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I am trying to determine the actual size of an object in image/video (CCTV video tracks), my thinking is, if this object is moving, the size appears on the screen changes, and depending on the position of this object (its distance from the camera), the size changing rate should be different, vice versa, if the object image size changing rate is known, it should be possible to deduce its actual position and actual size? If this is not right, "put a reference object in the scene" method is also acceptable.

However, the current problem is, I do not really understand the 3D real world projecting to 2D image geometrical math, perhaps with a reference object, some mapping equation f(x,y)→g(x_real, y_real, z_real) can be found? Or with the changing rate way, f(dr/dt, x, y)→g(x_real, y_real, z_real) is possible?

Please, any fundamental ideas, corrections or suggestions will be appreciated, thanks!