public void startPreview() {
        Camera theCamera = camera;
        if (theCamera != null && !previewing) {
            theCamera.startPreview();
            previewing = true;
            autoFocusManager = new AutoFocusManager(camera, settings);
            ambientLightManager = new AmbientLightManager(context, this, settings);
            ambientLightManager.start();
        }
    }
Asks the camera hardware to begin drawing preview frames to the screen.

Must be called from camera thread.
zxing-android-embedded/src/com/journeyapps/barcodescanner/camera/CameraManager.java