Use "Texture2D.ReadPixels" with QualComm ARCamera(Vuforia)

Hello, I am building a project with QualComm ARCamera(Vuforia) on Unity-Android.

Vuforia is a package for free on the asset store, put its camera into Heiarchy will show the webcam view directly.

I added a 3D model into the scene, then made a GUI.Button to photograph, use "Texture2D.ReadPixels" function. But finally I got an image that shows all webcam view --- there's no 3D model there!

I try another way. To create a renderTexture for "Texture2D.ReadPixels" reading. The result is "Only 3D model", all where webcam view is black.

I am confused in this sitiuation. Doesn't "Texture2D.ReadPixels" read the finally pixels which was projected on the screen? What can I do to solve this problem?

Thanks for any opinion.