Tag Archives: photography

f-spot, exiftool, exiv2, and exif header weirdness

I decided to revisit an old bug that I logged about a problem I have with f-spot handling of exif headers. Now I’m more confused than ever. I can see what’s happening, but not how or why.

The problem I’m having is that when I upload some photos to gallery websites the handling of exif data doesn’t appear consistent. For example, have a look at this photo. If you look at the right hand side of the screen where the photo title is shown it also shows the caption. It looks like this:

From Zenfolio gallery

Now have a look at the same photo uploaded to picasaweb (the sizes are slightly different – I think picasaweb changed them on the fly when I uploaded via google plus). In this case there is no garbled text and picasaweb has used the exif description field for the photo caption. i.e:

from picasaweb

So what’s happening here? Well, here’s what I think is happening. Picasaweb is extracting the caption from the exif field Description, and Zenfolio is extracting the caption from the field UserComment. Here’s what I get if I run exiftool on the image file (you might need to click on the image to see it properly):

exiftool -UserComment -Description test1.jpg

As you can see the UserComment field contains garbled characters.

A lot of my photos have this and the inconsistencies can cause me headaches. I decided on a brute-force scan of my photo collection to address the issue. I thought, why not just duplicate the Description field into the UserComment field? That way, whatever package or gallery reads the image file will probably get the caption. So here’s the script I used:

#!/bin/bash

#
#       Process all jpegs and overwrite UserComment field with Description
#       field where they differ.
#
find /jpegs -type f -iname '*.jpg' | while read fname
do
        field_usercomment=$(exiftool -UserComment "${fname}" | sed 's/^.*: //')
        field_description=$(exiftool -Description "${fname}" | sed 's/^.*: //')

        if [[ "${field_usercomment}" != "${field_description}" ]] ; then

                echo "$fname"
                echo "User Comment: ${field_usercomment}"
                echo "Description: ${field_description}"
                echo

                echo "${fname}"
                exiftool -overwrite_original -UserComment="${field_description}" "${fname}"
                echo
        fi
done

After a satisfying hour or two my photos all looked a bit tidier. So now I have a cludge – something that will get round the problem. But I’d really like to treat the cause, and not the symptom. So what’s the cause?

My first guess has always been the camera. I have a knackered old Nikon Coolpix that gives me problems. But I see this problem with other cameras too, my Canon DSLR and compact camera have acquired the garbled field in some pictures too.

So I turn to f-spot, the photo-management software I use in Linux Mint. I checked the exiftool output from a test file, then imported it into f-spot. I then added a tag so that f-spot would write out the exif data (I have f-spot configured to Store Tags and Description inside image files where possible) then had a look at the image file. Here’s what I got:

exiftool output

The output from the first invocation of exiftool is on the jpeg before importing into f-spot. The second example shows the output of exiftool run on the file after it has been imported into f-spot. It looks like f-spot does something to the image that mangles the UserComment field.

At this point I assumed (erroneously I think) that f-spot must be storing the UserComment and Description fields in its sqlite database. Using sqlite3 on the command line I had a look at the structure of the photos table:

dougie@phoenix ~ $ echo '.schema photos' | sqlite3 .config/f-spot/photos.db
CREATE TABLE photos (
    id            INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
    time            INTEGER NOT NULL,
    base_uri        STRING NOT NULL,
    filename        STRING NOT NULL,
    description        TEXT NOT NULL,
    roll_id            INTEGER NOT NULL,
    default_version_id    INTEGER NOT NULL,
    rating            INTEGER NULL
);
dougie@phoenix ~ $

There’s the description field, but no UserComment field. But it looks to me that f-spot is certainly writing to the UserComment field when it updates an image file. Perhaps it’s dependent on the type of contents of the Description field. It doesn’t, however, make any difference as far as I can tell what the contents of the Description field are. In this example the text ends with an exclamation mark, but I can’t detect any pattern in the text I put in Description fields that might provide a clue.

So if f-spot isn’t storing the UserComment field directly what, when, and why is it writing to the field in the image file? It seems that here we enter the murky world of XMP and I rapidly lose the tenuous grasp I had on the underlying technology up to this point.

Back in 2008 there was a discussion on the f-spot mailing list about how f-spot stores its data in the image file. Re-reading that thread it sounds as if f-spot writes its tags to the XMP metadata and not all utilities are able to read that. This was 2008 mind, and f-spot has jumped a few versions since then.

