Used by the routines at DRAW_OUTLINE and DRAW_WALLS.

The projection, and the scrolling: everything is placed relative to the knight, who therefore stays at the middle of the screen while the town moves under him. For a point at U and V, with dU and dV its distance from the knight's U and V (both turned round with the town if it is): DRAW_X = (dU + dV + $F0) / 2 and DRAW_Y = (dV - dU + $1B0) / 4, signed. So U runs right and down the screen and V right and up, 256 units a cell: a cell is a diamond 256 pixels wide and 128 lines high, bigger than the play area, and the knight's own point is at x 120, the middle of the part of the buffer that is shown, and line 108. DRAW_Y counts up the screen, as the buffer's lines do. It is Filmation's projection, x from U + V and y from V - U, at half the scale and with no height.

PROJECT_CELL projects the corner of the cell last looked up (CELL_LOOKED_UP, in the view's terms): its lowest U and V, where the wall along its first face starts. PROJECT_POINT, entered from PROJECT_THING with a U in HL and a V pushed, both turned already if the town is, projects any point.