If you recall, when we added the business card image to the Vuforia database it asked us to specify the real-world width of the object. We say it's 3.5 inches, but first we converted it into meters and entered 0.09 meters for the width. Then, when we used the image on the ImageTarget, it has the actual size in the scene.
The camera's position relative to the target is calculated at runtime by tracking the target. To track the target continuously we need to keep in mind the target's size and its distance from the device's camera. You can estimate the minimum size for a target by dividing the camera-to-target distance by 10. For instance, a 10 cm wide target would be detectable up to about 1 meter (10 cm x 10). ...
