1. An image transformation apparatus, comprising: a coordinate transformation unit configured to make a transformation to transform a projected image that is projected on a screen by an image projection apparatus having an optical axis tilted by an arbitrary angle with respect to the screen, into an image substantially equivalent to an image that is projected on the screen in a state where the optical axis of the image projection apparatus is perpendicular to the screen; and a luminance value substituting unit configured to substitute luminance values of coordinates before the transformation as luminance values corresponding to coordinate values after the transformation, wherein said coordinate transformation unit comprises: a part configured to carry out a −γ rotation process to rotate coordinates (X, Y) to coordinates (X′, Y′) using a formula ( X ′ Y ′ ) = ( cos ⁡ ( - γ ) - sin ⁡ ( - γ ) sin ⁡ ( - γ ) cos ⁡ ( - γ ) ) ⁢ ( X Y ) ; a part configured to carry out a tilt transformation process to transform the coordinates (X′, Y′) into coordinates (x′, y′) using a formula ⁢ ( x ′ y ′ ) = 1 1 ⁢ cos ⁢ ⁢ α + X ′ ⁢ ⁢ sin ⁢ ⁢ α ⁢ ( ⁢ X ′ Y ′ ⁢ ⁢ cos ⁢ ⁢ α ) ; and a part configured to carry out a β rotation process to rotate the coordinates (x′, y′) to coordinates (x, y) using a formula ( x y ) = ( cos ⁢ ⁢ β - sin ⁢ ⁢ β sin ⁢ ⁢ β cos ⁢ ⁢ β ) ⁢ ( x ′ y ′ ) , where α denotes a tilt angle of the optical axis of the image projection apparatus with respect to an axis perpendicular to the screen, β denotes an angle formed by a straight line that is a projection of the optical axis on the screen and a reference axis (x-axis) of the screen, y denotes an angle formed by a tilt direction of the image projection apparatus on the projected image and a reference axis (X-axis) on the projected image, (X, Y) denotes the coordinates on the projected image, and (x, y) denotes the coordinates on the screen, wherein the formulas are successively calculated to obtain the coordinates (x, y) before the transformation corresponding to the coordinates (X, Y) of the projected image after the transformation.