Take a look at pages 249-250 in Peter Krogh's "The DAM Book," Second Edition. As I said, I have personally experienced the integrity check of the DNG converter.
Hi Patrick,
I'm not contesting the usefulness of verifying that data remains unchanged. I'm just not convinced yet that a conversion to DNG is better at detecting a Raw image data change upon initial import than e.g. the native Raw converter that came with the camera.
In fact, step 1. in Krogh's suggested Ingestion workflow is; "1. Download: Move the images from the camera to the computer."
He later expands on that step as follows: (quoting a fragment under assumption of Fair use, colored emphasis is mine)
No matter what the download type, there is a possibility of transfer error producing corrupt files. Since download programs don't do a bit-for-bit comparison with the data on the card, you will want to do a visual inspection of the images before erasing the media card. Even if the software did a bit-for-bit comparison with the files on the card, you'd still want to do a visual inspection before erasing the card -- some read errors of the card data can be recovered using data recovery software.
Important to note at this stage is that there is
software that does do a bit-for-bit comparison between the data on the card and data after downloading it. The
ASMP website specifically mentions ChronSync on Mac and SyncBack on PC, as examples of programs that do that.
In addition, I'd have to assume that a native Raw converter would be better at repairing a corrupted Raw file, the structure and data of which they understand intimately, than another converter. Of course, when things are that problematic that the file structure is corrupted beyond repair, then any attempt to convert it with any converter would (most likely) fail.
In step 6. of Krogh's workflow (after Renaming, Applying bulk Meta data, Applying initial image settings, and making a Backup) he gets to : "6. Convert to DNG (optional). You can make a DNG in the ingestion process and take advantage of the data validation hash for added security." Nothing is mentioned about specific image data testing before the hash is calculated.
So, AFTER conversion to DNG, which calculates a hash based on the data it finds (regardless of it's validity at that time) and the DNG converter adds it to the file, it's nice to be able and verify if
at any later moment in time that image data section
of the DNG is still the same (that no 'bit rot' over time has occurred).
Since DNGs will be rewritten every time when changes are made during editing of the image processing parameters, it's logical that the data verification hash only covers the original image data section in the DNG. In fact, given the frequent re-writing it's essential to verify that at least the image data section survived. Re-writing increases the risk of corruption, so it should be mandatory to check for successful re-writes.
Alternatively, when storing the original camera Raws (non-DNG) which do not get re-written (so can only ever change due to errors in file-management or 'bit-rot'), one can also use a separate hash calculation and verification application to do the same verification, for single files or in bulk, without having to use an image editor (it can run as a background process).
Cheers,
Bart