I created a sample to access to background image as the document from here https://developer.vuforia.com/resources/dev-guide/unity-camera-image-access.
It runs well on the first time. I am able to take the background image in the first time running.
However, when I close the AR application (presh Home button on iPhone/iPad), then I open the AR application again. The background image taken on this time is not the real background image in the AR camera.
Does anyone know the reason? And how to get the correct background image in the second run?