Image processing device, image processing method and program with improved image contrast

An image processing method comprising the steps of:

• calculating a frequency of a pixel value for each of blocks formed of a plurality of pixels, dividing a dynamic range into N ranges (where N is an integer larger than one), and generating a first histogram formed by N bins in which a width of one bin corresponds to a plurality of pixel values;
• extracting the first histogram of a high luminance block formed by pixels having a high luminance value from among the plurality of blocks of an entire image;
• mixing the first histogram of each of the blocks and the first histogram of the high luminance block, and generating a second histogram for each of the blocks;
• calculating, based on the second histogram of a block including a target pixel and the second histogram of a block adjacent to the block including the target pixel, a frequency of a pixel value that depends on a pixel position in the block of the target pixel, and generating a third histogram formed by N bins for each pixel;
• and
• generating a relationship between the pixel value and an output value of the pixel value based on the third histogram such that a maximum cumulative frequency of the third histogram matches a maximum value of the output value of the pixel value, and calculating the output value from the pixel value of the target pixel.

A non-transitory computer-readable medium having stored thereon a computer-readable program that comprises instructions that command a computer to function as:

• a first histogram generation unit that calculates a frequency of a pixel value for each of blocks formed of a plurality of pixels, divides a dynamic range into N ranges (where N is an integer larger than one), and generates a first histogram formed by N bins in which a width of one bin corresponds to a plurality of pixel values;
• an extraction unit that extracts the first histogram of a high luminance block formed by pixels having a high luminance value from among the plurality of blocks of an entire image;
• a second histogram generation unit that mixes the first histogram of each of the blocks and the first histogram of the high luminance block, and generates a second histogram for each of the blocks;
• a third histogram generation unit that, based on the second histogram of a block including a target pixel and the second histogram of a block adjacent to the block including the target pixel, calculates a frequency of a pixel value that depends on a pixel position in the block of the target pixel, and generates a third histogram formed by N bins for each pixel;
• and
• an output value determination unit that generates a relationship between the pixel value and an output value of the pixel value based on the third histogram such that a maximum cumulative frequency of the third histogram matches a maximum value of the output value of the pixel value, and calculates the output value from the pixel value of the target pixel.