Out of the same thread came a hint on using exiv2. This is a tool I use a lot, especially for adjusting ‘date taken’ timestamps, but hadn’t found it any use for extracting the UserComment field. It turns out I hadn’t RTFM properly and it is possible if you tell exiv2 to look at XMP data. This leads to some interesting output. Here is the example file again, using both exiv2 and exiftool to examine the field contents:

dougie@phoenix ~ $
dougie@phoenix ~ $ exiftool -UserComment -Description /jpegs/2011/07/17/test1.jpg
User Comment                    : 桷⁹畲污湯⁧桴⁥敢捡⁨桷湥礠畯挠湡挠瑵琠敨挠牯敮ⅲ††††††††††††
Description                     : why run along the beach when you can cut the corner!
dougie@phoenix ~ $
dougie@phoenix ~ $ exiv2 -p a /jpegs/2011/07/17/test1.jpg
Exif.Image.ImageDescription                  Ascii      77  why run along the beach when you can cut the corner!                        
Exif.Image.Software                          Ascii      21  f-spot version 0.8.0
Exif.Image.ExifTag                           Long        1  162
Exif.Photo.DateTimeOriginal                  Ascii      20  2011:07:17 09:37:13
Exif.Photo.UserComment                       Undefined  84  why run along the beach when you can cut the corner!                        
Exif.Image.GPSTag                            Long        1  296
Exif.GPSInfo.GPSVersionID                    Byte        4  2.0.0.0
Exif.GPSInfo.GPSLatitudeRef                  Ascii       2  North
Exif.GPSInfo.GPSLatitude                     Rational    3  55deg 32' 25.410"
Exif.GPSInfo.GPSLongitudeRef                 Ascii       2  West
Exif.GPSInfo.GPSLongitude                    Rational    3  1deg 37' 58.960"
Exif.GPSInfo.GPSAltitudeRef                  Byte        1  Above sea level
Exif.GPSInfo.GPSAltitude                     Rational    1  9.5 m
Exif.GPSInfo.GPSTimeStamp                    SRational   3  09:37:13
Exif.GPSInfo.GPSMapDatum                     Ascii       7  WGS-84
Exif.GPSInfo.GPSDateStamp                    Ascii      11  2011:07:17
Xmp.xmp.CreateDate                           XmpText    19  17/07/2011 09:37:13
Xmp.xmp.Rating                               XmpText     1  0
Xmp.xmp.CreatorTool                          XmpText    20  f-spot version 0.8.0
Xmp.dc.description                           LangAlt     1  lang="x-default" why run along the beach when you can cut the corner!                        
Xmp.dc.subject                               XmpBag      2  Dougie Nisbet, Coastal Run - 2011
Xmp.exif.UserComment                         LangAlt     1  lang="x-default" why run along the beach when you can cut the corner!                        
dougie@phoenix ~ $

Interestingly, exiv2 is displaying the UserComment data in clear text, in two places, which I’m guessing must be something to do with the different types of metadata – XMP and EXIF.

So does this mean this has nothing to do with f-spot at all? If exiv2 is able to read the text, perhaps it’s something to do with the utility used to examine the image file? If I have a look at the file using geeqie and open up the EXIF window geeqie seems to understand the text too:

geeqie EXIF window

Perhaps this is an unrelated setting I need to have a look at. Language, Locale, who knows. The issue seems to crop up in different places so it may very well be unrelated to f-spot. For the time being I have a fairly straightforward work-around, I just need to remember to run a script to copy the description field into the UserComment field for any images I update in f-spot. It’s a bit of a nuisance but fairly easy to automate, and a lot less time-consuming than trying to get to the bottom of the mystery!

Time delay of adding caption to photo slideshow

photos make a great screensaver and the xscreensaver with glslideshow option is what I use. I often use it for ident practice, e.g. plants, birds etc. The image appears randomly on the screen and I think, I know what that is …

At the moment I have various scripts that can create ad-hoc collections of images based on the embedded tags. I also overlay a caption over the images using the convert bit of imagemagick and that works well.

What I want to do is have a time delay so that an image appears for say, 10 seconds, and then a caption appears over it. The filename would be ok. I can control how long the image appears in xscreensaver, and it’s surprisingly easy to train yourself to not-look at the filename or caption, but I’m curious as to whether it’s possible (without too much effort) to add the caption after some time-out period.