Hi Leonard
The current LibTIFF can be easily updated support an additional TIFF field with ID 52545 for the C2PA manifest data block for an image. But then, the second use case with multiple images, with the extra IFD having C2PA field only will still not be supported. Did you check that an IFD with a single TIFF field is also compatible to DNG and TIFF/EP specifications? - C2PA specification says: “The Digital Negative or DNG format provides camera manufacturers to provide their camera raw formats in a standardized manner. DNG is based on which is based on TIFF/EP” - E.g. TIFF/EP (ISO/DIS 12234-2) defines mandatory fields that have to be within a valid IFD. It says: “All mandatory tags shall be included in the IFD, since default values are not allowed in TIFF/EP, even in cases where default values are given in TIFF 6.0.” My second concern is the requirements about location of the C2PA field and its manifest data, or update of manifest data: - C2PA specification says: “To support update manifests, the C2PA Manifest Store should be located at the end of the file; this ensures that changes to the size of the C2PA Manifest Store do not impact any of the other tag offsets.” - LibTIFF does not have any interface (nor the possibility) to guarantee the location of IFD data and its associated data within the file. - For an update of IFD field contents (size or offset of the associated data) the IFD has to be re-written, which might change its location in the file. Regards Su Von: Tiff [mailto:[email protected]] Im Auftrag von Kemp Watson via Tiff Gesendet: Montag, 16. März 2026 19:27 An: Leonard Rosenthol <[email protected]> Cc: [email protected] Betreff: Re: [Tiff] Update: Content Credentials and TIFF Not sure I care for that solution for single-image TIFFs... seems to be "special-casing" right off the bat... >From what I can read, maybe the spec needs clarifications such as - in which IFD TIFFtag (52545) goes relative to the data it points to (must be in same IFD in my opinion) - whether data is in an IFD as "raw/image data" or is pure raw data in some other container format at end of file W. Kemp Watson, Managing Director Objective Pathology Services Limited Toronto, Canada On Sun, 15 Mar 2026 at 10:31, Leonard Rosenthol via Tiff <[email protected] <mailto:[email protected]> > wrote: 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 _______________________________________________ Tiff mailing list [email protected] <mailto:[email protected]> https://lists.osgeo.org/mailman/listinfo/tiff
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