Frigate Tip: Motion Masks

[hawkeye217]

The Frigate Tips series is a collection of posts where the Frigate developers give tips on how to best configure, use, and maintain your Frigate installation. This post's tip aims to bring clarity on when to use motion masks, something we've discovered is often misunderstood by new users and users coming from other NVR software.

What is a motion mask?

Motion masks in Frigate are used to prevent areas of a scene on a camera from triggering motion detection. They are most easily configured from the Frigate Web UI. The Frigate documentation says:

You want to mask out your timestamp, the sky, rooftops, etc. Keep in mind that this mask only prevents motion from being detected and does not prevent objects from being detected if object detection was started due to motion in unmasked areas.

This is a really important point that we've found that many users misunderstand: a motion mask only prevents motion from being detected and does not prevent objects from being detected.

Motion masks don't prevent objects from being detected

Motion masks should not be used to block out areas where you aren't interested in detecting things.

Yes, you read that correctly (though there are some caveats). Motion masks should be used very sparingly. In order to understand why, it's important to understand how Frigate works.

Again, from the Frigate documentation:

Frigate uses motion detection as a first line check to see if there is anything happening in the frame worth checking with object detection.

Once motion is detected, it tries to group up nearby areas of motion together in hopes of identifying a rectangle in the image that will capture the area worth inspecting. These are the red "motion boxes" you see in the debug viewer.

For a simple example, take this scene of a street in Orlando, Florida. A car is driving along the road, and Frigate's Debug view shows red boxes to indicate that motion is being detected on the frame in the areas where the car is moving. These regions of motion are grouped together and passed onto Frigate's object detector, which identifies the object in the frame as a car. Frigate's Debug view then shows the label and colored bounding box around it.

Let's imagine a scenario with this scene where you're only interested in people that are walking on the sidewalk on the right side of the frame, and you only want those people to be shown in the Review pane in Frigate.

It's often assumed that this is where a motion mask would be the solution. A new user may be tempted to add a motion mask like this:

However, this is not the case, and this kind of a motion mask is actually detrimental to the way Frigate is designed to work.

Again, from the documentation:

With too much masking, tracking is hampered and if an object walks from an unmasked area into a fully masked area, they essentially disappear and will be picked up as a "new" object if they leave the masked area. This is important because Frigate uses the history of scores while tracking an object to determine if it is a false positive or not.

So take this hypothetical situation:

A person walks underneath the clock (which, if using the masking example above, is in a masked area) and proceeds down the sidewalk we're interested in (which is now in the unmasked area).

Frigate would detect motion just outside of the masked area and begin tracking the person as they proceed down the sidewalk.

Seems like this would work... But wait.

What if the person realized they had dropped something as they were walking underneath the clock? They would turn around and proceed back into the masked area. Perhaps they spend a couple of minutes there talking to someone else, and then continue back down the sidewalk of interest.

Because of overmasking, the person of interest would actually be tracked by Frigate as an entirely new person as they leave the masked area and proceed down the sidewalk the second time, potentially creating two separate review items. In a high-traffic area, this would certainly not be ideal.

So, what's the solution?

Use required_zones instead of motion masks

If you're only interested in keeping track of objects in a particular area of your scene, use required_zones.

From the documentation:

Zones allow you to define a specific area of the frame and apply additional filters for object types so you can determine whether or not an object is within a particular area. Presence in a zone is evaluated based on the bottom center of the bounding box for the object. It does not matter how much of the bounding box overlaps with the zone.

Using a zone for the sidewalk and then restricting alerts and/or detections to objects in that zone is the correct solution here. This can easily be done with the Frigate UI (0.14 and later).

Create a zone called "sidewalk" in Frigate's mask/zone editor and then restart Frigate:

After restarting, navigate to Settings → Camera Settings, choose the camera in the top right, and then check the boxes to restrict alerts and/or detections to the "sidewalk" zone you just created. Then save and restart Frigate once more.

With this configuration, Frigate will track objects outside of the sidewalk zone, but only the person objects appearing in the sidewalk zone will show as alerts in the UI.

When creating zones for your cameras, remember that an object's presence in a zone is evaluated based on the bottom center of the bounding box. The example in the documentation shows that more clearly.

Applicable situations for motion masks

Motion masks are used to prevent unwanted types of motion from triggering object detection. A great use for a motion mask would be to cover a camera timestamp, to cover the sky and any moving clouds, or to cover the top of a tree for moving branches and leaves.

Official Frigate documentation

Masks

Zones

Review - for configuring alerts and detections (Frigate 0.14 and later)

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If this has been a helpful post, give it an upvote and thumbs up. If you have any questions, leave them down below for us!

-The Frigate dev team (Blake @blakeblackshear, Nick @NickM-27, Josh @hawkeye217)