The planar perspective matrix is a 3x3 homography matrix used to correlate the top view of a plane to different perspective views. Its most important property is that the 3rd dimension is eliminated. The derivation of this matrix is based on the 3x4 perspective projection matrix. Element values are affected by the camera calibration matrix and translation and rotation of the camera as well. The simple case happens when the focal length is equal to the unity and no translation or rotation is involved.