Fitts' law is the most widely applied model in the field of HCI. However, this model and its existing extensions are still limited for 2D pointing task especially when the effects of movement direction (Θ) remain in the task. In this paper, we employ the concept of projection to account for the effects of target width (W) and height (H) on movement time so that we seamlessly integrate the four factors, i.e. Θ, amplitude (A), W and H, into the new extension of Fitts' law, which can uncover not only the periodicity of the asymmetrical impacts of W and H with the variation of Θ but also their interrelation. Carrying out two experiments, we verify that the vertical projection of W and the horizontal projection of H in the line of movement direction can be viewed as the determinants of movement time. Finally, we offer recommendations for 2D pointing experiments and discuss the implications for interface designs.