Patent ID: 7391896

Claim:
An image processing apparatus which executes image processing for an original image, comprising: defining means for defining a gray level conversion curve to be used for gray level conversion on the basis of a value associated with a contrast of an image resulting from gray level conversion of the original image, the value being calculated from pixel values of the original image; and gray level conversion means for converting a gray level of the original image by using the gray level conversion curve defined by said defining means; wherein said defining means defines the gray level conversion curve on the basis of a contrast improvement factor defined by the gray level conversion curve; wherein to define the gray level conversion curve, the defining means calculates the contrast improvement factor C(d) given by: C ( d )=∫ dy ∫ dx F ( d,c )′( f 1 ( x,y )) dx dy/∫ dy ∫ dx Sgn ( f 1 ( x,y )) dx dy where Sgn (x,y) is given by: Sgn ( x,y )=0 when f ( x,y )≧ Th 1 Sgn(x,y)=1 otherwise where Th 1 is a constant: wherein F(d,c)' indicates the differential value of the gray level conversion curve, f 1 (x,y) indicates pixel values of the original image, d is a parameter that indicates a translation amount of the gray level conversion curve with respect to the pixel values and c is the contrast of the gray level conversion curve, which indicates a tilt amount of the gray level conversion curve.