A plurality of grid points for forming a quadrangle is quasi-formed on a medium prior to deformation, after which position information is acquired that relates to the positions of the grid points moved in conjunction with deformation of the medium. The area difference of two triangles divided by a first diagonal is computed in a quadrangle formed by the grid points after deformation of the medium, and the area difference of two triangles divided by a second diagonal different from the first diagonal is computed in the quadrangle. The quadrangle quasi-formed on the medium prior to deformation is divided using one of the first and second diagonals that yields the larger area difference, and the formation amount of colorant formed on the medium is corrected according to the deformation ratio after deformation for the triangles formed by the division.