These are valid DNGs and should work with any application that fully supports the DNG spec or uses the Adobe DNG SDK. But they are not supported by the GainMap implementation in either ART or Darktable master.
The typical smartphone DNGs that are supported by those apps have the GainMaps in OpcodeList2, which are applied to each color channel separately before demosaicing, and correct both vignetting and color shading at the same time.
But these DJI DNGs work differently - in OpcodeList3 (to be applied after demosaicing) they have two separate opcodes. There is FixVignetteRadial which fixes the vignetting and has no effect on color, and there is a GainMap which only affects color shading.
Could someone modify your DNGpreprocess project to target that data and apply it the what you did to std DNG before it was merged into DT??? Or would it be a start from scratch project??
It would be a simple one line change to have the Adobe DNG SDK also perform the OpcodeList3 processing in addition to the OpcodeList2 processing, but that would mean the Adobe DNG SDK also has to demosaic the image in between. I’m not sure how it performs demosaicing but it doesn’t appear to have any sophisticated algorithm like AMaZE or RCD, so there could be some loss of quality.
As for supporting an image like this in Darktable - it probably would be worthwhile to add support for the FixVignetteRadial opcode as it’s also used by some iPhones. Some of the code from the recent change to support GainMaps could also be reused to make other DNG opcodes available to other modules, so the lens correction module could use the FixVignetteRadial data for vignetting correction instead of lensfun.
The GainMap in OpcodeList3 could be trickier - one possibility is to add a separate GainMap implementation either in the demosaic module or in a new module that comes after it. Another possibility is to take that GainMap in OpcodeList3 and transform it into equivalent OpcodeList2 GainMaps that have approximately the same effect but are applied before demosaic. It might not be exactly the same result bit for bit as if it was applied at the correct place, but would probably be close enough.
Thanks for taking the time to explain it. I noticed in ART that you can select embedded for lens correction and it seems to apply one. I was going to ask you if it was a similar thing is grabbing from the meta data…
Here are a couple of .dng files taken with my Fairphone 4 - they appear to have Exif entries for Opcode List 2 and Opcode List 3, so hope they might be of use regarding embedded DNG Gainmaps. Kind regards, Paul
Thanks! These have the typical smartphone GainMaps in OpcodeList2, which are supported by ART and the current Darktable master. The OpcodeList3 has a WarpRectilinear opcode for distortion correction - Darktable does not support that and I’m not sure what other apps do.
In the “raw black/white point module”, if the “embedded GainMap” option appears in the “flat field correction” dropdown, then it is using it. You can switch it back and forth from disabled to see the effect of the GainMap.
For the existing photos - select them in lighttable, under selected images module go to metadata tab and press “refresh EXIF”. The dropdown should appear after that.
I wonder if there is a chance to translate the opcode processing into a lensfun profile. A while ago I tried to figure out what a Mini 3’s opcode list means from the dng spec, but was not yet able to solve this puzzle. I did not even understand if it contains lens calibration data, but at least the amount of entries makes it likely.
In particular, this is how the opcode list of your image starts according to exiv2:
Undefined 12564 0 0 0 2 0 0 0 9 1
I have no clue, what the “Undefined” means, for other tags it describes the data type, and the 12564 is likely the count. Next, the first opcode should start but there is no opcode “0”. Any hint welcome!
The OpcodeList3 in this DNG contains a GainMap opcode which applies both vignetting and color shading correction. It also contains a WarpRectilinear opcode which in theory could correct distortion and CA, but in this file the tag values do nothing and it could have been left out.
The file is valid, but it won’t work with the GainMap implementation that I added to darktable 4.0 because that implementation only works with GainMap in OpcodeList2 (before demosaic) and this one is in OpcodeList3 (after demosaic). At some point I’d like to try to fix that, maybe by converting it to an OpcodeList2 GainMap that has almost the exact same effect.
It would be possible to convert vignetting, distortion, and CA correction from DNG opcodes to lensfun profiles, but the color shading can’t be handled by lensfun. It can only handle vignetting correction which is applied to all channels equally, not color sensitivity that varies from the center of the image to the edge.
Comparing the JPG to the corresponding DNG it looks like there’s either no distortion or the correction is already baked into the DNG.
Getting the GainMap to work in darktable would be awasome! I’m currently using Davinci to load and edit the DNG which does seem to apply the GainMap, but it’s not ideal
