Image processing apparatus combining plural sets of image data and method for controlling the same

A method for controlling an image processing apparatus, comprising:

• an input step, in which an input unit inputs a plurality of sets of M-bit image data that are to be combined;
• a determining step, in which a determining unit determines reference image data serving as an exposure reference from among the plurality of sets of input M-bit image data;
• a composition step, in which a composition unit combines the plurality of sets of M-bit image data, and generates one set of N-bit composite image data (N>M);
• a characteristic parameter setting step, in which a characteristic parameter setting unit sets a characteristic parameter of tone compression on the N-bit composite image data such that a predetermined reference luminance in the reference image data does not change;
• and
• a generating step, in which a generating unit generates M-bit composite image data by compressing a luminance tone of an image component contained in the N-bit composite image data and having a frequency lower than a predetermined frequency based on the set characteristic parameter,
• wherein the method further comprises at least one of:
• a shadow pixel ratio determining step, in which a shadow pixel ratio determining unit determines a shadow pixel ratio, which is the ratio of the number of pixels having a luminance value smaller than a first predetermined luminance value with respect to the total number of pixels, in the reference image data;
• and
• a highlight pixel ratio determining step, in which a highlight pixel ratio determining unit determines a highlight pixel ratio, which is the ratio of the number of pixels having a luminance value larger than a second predetermined luminance value with respect to the total number of pixels, in the reference image data;
• wherein, in the characteristic parameter setting step, the characteristic parameter setting unit sets a characteristic parameter that reduces a shadow detail loss of a pixel of the N-bit composite image data having a pixel value smaller than the predetermined reference luminance according to the shadow pixel ratio, and sets a characteristic parameter that reduces a highlight detail loss of a pixel the N-bit composite image data having a pixel value larger than the predetermined reference luminance according to the highlight pixel ratio.