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Dylan Armstrong

Personal Blog by Dylan Armstrong Just some random, potentially useful, things I've written about.

Exif Data

June 02, 2021

I have decided to update my photo site to have exif data below each picture. This will require about 65 GB of data egress from S3 to run exiv2 on each original image file.

To start, I grabbed a small folder, in my case rwanda-2018 with:

aws s3 sync s3://bucket/rwanda-2018 rwanda-2018

Then I took a look at the exif data to see what I actually wanted to store.

The tool used, exiv2 can be intalled with brew install exiv2.

Here’s a link to the example image with exif data

exiv2 -p a IMG_1768.jpeg
Exif.Image.Make                              Ascii       6  Apple
Exif.Image.Model                             Ascii       9  iPhone 7
...
Exif.Image.DateTime                          Ascii      20  2018:11:15 07:32:58
...
Exif.Photo.PixelXDimension                   Long        1  4032
Exif.Photo.PixelYDimension                   Long        1  3024
...
Exif.GPSInfo.GPSLatitudeRef                  Ascii       2  South
Exif.GPSInfo.GPSLatitude                     Rational    3  1deg 39' 28"
Exif.GPSInfo.GPSLongitudeRef                 Ascii       2  East
Exif.GPSInfo.GPSLongitude                    Rational    3  30deg 6' 42"

At this point, I can see that the image has information that I’d like displayed.

Namely, the make, model, resolution, and GPS coordinates of the photo.


So the next step was to determine how to store this information:

  1. RDS would cost about $13/month to run from some quick napkin math.
  2. A _exif file stored in S3 that is called along with every image via fetch was unattractive as:
    • It would require javascript which this photo site mostly avoids
    • It would be slow, and not available on initial render
    • Every single image would have to request ~20 per page for every user
  3. Sqlite3 local database (spoiler: I went with this one)

I ended up going with a local sqlite3 database, it’s dead simple and is no nonsense.

So here’s the quick schema I came up with:

create table images (
  id integer primary key,
  -- The name of the album, such as turkey-2020
  album text not null,
  -- The img file name, such as IMG_1414.jpeg
  file text not null,

  -- Do not allow duplicates
  -- So turkey-2020/IMG_2020.jpeg conflicts with turkey-2020/IMG_2020.jpeg
  -- But not rwanda-2018/IMG_2020.jpeg
  unique (album, file)
);

I decided to split exif data from image data in case I ever wanted to extend in the future with more information such as tags.

Some might have gone with a fully flat table where images also stored exif data, but it just didn’t feel right. On a small application like this (especially where there’s only a one-to-one data mapping) that’s just personal preference, and I wouldn’t say one way was absolutely correct.

create table exif (
  id integer primary key,

  -- I truncated fields that aren't being used on the actual application
  -- ...
  datetime timestamp,
  -- ...
  gps_altitude text,
  gps_altitude_ref text,
  gps_latitude text,
  gps_latitude_ref text,
  gps_longitude text,
  gps_longitude_ref text,
  -- ...
  make text,
  model text,
  pixel_x_dimension text,
  pixel_y_dimension text,
  -- ...

  image integer,
  foreign key (image) references images (id) on delete cascade
);


I figured I would be frequently dropping the database while doing development so I wrote a quick helper python script that runs the above commands called setup-db.py.

It uses better-sqlite3 and just runs those exact inserts.

Then I began work on an insert.py script that will run pyexiv2 on each image in an album. This one was a little more bothersome, just due to the busy work of typing out 25 columns.

The exact keys that we want are the same as the exiv2 output above.

Parsing dates was a little tricky as the exif dates come with : separators for year:month:day. I have no idea why?

  # Get is a helper and will return None or the value
  date = get(data, 'Exif.Image.DateTime'),
  if date:
      try:
          # Uganda pictures are coming back as a tuple
          if isinstance(date, tuple):
              date = date[0]
          date = datetime.strptime(
              date,
              '%Y:%m:%d %H:%M:%S'
          )
      except:
          # Inserting null is fine
          date = None

After syncing every album folder from S3, I ran the insert script on every album with:

for f in $(ls /tmp/photos); do python3 scripts/insert.py /tmp/photos/$f; done

And it nicely inserted 6003 images with all of their exif data like I was hoping for.

Both scripts are in the GitHub repo.


Searching for images then is a pretty simple SQL query:

select
  i.file,
  e.gps_latitude,
  e.gps_latitude_ref,
  e.gps_longitude,
  e.gps_longitude_ref,
  e.datetime,
  e.make,
  e.model,
  e.pixel_x_dimension,
  e.pixel_y_dimension
from images i
join exif e
on
  i.id = e.image
where
  album = ?

I updated the server to populate an exif cache for each album on load.

Ending up with a nice cache that looks like this:

exifCache[albumName][fileName] = {
  make: 'Apple',
  model: 'iPhone 7',
  // ...
};

So on every page request, I get the exif information for each image.

// Images is Array<String>
const data = images
  .slice((page - 1) * imagesPerPage, page * imagesPerPage)
  .map((image) => {
    // rwanda-2018/IMG_1768.jpeg -> IMG_1768.jpeg
    const file = image.split('/')[1];
    // This can be undefined
    const exif = exifCache[album][file];
    return {
      exif,
      // Force a boolean, so we don't show a figcaption when exif is empty
      hasExif: !!exif,
      image: `https://${domain}/${image}`,
      thumb: `https://${domain}/${image.replace(/\./, '_thumb.')}`,
    };
  });

And that’s really about all there was to it.

This can be seen here with source code.