Gradation corrector

Claim 1

A gradation corrector comprising: a histogram memory device (8) for storing the luminance histogram of a video luminance signal; a limiter circuit (31), connected with said histogram memory, for processing the data supplied from said histogram memory (8); a total frequency detecting circuit (33) for detecting the total frequency of the luminance histogram on the basis of the output signal from said histogram memory (8); a luminance distribution detecting circuit (34) for detecting the luminance distribution of said luminance histogram on the basis of the output signal from said total frequency detecting circuit (33) and that from said histogram memory (8); a dispersion coefficient calculating circuit (35) for calculating the expanse of said luminance histogram on the basis of the output signal from said luminance distribution detecting circuit (34) and that from said total frequency detecting circuit (33); an adding value calculating circuit (36) for calculating a fixed value to be added when a cumulative histogram is acquired on the basis of the calculation result supplied from said dispersion coefficient calculating circuit (35); an adder (37) for adding the calculation result from said adding value calculating circuit (36) to the output signal from said histogram memory (8); a minimum value detecting circuit (38) for detecting the minimum luminance level of said luminance histogram on the basis of the output signal from said histogram memory (8); an average luminance level detecting circuit (32) for detecting the average luminance level of the video signal inputted to said histogram memory (8); an accumulation starting luminance level calculating circuit (39) for calculating the luminance level where accumulation should be started when the cumulative histogram is to be acquired, on the basis of the output signals from said average luminance level detecting circuit (32), said minimum value detecting circuit (38) and said dispersion coefficient calculating circuit (35); an accumulation stopping luminance level calculating circuit (40) for calculating the luminance level where accumulation should be started when the cumulative histogram is to be acquired, on the basis of the output signal from said average luminance level detecting circuit (32); a histogram accumulation circuit (9) for accumulating the outputs from said adder (37) while they are controlled by the output signals from said accumulation starting luminance level calculating circuit (39) and said accumulation stopping luminance level calculating circuit (40); a cumulative histogram memory device (10) for storing the accumulation result; a maximum cumulative value detecting circuit (41) for detecting the maximum value of the cumulative histogram on the basis of the output signal from said cumulative histogram memory (10); a look-up table operating circuit (11) for normalizing the output signal from said cumulative histogram memory (10) on the basis of the output signal from said maximum cumulative value detecting circuit (41); a look-up table memory device (12) for storing the operation result in said look-up table operating circuit (11) to convert the inputted video luminance signal on the basis of the corresponding data and outputting the converted signal; and a timing control circuit (14) for controlling the operation of each of said circuits.