Whoops, my posts disappeared into the overnight black hole the other day. Re post here...
There's another reason writing back to the RAW format is stupid: unless you're shooting with a digital back, the RAW format is limited to 12 or 14 bits. If you're exposure blending to increase captured DR, limiting yourself to 12 bits is going to leave you with quantization noise problems in the shadows 4 stops sooner than a 16-bit format.
Doesn't apply in my case, I'm only copying the R, G and B channels from individual exposures. No tone curves, no expansion of the DR of any individual channel data, just direct copies. So 12-bits in, 12-bits out. No quantization problems.
The maximum increase in captured DR cannot be any greater than using a magenta filter,
Not really true. A magenta filter affects R and B identically. On my camera, in daylight, the R channel is 2/3 stop lower than the B channel. Under clouds or in open shade it is even worse. So a magenta filter does little to help my R channel - the most important channel for my B&W conversions. Yeah, in theory I could carry around a whole set of CC filters and stack them based on lighting conditions... Actually, for B&W conversions I may use a Wratten 15. It cuts at 530nm, right in the middle of the G channel. It gives me equal R and G channels in daylight and has the added benefit of taking most of the G channel from the red end of its spectral response. Of course I lose the B, but I'm usually dropping that in my B&W conversions anyway.
and you have an extremely kludgy workflow with all the disadvantages inherent to blending multiple captures (alignment corrections, problems with subject movement, etc.). All of the disadvantages + none of the advantages = don't bother. If you want to increase captured DR, blending bracketed linear RGB images is the best way to go.
I wouldn't be surprised at all to discover that if I'm going to bother with multiple exposures that in the end a standard HDR merging approach is as good or superior to what I'm proposing to try - just as you suggest. That doesn't mean I wouldn't want to try, and that is why I'm asking about tools for repacking RAW data that has been modified.
a. convert the raw file in uncompressed DNG,
e. create another DNG file; the part before and after the raw data needs to be copied around the updated raw data. This is the easiest step, it can be cone in MS command mode by COPY.
Good idea, I hadn't realized there was an uncompressed DNG format. That would be a pretty easy task to do extraction and then just write back to the same offset. Thanks for the suggestion!
However, difficulties may arise:
Yes, many! I'm only trying to do something for B&W conversion so at least many of the color disasters that might occur are less of an issue.
Ken, which camera are you thinking of?
At the moment I'm doing this on a Panasonic G1.