Virtual Scene Scale Factor in Unity

This article explains how to use an arbitrary scale in your virtual Unity scene despite Vuforia Engine requiring all targets to be configured accurately in meter scale.

Vuforia Engine requires the size of all targets to be defined in meters. It is important that the size is configured as accurately as possible since it is used to compute the pose of the target – its position and orientation in space.
Vuforia Fusion unites tracking information from the camera feed and the device sensors. If the size of a target is not configured correctly, poses computed from the camera image and the sensors do not match, which will cause various tracking issues.

Starting with Vuforia 9.0, there is a simple mechanism to transform all Vuforia Targets into whatever virtual scene scale you are using. It is called Virtual Scene Scale Factor and can be found in the Vuforia Configuration.

Setting the Physical Scale of a Model Target

The physical size of a target is set in the dataset, but can also be configured in the Unity inspector for a particular target:

Because targets need to be configured in meter scale, by default all virtual content used as augmentations also needs to be scaled in meters to match the same size.

This however may cause various issues with your digital assets. For example, you might want to create an AR experience where a virtual character interacts with a toy car model using Vuforia Model Targets.

Example Scenario

In this scenario, your virtual content will likely be in a different scale than the virtual content: The toy car in this example is 1:18 in scale, but the digital character is 1.75m tall.

Scaling down the digital assets to match the physical size of the target may be possible, but often causes issues in Unity, in particular if you are using physics or particle systems.

Instead, use the Virtual Scene Factor setting on your Vuforia Targets into whatever virtual scene scale you are using that can be found in the Vuforia Configuration.

The Virtual Scene Scale Factor configures how many Unity scene units correspond to one meter in the physical world. Poses reported by Vuforia will be transformed accordingly, but scene content (other than preview assets of Vuforia targets) will not be scaled.

In our example, we want the poses calculated from our 1:18 toy model to be transformed to a virtual scene that contains assets matching the size of a real-sized car. This can be done by setting the Virtual Scene Scale Factor to 18. Preview Models, e.g. for Model Targets or Image Targets will be automatically adjusted in size as seen in the screenshot above, but no application content will be changed.

At runtime, the Virtual Scene Scale will now automatically be applied, allowing you to build your scene in whatever scale you need while still setting the correct physical scale for Vuforia targets.

NOTE: The virtual scene scale factor is not supported on the HoloLens and Magic Leap devices. On those platforms, head tracking poses are not controlled by Vuforia Engine and all scene content is required to be in meters. Therefore, the Virtual Scene Scale Factor is forced to 1.0.

Setting the Virtual Scene Scale Factor at runtime.

In some cases, it might be necessary to change the Virtual Scene Scale Factor at runtime.
For instance, in a similar example as above, you might be building your AR experience for a real-size car. However, for testing your application, you are using a toy model of the same car.
By calling the following value from any script, you can change the Virtual Scene Scale Factor at runtime to toggle between the real and toy-sized car models:

VuforiaConfiguration.Instance.Vuforia.VirtualSceneScaleFactor = 18f;

NOTE: For this use case, you will also need to adjust the size of the target at runtime by calling the ObjectTarget.SetSize(..) method.