Image processing apparatus and image processing method

An image processing apparatus comprising:

• an acquiring means for converting a luminance value in accordance with preset nonlinear conversion characteristics and acquiring, from an imaging device, image data that is a data string formed of a first display value expressed by a first number of gradations;
• a compressing means for compressing the first display value in accordance with preset compression characteristics and outputting, as a second display value, a value expressed by a second number of gradations smaller than the first number of gradations;
• and
• a recognizing means for detecting a recognition target from an image expressed by compressed data that is a data string formed of the second display value, wherein:
• a ratio of the second number of gradations to the first number of gradations is set as a basic compression ratio, a luminance range including at least a recognition target range that is a luminance range where the recognition target is estimated to be present is set as a specified range, and the first display value corresponding to a boundary luminance value that is a minimum luminance value in the specified range is set as a boundary first display value;
• the conversion characteristics are set as a combination of linear characteristics that are predetermined for each luminance range;
• the specified range is set such that the recognition target range includes a changing point luminance value, the changing point luminance value being a luminance value at a changing point of the linear characteristics, the linear characteristics being changed at the changing point in accordance with the conversion characteristics;
• the compression characteristics are set such that the second display value is a sum of a compressed value and the boundary first display value in the recognition target range, the compressed value being obtained by compressing a value of not less than the boundary first display value among the first display values at a low compression ratio lower than the basic compression ratio;
• and
• the compression characteristics are set such that, at least, in the specified range, a relationship of the second display value converted in accordance with total conversion characteristics to the luminance value becomes linear, the total conversion characteristics being set as a combination of the compression characteristics and the conversion characteristics.