Sorry to have taken so long to respond to folks…. We’ve been in discussions in the C2PA about what (if any) changes we should make to our specification to better align with libTIFF and the TIFF community in general. Here is what we believe is the correct answer - we would welcome your feedback.
As previously mentioned by me and also mentioned in our specification, the C2PA believes that there should only be a single Content Credential in a given TIFF file - regardless of how many actual images may be present. This is consistent with all other formats that may contain multiple assets/images - APNG, JPEG XL, HEIC, etc. It is for this reason that we put our data into our own IFD, rather than associating it with some other (arbitrary) IFD. Based on our reading of the TIFF v6 spec, and in consultation with the maintainers of TIFF at Adobe, we consider this a valid position. However, in the case where a TIFF contains only a single image (and thus a single top-level IFD), we are going to change our spec to have the C2PA Manifest’s IFD be a sub-IFD to that single image - since that makes it clear that it is associated with that image. That will also make things more compatible with libTIFF for the most common scenarios. Before we finalize this decision for our next revision, we would welcome your input. Thanks, Leonard
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