Exiv2

In TIFF and TIFF-like images, some Exif.*.* tags are required to correctly display the primary image. These image tags contain image data rather than metadata.

When Exiv2 writes metadata, it must not add, modify or delete any image tags.

The Exiv2 write operation is a two step process:

First, it attempts non-intrusive writing, i.e., in-place updating of the image without re-writing the TIFF structure. The existing image is parsed into a TIFF composite tree and updated with the Exif tags from the Exiv2 Image. No tags are added or deleted in this step and modifying of image tags must be avoided.

Second, if non-intrusive writing is not possible, Exiv2 re-writes the TIFF structure. In this step, all image tags from the original image must be copied to the new TIFF structure. The Exif tags from the Exiv2 Image are used to complete the new TIFF structure. They must not be used to add, modify or delete any of the image tags from the original image.

It should still be possible to create TIFF or TIFF-like images from scratch. In this case (intrusive writing without an original image), the image tags set as Exif tags in the Exiv2 Image will be written to the TIFF structure.

Required is

• for each TIFF-like image format, a mechanism to determine if a tag is an image tag
• a way to access this mechanism from TiffParserWorker::encode and TiffEncoder
• logic in TiffParserWorker::encode to copy image tags from the original image to the new TIFF structure in case of intrusive writing
• logic in TiffEncoder::encodeTiffComponent to avoid adding, modifying and deleting image tags during both, non-intrusive and intrusive writing

• logic in TiffParserWorker::encode to copy image tags from the original image to the new TIFF structure in case of intrusive writing

That's turning out to be the messy part. Added a TiffCopier visitor and introducing a clone() function on TiffComponent now.

Getting there...

• eventually the "special purpose code" should be removed from the command line utility