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Face recognition data structures

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This article explains the data structures used in the Face service for face recognition operations. These data structures hold data on faces and persons.

Caution

Face service access is limited based on eligibility and usage criteria in order to support our Responsible AI principles. Face service is only available to Microsoft managed customers and partners. Use the Face Recognition intake form to apply for access. For more information, see the Face limited access page.

Data structures used with Identify

The Face Identify API uses container data structures to the hold face recognition data in the form of Person objects. There are three types of containers for this, listed from oldest to newest. We recommend you always use the newest one.

PersonGroup

PersonGroup is the smallest container data structure.

  • You need to specify a recognition model when you create a PersonGroup. When any faces are added to that PersonGroup, it uses that model to process them. This model must match the model version with Face ID from detect API.
  • You must call the Train API to make any new face data reflect in the Identify API results. This includes adding/removing faces and adding/removing persons.
  • For the free tier subscription, it can hold up to 1000 Persons. For S0 paid subscription, it can have up to 10,000 Persons.

PersonGroupPerson represents a person to be identified. It can hold up to 248 faces.

Large Person Group

LargePersonGroup is a later data structure introduced to support up to 1 million entities (for S0 tier subscription). It is optimized to support large-scale data. It shares most of PersonGroup features: A recognition model needs to be specified at creation time, and the Train API must be called before use.

Person Directory

PersonDirectory is the newest data structure of this kind. It supports a larger scale and higher accuracy. Each Azure Face resource has a single default PersonDirectory data structure. It's a flat list of PersonDirectoryPerson objects - it can hold up to 75 million.

PersonDirectoryPerson represents a person to be identified. Updated from the PersonGroupPerson model, it allows you to add faces from different recognition models to the same person. However, the Identify operation can only match faces obtained with the same recognition model.

DynamicPersonGroup is a lightweight data structure that allows you to dynamically reference a PersonGroupPerson. It doesn't require the Train operation: once the data is updated, it's ready to be used with the Identify API.

You can also use an in-place person ID list for the Identify operation. This lets you specify a more narrow group to identify from. You can do this manually to improve identification performance in large groups.

The above data structures can be used together. For example:

  • In an access control system, The PersonDirectory might represent all employees of a company, but a smaller DynamicPersonGroup could represent just the employees that have access to a single floor of the building.
  • In a flight onboarding system, the PersonDirectory could represent all customers of the airline company, but the DynamicPersonGroup represents just the passengers on a particular flight. An in-place person ID list could represent the passengers who made a last-minute change.

For more details, please refer to the PersonDirectory how-to guide.

Data structures used with Find Similar

Unlike the Identify API, the Find Similar API is designed to be used in applications where the enrollment of Person is hard to set up (for example, face images captured from video analysis, or from a photo album analysis).

FaceList

FaceList represent a flat list of persisted faces. It can hold up 1,000 faces.

LargeFaceList

LargeFaceList is a later version which can hold up to 1,000,000 faces.

Next steps

Now that you're familiar with the face data structures, write a script that uses them in the Identify operation.