Playing with exif

Exif information is most commonly encountered in pictures from digital cameras. I don’t have a lot of requirements surrounding exif, just the following:

Losslessly rotate a jpeg image, and also rotate its exif thumbnail and set the exif rotation flag to “Doesn’t need to be rotated”.
Set the file’s modify time to be the same as its exif timestamp (necessary if there’s been any transformation on the image).
Change the create time in the exif header if it somehow got wrong. I had to do this when my camera forgot the date and time completely.
Copy exif information from a RAW file and into a corresponding jpeg (since RawStudio stupidly doesn’t do this for me).
Totally delete the exif information before putting images on the web.

First Go

I had been using jpegtran to rotate my jpegs, and then jhead to update the mtime. I wasn’t totally happy with jhead in particular, because it seems too windowsy and not very complete. Anyway, image rotation worked well with jpegtran -rotate 90 etc, and jhead did its job: jhead -ft image.jpg sets the mtime to what exif says, and jhead -de image.jpg deletes the header altogether. But there are better solutions!

exiftran

Exiftran is a part of fbida. I stumbled across it and discovered that, unlike jpegtran, it updates the exif information. So now my image rotation commands are:

Rotate 270 degrees (ie anti-clockwise):

% exiftran -2 -o rotated.jpg original.jpg

Rotate 180 degrees

% exiftran -1 -o rotated.jpg original.jpg

Rotate 90 degrees (ie clockwise)

% exiftran -9 -o rotated.jpg original.jpg

And the one I like the best:

Rotate automatically based on the exif rotation tag

% exiftran -a -o rotated.jpg original.jpg

Exiftran can also fix up the mess jpegtran left behind, by using the -ni switch (to not rotate the jpeg, but still rotate the thumbnail and update the rotation tag). The -i switch tells exiftran to write the rotated image out to the original filename, and -p tells it to keep the original mtime when using -i. Someday I may stop using gqview for rotation altogether, and simply do

% exiftran -aip *.jpg

exiv2

Exiv2 is a program and library for manipulating the exif header. And it doesn’t just work on jpegs, but on any file with exif, such as TIFFs and RAW files. exiv2 is a complicated beast. The options to solve my problems are:

Set the mtime of the file to the exif creation time (like the old jhead -ft did).

% exiv2 mv -T abc.jpg

Copy the exif headers from the Nikon RAW files in /tmp to the correspondingly named jpegs in /data. Not all the exif data makes sense, since the RAW file has been converted to a jpeg, but as far as I’m concerned, it’s close enough. I mostly need the time, and possibly the aperture and shutter speed etc., and that’s all still valid.

% exiv2 in -k -l /tmp -S .nef /data/*.jpg

Remove exif headers from the jpeg file.

% exiv2 rm *.jpg

Hex Editor

Neither jhead nor exiv2 can cope with the timestamp used by one of my cameras when the camera’s date hasn’t been set. I have this problem because somehow the camera forgot the date and time and took a bunch of photos without them. As a result I’ve been getting this message:

Failed to parse or convert timestamp +0000:00:00 00:00:00

Goodness knows why none of these programs can only adjust the date, rather than setting it. Anyway, I’ve been able to solve the problem using a hex editor. Thankfully the timestamps (there are 3 of them) are quite easily found and edited in plain text.