creation of) accessible formats of digital media.  These include some of the following types of users as examples:
•    Rights owners (e.g. publishers, authors, photographers, artists)
•    Alternate format production specialists
•    Contracted volunteers
•    General populace of volunteers

Accessible Media Facilitator Use Cases are targeted towards all users that will participate in finding and distributing accessible media to 3rd parties (the Consumers below).  These include some of the following types of users as examples:
•    Educators
•    Parents
•    Disabilities Specialists

Consumer / Reader Use Cases are targeted towards all users that will be the ones needing to have digital media in alternate formats.  These include some of the following types of users as examples:
•    Blind Users
•    Low Vision Users
•    Dyslexic Users
•    Cognitively Disabled Users

Information Requirements

Once the use cases were developed, the information needed to support them was identified and classified into two elements: metadata and content.  Metadata includes such factors as: purpose, age/grade level, version of content model, links to specific image description repositories, quality rating, and concept (of the image).  Content includes such items as: short description, long description, SVG reference, description of tactile, alternative image, alternative image description, and simplified description.


In order to make the relationships between use cases, information requirements and metadata clearer, consider this example.
Below are two use cases.  The first is an alternative media producer who wants to be provided the purpose and rights information for the image about which the alternative will be created. The second is an original publisher who wants to provide the purpose (reason) for which the graphic element is created.

  • Case 1 – alternative media producer: Be provided the purpose and rights information for the image.
  • Case 2 – original publisher: There is text that provides the purpose for the graphical item. Depending on the context, graphical content can have different purposes …

From the use cases it is evident that there is an information requirement for a metadata element called “purpose” which describes why the image is included in the book and therefore sheds some light on the values of producing an alternative for it.

Now, a mock-up using the metadata example can be created to illustrate how the data element might apply in actual practice.

Illustrates the process of evaporation and precipitation and the natural purification of water.

This process is extended to create the set of metadata elements, supported by use cases, and illustrated by tangible examples.


Content describes the actual information of the image alternative and could include such elements as: short description, long description, SVG reference, description of tactile image, etc.

Example of Metadata and Content Elements

Using this illustration of the hydrologic cycle as an example, a visual graphic could have the following descriptive elements as outlined in the Content Model:

The hydrologic cycle; explanations are contained in the text.

About this description

Author:  John Doe, Ph.D. in Water Engineering
Target Age:   9-12
Target Grade:  4-7


Figure 1.1, The Hydrologic Cycle, shows the processes involved in the water cycle, including evaporation, formation of clouds, precipitation, condensation, infiltration and the paths taken by the water when it has been returned to the ground.

Long Description

The diagram shows the processes of evaporation, condensation, evapotranspiration, water storage in ice and snow, and precipitation. A large body of water (an ocean), sky, earth surface and cross section of the earth structure are shown to illustrate the processes. The diagram also shows what happens to the water which has returned to the earth as precipitation, including surface runoff, infiltration of the water into the ground surface, percolation, ground water flow, and the movement of water flowing into an ocean. The water table and ground water flow are also shown.

The parts of the diagram, from upper left, moving across the top to the right are: a mountain showing water storage as ice and snow, the sky, clouds and the sun; in the lower right and moving to the left across the bottom to the left are: the ocean…

Annotation added by teacher

In the winter if it is below the freezing point, precipitation may take the form of snow rather than rain.

Simplified Language Description

The image shows the water cycle: how it forms clouds and is returned to the earth as rain, and how it goes into and moves over the earth’s surface.

Tactile Image

[Tactile image]
In the upper left corner of the tactile illustration, the mountain covered with ice and snow is represented by…

Simplified Image

[Simplified image]
Moving from the top left corner of the image …

Access Rights Note: This resource is for use by academic institutions only.

Example Content Model XML

Content Model Draft Candidate

To see the details of the draft candidate, visit the following links at the DAISY web site.

The feature resource page

The profile resource page

Schema documentation

Description of the metadata vocabulary

You can go to the DAISY Authoring and Interchange working area to navigate all these pages

The DIAGRAM Content Model in Tobi

Tobi is a software tool that allows you to convert a DAISY or EPUB 3 file into a digital talking book (DTB).  Tobi uses the DIAGRAM Content Model as a basis for allowing users to add long descriptions and other image alternatives to a file. Development is ongoing. Click here to see the DIAGRAM-Tobi image description report April 2013