Image processing to set an output gradation number based on an information-loss evaluation value

An image processing apparatus which processes input image data and generates output image data, the image processing apparatus comprising:

• a feature-value calculating unit that calculates a feature value from the input image data,
• an output-gradation-number setting unit that sets, as a number of output gradations, any one gradation number of a plurality of candidates for the number of output gradations based on the feature value calculated by the feature-value calculating unit;
• and
• an image processor that processes the input image data and that generates the output image data having the number of output gradations which is set by the output-gradation-number setting unit,
• wherein the feature-value calculating unit uses, as a first evaluation image, the input image data itself or image data newly generated by performing given image processing on the input image data,
• wherein the feature-value calculating unit generates a second evaluation image whose number of gradations is less than the number of gradations of the first evaluation image, based on the input image data itself or image data newly generated by performing the identical given image processing or other given image processing on the input image data, and
• wherein the feature-value calculating unit calculates the feature value based on a difference between the first evaluation image and the second evaluation image.

An image processing method comprising:

• calculating a feature value from input image data;
• setting, as a number of output gradations, any one gradation number of a plurality of candidates for the number of output gradations based on the calculated feature value;
• and
• processing the input image data and generating output image data having the number of output gradations which is set,
• wherein the feature value is an information-loss evaluation value produced if the output image data is generated, and
• wherein the number of output gradations is set based on the information-loss evaluation value.