A first unit obtains a gradation conversion characteristic for equalizing a
histogram of an input image. A second unit obtains an inverse
characteristic of a gradation conversion characteristic for equalizing a
histogram of an image to which a predetermined gradation conversion is
applied. A third unit composites the gradation conversion characteristic
obtained by the first unit and the inverse characteristic obtained by the
second unit. A fourth unit performs fitting on the composited
characteristic obtained by the third unit with a lower degree function.
Gradation of the input image is converted based on the composited
characteristic after the processing by the fourth unit.
