Recommendations for Designing Multi Targets

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Multi Targets consist of multiple Image Targets. All the qualities, properties, methods, and measurements for Image Targets are also valid for Multi Targets. However, given the 3D geometry of Multi Targets, other factors are important to consider, such as depth and geometric consistency.

Attributes of Multi Targets

An ideal Multi Target has the same requirements as an ideal Image Target. We therefore recommend to consult the Best Practices for Designing and Developing Image-Based Targets that will explain everything you need to know for a good trackable image.

A cereal box and similar product packages are good use cases for Multi Targets. Since Multi Targets are tracked as a single trackable item, performance is substantially improved (compared to tracking multiple single-Image Targets at the same time).

The factors that differ from an Image Target are the dimensional parameters of a Multi Target. This guide will explain the following concepts of defining these parameters and maintaining its geometric consistency.

Depth

We recommend the depth of the Multi Target be at least half the width of the front side. Since Multi Targets are detected and tracked as shapes, it is important to present enough rich details on the side to maintain a steady tracking experience. A side of a Multi Target that is too slim might fail and loose tracking.

A DVD case is a good example of a 3D object that has small depth. The spine label on a DVD case is usually too thin to find features at a distance. So, the Multi Target would be lost when turning the DVD cover around.

TIP: You can detect and track the DVD case by using one target image on each side as it is not required to fill all sides of a Cuboid with images. Providing only a front and back target image on a Multi Target is sufficient for a user to track content on either side of the 3D object.

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Geometric consistency

The other important factor to consider is the spatial relationship among the parts of the Multi Target that should not change during use. For example, if one side of a Multi Target moves when opening the top of the cereal box, Vuforia Engine assumes the top part is still in place. This inconsistency during use can decrease the tracking performance of the Multi Target.

TIP: To overcome such a situation, leave out the moving target part on the Cuboid, or create the moving target as a separate Image Target. This method allows the application to track a moving part separately (even if ripped apart) without influencing the tracking performance of the Multi Target.

The Parameters of Multi Targets

The parameters of a Multi Target are defined in the Target Manager when creating a new Cuboid for the Multi Target. The following table provides guidelines for defining these values.

Parameter

Description

Dimensions

Width, Height, Length are the input fields and represent the actual size of the Multi Target in 3D scene units.

Once a Multi Target is created, its size can no longer be changed directly. Only the sizes of the individual target images that make up the Multi Target.

TIP: You can specify the target size when creating the target online or afterwards in the Database Configuration XML file that is generated by the Target Manager.

The target size parameters are important, since the pose information returned during tracking will be in the same scale. For example, if the target image is 16 units wide, then moving the camera from the left border of the target to the right border of the target changes the returned position by 16 units along the x-axis.

TIP: The size of the Multi Target can be changed at runtime. This gives you a flexible way to define or update the scale of the Multi Target in your 3D scene.

NOTE: Developers looking to use Multi Targets in combination with Device Tracking should specify the target size in meters.

Name

Name must be unique to a database. When a target is detected in your application, this will be reported in the API.

Maximum string length: 64 characters

Character set can consist of a-z, A-Z, 0-9, _-

Multi Target Orientation

In addition to a Multi Target’s scale, you should also consider the rotational translation of the Multi Target.

  • Translation - Translates the origin of one part of an target image by the defined scene units along the (x,y,z) axes.
    • (x, y, z) translation in scene units are measured along the three spatial axes as seen in figure A.
  • Rotation - Rotates the origin of a Multi Target part by the defined angle, see figure B. There are several formats that can define such rotation:
    • (x,y,z angle deg) - rotation in decimal degrees along an axis defined by the vector (x,y,z).
    • (x,y,z, angle rad) - rotation in radians along an axis defined by the vector (x,y,z).
    • (qx,qy,qz,qw) – let quaternion define the rotation.
Vuforia Image

Figure A – Multi Target with coordinate axes.

 

Vuforia Image

Figure B – The individual parts are placed using translation and rotation pairs; Figure B shows the transformation of the "right" part of the box.

Learn More

Spatial Frame of Reference

How to Access and Modify Multi Targets at Runtime

Multi Targets