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Dng fixes #285

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merged 2 commits into from Apr 11, 2022
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Dng fixes #285

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DNG: Add DateTimeOriginal exif tag
davidplowman Apr 7, 2022
a6b5812
DNG: workaround for bogus IDF1 directory at end of file
davidplowman Apr 7, 2022
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16 changes: 16 additions & 0 deletions image/dng.cpp
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Expand Up @@ -304,6 +304,14 @@ void dng_save(std::vector<libcamera::Span<uint8_t>> const &mem, StreamInfo const
// Create a separate IFD just for the EXIF tags. Why we couldn't simply have
// DNG tags for these, which would have made life so much easier, I have no idea.
Comment on lines 304 to 305
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@kmilos kmilos May 12, 2022 •
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But you do - it's called TIFF/EP (which DNG extends if you've read the spec carefully, as do other raw containers like NEF) and these tags go straight into IFD0 and replicate most of Exif tags. Everything is in these two specs:

http://www.barrypearson.co.uk/top2009/downloads/TAG2000-22_DIS12234-2.pdf
https://helpx.adobe.com/content/dam/help/en/photoshop/pdf/dng_spec_1_6_0_0.pdf

So typically DNGs don't have an ExifIFD unless there are really some other tags needed that don't exist in TIFF/EP+DNG. Usually you just have IFD0 w/ preview and all the metadata, and SubIFD w/ the raw image.

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Hi, thanks for that information. In fact I do seem to remember trying to get it to accept those tags in IFD0 but I never got it to work. I always got "unknown tag" errors, unless I was putting them in the EXIFID. Do you know if libtiff supports this feature? Perhaps libtiff isn't the best library to use, do you know of any alternatives? Thanks again!

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@kmilos kmilos May 12, 2022 •
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Ah, haven't tried that in a while, might be a problem indeed. Perhaps that's best raised w/ libtiff as an unnecessary limitation.

As a workaround, you could close the file w/o these "Exif" specific tags w/ libtiff, then add them via exiv2 library API to the Exif.Image directory (i.e. IFD0), rather than Exif.Photo (ExifIFD). (There is also libexif which might be lighter but don't know if it'll do the job...)

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To be honest, I've found writing DNG files in C/C++ to be unreasonably difficult. Given that dcraw, RawTherapee, DarkTable and the Python libraries we use in our own software all seem to accept them, I'm not very keen to spend more time on it. Moving forwards we're going to be making ever more use of Picamera2 which supports DNG using the PiDNG Python library.

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You could use the Adobe DNG SDK then, perhaps that is easier...

In any case, requested w/ libtiff.

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Thanks for submitting that, let's see what happens!

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@davidplowman The PiDNG description at https://github.com/schoolpost/PiDNG kind of implies it should have generic DNG support, any reason it was named PiDNG as opposed to PyDNG?

Thanks for mentioning that, it looks like it might be the solution to some of my headaches of writing DNGs from Python after reading raw data with rawpy.

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I think the PiDNG library was originally for the Pi but it has expanded since. You'd have to check with the author who has no affiliation with Raspberry Pi but has very generously been happy to help us make use of his work.

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@kmilos kmilos May 12, 2022 •
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Looks like it went the other way for some reason: https://github.com/schoolpost/PyDNG

TIFFCreateEXIFDirectory(tif);

time_t t;
time(&t);
struct tm *time_info = localtime(&t);
char time_str[32];
strftime(time_str, 32, "%Y:%m:%d %H:%M:%S", time_info);
TIFFSetField(tif, EXIFTAG_DATETIMEORIGINAL, time_str);

TIFFSetField(tif, EXIFTAG_ISOSPEEDRATINGS, 1, &iso);
TIFFSetField(tif, EXIFTAG_EXPOSURETIME, exp_time);

Expand All @@ -317,6 +325,14 @@ void dng_save(std::vector<libcamera::Span<uint8_t>> const &mem, StreamInfo const
TIFFSetField(tif, TIFFTAG_EXIFIFD, offset_exififd);
TIFFWriteDirectory(tif);

// For reasons unknown, the last sub-IFD that we make seems to reappear at the
// end of the file as IDF1, and some tools (exiftool for example) are prone to
// complain about it. As far as I can see the code above is doing the correct
// things, and I can't find any references to this problem anywhere. So frankly
// I have no idea what is happening - please let us know if you do. Anyway,
// this bodge appears to make the problem go away...
TIFFUnlinkDirectory(tif, 2);

TIFFClose(tif);
}
catch (std::exception const &e)
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