Solving TIFF malformation using exiftool
The issue page describes the problem as (essentially) TIFF files being unusable, despite being "validated" by tools like JHOVE.
A lot of time was spent investigating the details of the file structure, for the images which could not be opened; which ultimately was a dead-end.
Eventually, we realised that the images were claiming to be 16-bit greyscale, but in fact, were actualy 8-bit greyscale. (Which, in retrospect, should have been more obvious, based on the rare error messages tools would give us.)
This meant that detection/correction suddenly became a lot easier: check for a discrepancy between image dimension and bit-depth, and the actual file size. (See below.) Similarly, because the pixel data was actually correct, we can correct the images by setting the correct value for the "BitsPerSample" tag.
- What does "valid" mean for a file?
- Verified externally by a tool?
- Matching a spec?
- Internally consistent?
- What do we learn from this?
- Don't always assume your vendors/digitisers are doing the job right.
- Don't always assume that "successful validation" is meaningful. (Also: learn the limitations of your tools.)
- The only thing better than double-checking is triple-checking.
- KNOW WHAT YOU ARE "PRESERVING"!