A method of determining transformation parameters describing a motion model of camera motion from an input video signal in a video compression system, the method comprising: determining a motion vector field (300, 400, 500, 600) comprising motion vectors (302, 402) from consecutive frames of the input video signal, the motion vector field (300, 400, 500, 600) describing motion of pixels in the video signal; projecting x and y components of the motion vectors (302, 402) on to x and y axes of the coordinate system of the motion vector field (300, 400, 500, 600) to provide projection functions (308, 310, 408, 410) of the motion vectors (302, 402) on respective axes; and determining the transformation parameters from the projections of the motion vectors (302, 402), characterised in that: projecting x-components of the motion vectors comprises: determining an average of the vector x-components of motion vectors in each column of the motion vector field to provide a projection function pv x (x); and determining an average of the vector y-components of motion vectors in each column of the motion vector field to provide a projection function pv x (y), projecting y-components of the motion vectors comprises: determining an average of the vector x-components of motion vectors in each row of the motion vector field to provide a projection function pv y (x); and determining an average of the vector y-components of motion vectors in each row of the motion vector field to provide a projection function pv y (y), determining the transformation parameters comprises calculating one or more of a translation parameter, a scale parameter and a rotation parameter from one or more of the projection functions (308, 310, 408, 410). determining a motion vector field (300, 400, 500, 600) comprising motion vectors (302, 402) from consecutive frames of the input video signal, the motion vector field (300, 400, 500, 600) describing motion of pixels in the video signal; projecting x and y components of the motion vectors (302, 402) on to x and y axes of the coordinate system of the motion vector field (300, 400, 500, 600) to provide projection functions (308, 310, 408, 410) of the motion vectors (302, 402) on respective axes; and determining the transformation parameters from the projections of the motion vectors (302, 402), projecting x-components of the motion vectors comprises: determining an average of the vector x-components of motion vectors in each column of the motion vector field to provide a projection function pv x (x); and determining an average of the vector y-components of motion vectors in each column of the motion vector field to provide a projection function pv x (y), projecting y-components of the motion vectors comprises: determining an average of the vector x-components of motion vectors in each row of the motion vector field to provide a projection function pv y (x); and determining an average of the vector y-components of motion vectors in each row of the motion vector field to provide a projection function pv y (y), determining the transformation parameters comprises calculating one or more of a translation parameter, a scale parameter and a rotation parameter from one or more of the projection functions (308, 310, 408, 410). determining an average of the vector x-components of motion vectors in each column of the motion vector field to provide a projection function pv x (x); and determining an average of the vector y-components of motion vectors in each column of the motion vector field to provide a projection function pv x (y), projecting y-components of the motion vectors comprises: determining an average of the vector x-components of motion vectors in each row of the motion vector field to provide a projection function pv y (x); and determining an average of the vector y-components of motion vectors in each row of the motion vector field to provide a projection function pv y (y), determining the transformation parameters comprises calculating one or more of a translation parameter, a scale parameter and a rotation parameter from one or more of the projection functions (308, 310, 408, 410). determining an average of the vector x-components of motion vectors in each row of the motion vector field to provide a projection function pv y (x); and determining an average of the vector y-components of motion vectors in each row of the motion vector field to provide a projection function pv y (y), determining the transformation parameters comprises calculating one or more of a translation parameter, a scale parameter and a rotation parameter from one or more of the projection functions (308, 310, 408, 410).