This is the shift required so that the frames with orientation 0 are centred correctly. This shift is needed to correctly align objects when rendering, as the logical centre of the frame will not necessarily be the absolute centre of the frame (width/2, height/2). For example, the centre of a critter will be the position of its feet, while the centre of a tree will be its base. A positive value means shift the frame to the right of the screen when rendering.