where V_p is the number of pixels that the image of the object passed through per unit time on the imaging plane of the first image capturing module; p₂ is the size of a single pixel of the first image capturing module, the unit of which may be millimeter; f₂ is the focal length of the first image capturing module, the unit of which may be millimeter; p₁ is the size of a single pixel of the second image capturing module, the unit of which may be millimeter; f₁ is the focal length of the second image capturing module, and the unit of which may be millimeter; h is the distance between the lens of the first image capturing module and the object, the unit of which may be millimeter; and H is the distance between the lens of the second image capturing module and the object, the unit of which may be millimeter.

In order to accurately obtain the distance h between the object and the lens of the first image capturing module and the distance H between the object and the lens of the second image capturing module, in this embodiment, ranging modules selected from ultrasonic ranging modules or laser ranging modules can be used to obtain the distance h between the lens of the first image capturing module and the object and to obtain the distance H between the lens of the second image capturing module and the object in real-time.

In this embodiment, the direction of motion of the object may be selected as from the first image capturing module towards the second image capturing module.

The first ranging module, the second ranging module and the control module 19 of the device of this embodiment are also applicable to the fifth embodiment.