Use Case
You have a paged object, such as a printed book or early manuscript, that has an image missing. You want to include a Canvas in the page flow to acknowledge the missing sequenced image and also to make sure the recto/verso paging functionality isn’t thrown off by altogether eschewing a Canvas at the point of the missing image.
Implementation notes
This recipe attempts to address desires to have a good user experience for people interacting with IIIF resources presented sequentially but lacking one or more images. The recipe also contemplates software consumption of the same. None of the Manifest information described here is required by the IIIF Presentation API, but it may bring benefits to those accessing your work. Nothing in the IIIF Presentation API makes any provision for specifying how or even whether a Canvas replaces what is supposed to appear at that point of the page flow, as the API is agnostic both about your content and its semantics as well as any semantics of your Manifest’s structure.
To maintain a sequence presentation of a paged object with missing images, we suggest adding a content-less Canvas with, at a bare minimum, the id, type, height, width, and items properties. The items property would be written as empty specifically to assert there is no image available. In addition, while the spec doesn’t require the presence of items, the IIIF Presentation API Validator will fail to validate a Canvas without items as it differentiates between Canvas references found in Ranges and Canvas references found not in Ranges by the presence of items.
Using an empty Canvas is a very lightweight and authentic way to show missing content. That is, having a content-free Canvas takes modest effort and can represent the lack of content in a real-world object or an error in creating a digital version. However, with only the bare minimum properties neither viewers, nor people interacting with the resources, nor code interacting with it have any indication of why the image is absent. Consequently, as shown in this recipe’s Manifest, it’s a good idea to make use of the label property to contain useful text.
Additional Information
For a representation of an absent IIIF resource, accessibility to non-visual interactions is particularly important. Using both the minimum suggested properties thoughtfully and additional other properties as the situation demands will be more likely to provide an equitable experience for people with visual disabilities. As an example, note that the metadata property can contain as many arbitrary pairs of label and value as needed to convey accessible information about the missing resource. Note that because viewers are not required to display the metadata in any predictable way or at all, metadata content cannot be assumed to be always readable by both visual and non-visual means.
If the best approach is to redirect visitors away from the resource to explain the absence, other Manifest and Canvas properties can be used to provide content pointers to more information about missing image(s). For instance, homepage can direct them to a webpage that talks about the object and its physical state.
Restrictions
No known restrictions.
Example
For this example, we are using views from a paged Ethiopic manuscript. One verso page has been imagined missing, and represented in the Manifest as discussed above.
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