I don't think either spec says that the data is stored as an IFD. I
think they both say as an entry in an IFD.
With regard to IFD's as non-images, the TIFF 6 spec says, "There may be
more than one IFD in a TIFF file. Each IFD defines a /subfile/," and
later, "TIFF readers must be prepared for multiple images (subfiles) per
TIFF file, although they are not required to do anything with images
after the first one."
The ICC profile technote says, "as a single TIFF field or Image File
Directory (IFD) entry in the IFD". I read the "or Image File Directory
(IFD) entry in the IFD" as an alternate wording for "single TIFF field"
since it says "entry in the IFD". It then goes on to describe an IFD
entry for the profile.
The TIFF 6 spec says, "A TIFF field is a logical entity consisting of
TIFF tag and its value. This logical concept is implemented as an IFD
Entry, plus the actual value if it doesn’t fit into the value/offset
part, the last 4 bytes of the IFD Entry. The terms TIFF field and IFD
entry are interchangeable in most contexts."
The C2PA spec requires an IFD with a single entry containing their tag
(52545) pointing to the data at the end of the file. I think that means
it's not a valid TIFF file, since that IFD has no image data or tags.
It seems like it wouldn't be hard to special-case, but I'm not sure why
they didn't use an entry in the first IFD, as Even mentioned.
Phillip
On 3/4/2026 5:05 PM, Leonard Rosenthol via Tiff wrote:
An IFD with that tag - see
https://archive.color.org/files/technotes/ICC-Technote-ProfileEmbedding.pdf
Leonard
*From: *Kemp Watson <[email protected]>
*Date: *Wednesday, March 4, 2026 at 5:00 PM
*To: *Leonard Rosenthol <[email protected]>
*Cc: *Even Rouault <[email protected]>, [email protected]
<[email protected]>
*Subject: *Re: [Tiff] Libtiff failing on TIFFs with embedded C2PA
Manifests?
*EXTERNAL: Use caution when clicking on links or opening attachments.*
Isn't an ICC profile supposed to go in a TIFF tag, not an IFD? Tag 34675.
W. Kemp Watson, Managing Director
Objective Pathology Services Limited
Toronto, Canada
On Wed, 4 Mar 2026 at 15:51, Leonard Rosenthol via Tiff
<[email protected]> wrote:
Looping back on this.
Is there consensus that libTIFF’s implementation - assuming that
all IFD’s point to images - is incorrect as per the TIFF
specification and well known existing usage (e.g., ICC profiles,
C2PA Manifests etc.)?
If so, can we get a fix out ASAP?
If not, what is the argument for why the current implementation is
correct?
Thanks,
Leonard
*From: *Even Rouault <[email protected]>
*Date: *Monday, March 2, 2026 at 1:43 PM
*To: *Leonard Rosenthol <[email protected]>,
[email protected] <[email protected]>
*Subject: *Re: [Tiff] Libtiff failing on TIFFs with embedded C2PA
Manifests?
*EXTERNAL: Use caution when clicking on links or opening attachments.*
Seems to me like a weird design to hijack a IFD (*Image* File
Directory) to put something different in it. Why not having used a
TIFF tag in the first IFD for example ? But I assume my question
is mostly moot now if that C2PA spec has progressed beyond being a
draft?
I guess if someone wanted those files to be read by libtiff, the
best course of action would be to silently ignore that final IFD,
or just emit a warning instead of an error ( I assume there must
be a spefic TIFF tag to recognize it?)
Even
Le 02/03/2026 à 19:30, Leonard Rosenthol via Tiff a écrit :
A person reported
(https://github.com/contentauth/c2pa-rs/issues/1894) that
libtiff-based apps are failing to process TIFF files with
embedded C2PA Manifests
(https://c2pa.org/specifications/specifications/2.3/specs/C2PA_Specification.html).
I’ve commented there that I don’t agree with their position
on the topic, nor can I verify their results.
Would love for any/all of you to comment on the situation -
either here or there.
Thanks,
Leonard
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