[Digikam-devel] [Bug 211758] Saving TIFF-16 compressed using exiv2 0.18 shows odd results

[Andreas Huggel]

https://bugs.kde.org/show_bug.cgi?id=211758





--- Comment #41 from Andreas Huggel <ahuggel gmx net>  2010-04-13 04:28:17 ---
Re comment #37: Thumbnails

The standards regarding this are conflicting. There are at least four different
specifications to consider:

TIFF/EP and DNG
---------------
Recommend thumbnail in IFD0, main image in a sub-IFD of IFD0.
Chained IFDs are used for multiple non-identical images (each of which using
this sub-IFD tree structure)

Adobe PageMaker TIFF Technical Notes
------------------------------------
(also referred from the Photoshop Technical notes)
Recommend main image in IFD0, thumbnail in a sub-IFD of IFD0.
Chained IFDs are used as above.

Original TIFF
-------------
Only provides for chaining, the sub-IFD tag doesn't exist in the original TIFF
specification. It doesn't mention thumbnails, but the most obvious place to
store one is in a chained IFD, as specified in the Exif standard.

Exif
----
IFD0 contains only metadata, no image data. The Exif thumbnail is stored in a
chained IFD1. In reality, the Makernote structure of many cameras contain
further thumbnails.

---

The thumbnail that Marcel disabled was in a chained IFD1 and not recognized by
GIMP as such - GIMP looks at a chained IFD as a second non-identical image
("page") within the TIFF file, as recommended by the TIFF/EP and DNG standards.

Exiv2 only has support to read thumbnails which already exist in a TIFF image.
It doesn't generate thumbnails from the main image (e.g., after that was
modified). I'm not familiar with libtiff, but that would be the next place to
look for such a feature it seems: 

Does libtiff have a feature to generate a thumbnail from the main image and
store both main image and thumbnail in the appropriate places within the TIFF
image?

Andreas

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