What is this?

This document contains information on some inner workings of the Apple Photos program. All the information here was compiled by me looking at the library folder, examining the sqlite databases, monitoring changes in files, hexdumping, etc. while playing around with my personal photos collection.

Do what you want with this, but keep in mind it is unofficial and solely based on my personal observations, therefore it is incomplete and some parts could be plain wrong.

Apple may change how their software works at any time and without notice.

Start here

First stop is to navigate to your photos library which is usually located under Pictures in your home folder

$ cd ~/Pictures/Photos \Library.photoslibrary

Depending on how old your library is and weather you've migrated from iPhoto or started using Photos straight away, the contents of the library might differ slightly. Some important ones are:

• database This is where Photos stores it's sqlite3 databases with the most important one being Photos.sqlite
• originals Yep, that's all your original, unmodified photos in there
• resources This directory seems to hold information on quite a few things:
  • files from your iCLoud Shared Albums, both yours and other people's
  • edited photos and editing information
  • resized copies of original photos
  • audit logs
  • project files
  • other iCloud related files
• private Looks like these are config, state and cache files. This dir also contains files related to other services in the Apple Photos ecosystem, like the photoanalysisd (you know, the one that always eats a 100% of your CPU exactly when you are in the middle of doing some important CPU intensive work...)

Database

The Photos.sqlite database contains lots of useful information, let's poke around. Browsing the Z_PRIMARYKEY table reveals what I would refer to as Entities. Each record has a name, an id (Z_ENT), Z_SUPER which maps to the entity's parent and some cached counters. I assume these are the various classes in Photos code.

It's important to note the Z_ENT for an entity, as this is used in other tables to identify the type of a record. For example, on my mac, an Album's id is 26.

Other tables of interest are:

• ZGENERICASSET Records in this table represent your photos
• ZGENERICALBUM Album and folder records are stored here
• Z_XXASSETS It's a many-to-many relationship table that links photos to albums. The XX part is a number that corresponds to the Album entity from the Z_PRIMARYKEY table, so in my case the name of the table is Z_26ASSETS.
• ZADDITIONALATTRIBUTES contains metadata about your images, like dimensions, original filename, the import session, location data, etc.
• ZEXTENDEDATTRIBUTES has some more juicy metadata, some of which seems to be pulled off of the EXIF, like aperture, shutter speed, iso, focal length, sample rate for videos, camera and lens make and model, flash info, metering mode and more location data
• ZCOMPUTEDATTRIBUTES This one seems to contain magic numbers which I assume are used internally by Photos to further classify your images, like how well you framed your shot, or if it has nice bokeh effect, etc. All some complicated AI shit.

These ATTRIBUTES tables have foreign keys that link to the corresponding photo (or asset) from the ZGENERICASSET table.

I've only had a short peek at other tables, but once you get familiar with the ones mentioned above, it should be quite straightforward to navigate your way around the database if you want to explore further.

File locations

Identifying an image is done by looking it up in the ZGENERICASSET table. For example, a photo with ZUUID of D3FE5399-FA6A-47B0-976F-E7CC39EEF747 and a ZDIRECTORY equal to D (note that the directory name is the first character of the uuid) will be available under D/D3FE5399-FA6A-47B0-976F-E7CC39EEF747.jpg (also look at ZFILENAME for the exact filename including the extension).

Apple Photos does non-destructive edits to your images and original copies are kept in the originals folder.

Edited copies of photos are available in the resources/renders folder along with a plist file for each photo (sharing the same ZUUID) describing the edits. Edit information in these plists is base64 encoded, but decoding and hexdumping results in unreadable gibberish. I assume it's a proprietary encoding.

Resized photo versions are available in the resources/derivatives folder. These smaller versions include the latest edits applied to the photo.

• a medium sized version is available directly under resources/derivatives
• a tiny version (thumbnail) is available in resources/derivatives/masters

To sum up, assuming we made some edits to our photo, the following files will be available to us:

1. Original unmodified image: ~/Pictures/Photos Library.photoslibrary/originals/D/D3FE5399-FA6A-47B0-976F-E7CC39EEF747.jpg
2. Image with latest edits applied: ~/Pictures/Photos Library.photoslibrary/resources/renders/D/D3FE5399-FA6A-47B0-976F-E7CC39EEF747_suffix.jpg
3. Smaller version with latest edits applied: ~/Pictures/Photos Library.photoslibrary/resources/derivatives/D/D3FE5399-FA6A-47B0-976F-E7CC39EEF747_suffix.jpg
4. Tiny version with latest edits applied: ~/Pictures/Photos Library.photoslibrary/resources/derivatives/masters/D/D3FE5399-FA6A-47B0-976F-E7CC39EEF747_suffix.jpg

NOTE 1: _suffix may be different for you and I haven't yet figured out how it's generated.

NOTE 2: If there were no edits done to the original photo, then pt. 2 from the list above will not be available.

Shared files

Copies of images you have shared vie Photos are made in resources/cloudsharing. This is also where the files that have been shared by someone else are stored.

The locations structure is similar to the one for your ordinary photos:

• data/YOURPERSONID contains folders for each shared album and inside each folder are the images for that album along with a plist file that contains the album's name and some other metadata.
• resources/derivatives/masters contain resized versions of the images grouped in folders

YOURPERSONID folder can be obtained by reading the personID file.

Another interesting observation I made is that not all photos you share get immediately copied into the cloudsharing directory. For example, I noticed that the copying happened on demand, e.g. when you go and actually browse the files in the Shared Album via Photos app.

Albums

Albums are recorded in ZGENERICALBUM. I've noticed that this table contains records of multiple Entity types. I tend to keep my photos quite organised and I've accumulated a bunch of folders with subfolders and then albums in them.

Browsing that table, it quickly becomes obvious what the fields are used for:

• Z_ENT refers to the Entity, like a Folder, a CloudSharedAlbum, Album, etc.
• ZTRASHEDSTATE nope, Photos doesn't immediately delete your stuff, just marks it as deleted
• ZKEYASSET, ZXX_ASSET and ZXX_CUSTOMKEYASSET is a reference to the cover photo on your album. XX here again is a reference to an entity record from Z_PRIMARYKEY
• ZTITLE, ZUUID shouldn't need any explanation

The table describing the relationship between albums and photos/videos is Z_XXASSETS, where XX is again the id of your Album entity. The Z_XXALBUMS in this table is the foreign key to the ZGENERICALBUM record and Z_XXASSETS to the Z_GENERICASSET (XX in this case is the id of the GenericAsset entity). What is Z_FOK_XXASSETS? I don't know.