how to derive H_g2cam?

[hitbuyi]

in PersFormer.py, how to derive the H_g2cam? what's the definitions of cam and ground respectively?

       # homograph ground to camera
        H_g2cam = np.array([[1,                             0,               0],
                            [0, np.sin(-cam_pitch), args.cam_height],
                            [0, np.cos(-cam_pitch),               0]])
        H_g2cam = torch.from_numpy(H_g2cam).unsqueeze_(0).expand([self.batch_size, 3, 3]).type(torch.FloatTensor)