沿对角线测量的 Scene 视图大小。
         Use with pivot, rotation, and  CameraSettings.fieldOfView to calculate camera transformation.
                  
                   
                  
                The image above shows the math used to calculate the actual field of view. Here BC = size, and you need to find AB. ACB is a right angle, and BAC is half the field of view. That creates the expression: sin(BAC) = BC/AB, and so AB = BC/sin(BAC).