code: public void start() throws CameraAccessException, IOException, IllegalAccessException
      {
          if (mTryOpenCamera)
          {
              mCamera = openCamera();
              mCameraOwner = new GVRCameraSceneObject(getGVRContext(), mWidth / 2, mHeight / 2, mCamera);
              mCameraOwner.getTransform().setPositionZ(-200.0f);
              mTryOpenCamera = false;
          }
          if (isEnabled() && (mCamera != null))
          {
              mCamera.startPreview();
              if (!onStart())
              {
                  throw new IllegalAccessException("Cannot access body tracker");
              }
          }
      }
docs: Start body tracking from camera.
  <p>
  This function opens the camera and begins capturing camera previews.
  It will throw exceptions if the camera cannot be accessed.
  @throws CameraAccessException
  @throws IOException
  @throws IllegalAccessException