Video signal processing method and apparatus using histogram

1. A video signal processing method comprising the steps of:

extracting low-frequency components of a luminance signal from an input video signal at frequencies equal to or lower than a reference frequency, for pixels contained in a specific area of an image carried by the input video signal;

allocating the low-frequency components of the luminance signal to a plurality of gradation levels that range from maximum to minimum luminance levels exhibited by the luminance signal to produce luminance histogram data that indicates the distribution of the gradation levels for the pixels;

allocating the gradation levels to the pixels based on the luminance signal to produce high-frequency component histogram data that indicates the distribution of the gradation levels for specific pixels among the pixels, the specific pixels carrying high frequency components of the luminance signal, the high-frequency components being equal to or higher than a reference high frequency;

correcting the luminance histogram data in accordance with the distribution of the gradation levels in the high-frequency component histogram data to produce corrected luminance histogram data;

producing a gradation correction curve for correcting the gradation exhibited by the input video signal using the corrected luminance histogram data; and

correcting the low-frequency components of the luminance signal of the input video signal based on the gradation correction curve, with no correction of the high-frequency components of the luminance signal.

2. The video signal processing method according to claim 1 , wherein the step of correcting the luminance histogram data includes the steps of:

determining whether a first value based on the luminance histogram data at each gradation level is equal to or larger than a first reference value;

determining whether a second value based on the luminance histogram data at a specific gradation level is equal to or larger than a second reference value, when the first value based on the specific gradation level is determined as equal to or larger than the first reference value; and

correcting luminance data at the specific gradation level in the luminance histogram data based on the high-frequency component histogram data, when the second value at the specific gradation level is determined as equal to or larger than the second reference value.

3. The video signal processing method according to claim 1 , wherein the step of producing the gradation correction curve includes the steps of:

obtaining gain-correcting values that becomes smaller as variances becomes larger, the variances becoming larger as the corrected luminance histogram data converges on a particular gradation level of the gradation levels whereas smaller as the luminance histogram data diverges on a wide range of the gradation levels;

obtaining offset values based on the corrected luminance histogram data; and

applying the gain-correcting values and a predetermined constant gain to the offset values to obtain a plurality of values that constitute the gradation correction curve.

4. The video signal processing method according to claim 1 further comprising the steps of:

generating chrominance gains in accordance with ratios of the corrected low-frequency components to the luminance signal components of the input video signal; and

multiplying a chrominance signal of the input video signal by the chrominance gains to obtain a corrected chrominance signal.

5. A video signal processing apparatus comprising:

a low-frequency component extractor to extract low-frequency components of a luminance signal from an input video signal at frequencies equal to or lower than a reference frequency, for pixels contained in a specific area of an image carried by the input video signal;

a luminance histogram data producer to allocate the low-frequency components of the luminance signal to a plurality of gradation levels that range from maximum to minimum luminance levels exhibited by the luminance signal to produce luminance histogram data that indicates the distribution of the gradation levels for the pixels;

a high-frequency component histogram data producer to allocate the gradation levels to the pixels based on the luminance signal to produce high-frequency component histogram data that indicates the distribution of the gradation levels for specific pixels among the pixels, the specific pixels carrying high-frequency components of the luminance signal, the high-frequency components being equal to or higher than a reference high frequency;

a processor to correct the luminance histogram data in accordance with the distribution of the gradation levels in the high-frequency component histogram data to produce corrected luminance histogram data and to produce a gradation correction curve for correcting the gradation exhibited by the input video signal using the corrected luminance histogram data; and

a gradation corrector to correct the low-frequency components of the luminance signal of the input video signal based on the gradation correction curve, with no correction of the high-frequency components of the luminance signal.

6. The video signal processing apparatus according to claim 5 , wherein the processor corrects the luminance histogram data by:

determining whether a first value based on the luminance histogram data at each gradation level is equal to or larger than a first reference value;

determining whether a second value based on the luminance histogram data at a specific gradation level is equal to or larger than a second reference value, when the first value based on the specific gradation level is determined as equal to or larger than the first reference value; and

correcting luminance data at the specific gradation level in the luminance histogram data based on the high-frequency component histogram data, when the second value at the specific gradation level is determined as equal to or larger than the second reference value.

7. The video signal processing apparatus according to claim 5 , wherein the processor produces the gradation correction curve by:

obtaining gain-correcting values that become smaller as variances become larger, the variances becoming larger as the corrected luminance histogram data converges on a particular gradation level of the gradation levels whereas smaller as the luminance histogram data diverges on a wide range of the gradation levels;

obtaining offset values based on the corrected luminance histogram data; and

applying the gain-correcting values and a predetermined constant gain to the offset values to obtain a plurality of values that constitute the gradation correction curve.

8. The video signal processing apparatus according to claim 5 further comprising:

a chrominance gain generator to generate chrominance gains in accordance with ratios of the corrected low-frequency components to the luminance signal components of the input video signal; and

a multiplier to multiply chrominance signal of the input video signal by the chrominance gains to obtain a corrected chrominance signal.

9. A video signal processing method comprising the steps of:

extracting low-frequency components from an input video signal at frequencies equal to or lower than a reference frequency, for pixels contained in a specific area of an image carried by the input video signal;

allocating the low-frequency components, based on luminance signal components of the input video signal, to a plurality of gradation levels that range from maximum to minimum luminance levels exhibited by the luminance signal components, thus producing luminance histogram data that indicates the frequency of the gradation levels for the pixels;

allocating the gradation levels to the pixels based on the luminance signal components to produce high-frequency component histogram data that indicates the frequency of the gradation levels for specific pixels among the pixels, each specific pixel carrying a high-frequency component equal to or higher than a reference high frequency;

correcting the luminance histogram data in accordance with the frequency of the gradation levels in the high-frequency component histogram data to produce corrected luminance histogram data;

producing a gradation correction curve for correcting the gradation exhibited by the input video signal using the corrected luminance histogram data; and

correcting the luminance signal components of the input video signal based on the gradation correction curve,

wherein the step of correcting the luminance histogram data includes the steps of:

determining whether a first value based on the luminance histogram data at each gradation level is equal to or larger than a first reference value;

determining whether a second value based on the luminance histogram data at a specific gradation level is equal to or larger than a second reference value, when the first value based on the specific gradation level is determined as equal to or larger than the first reference value; and

correcting luminance data at the specific gradation level in the luminance histogram data based on the high-frequency component histogram data, when the second value at the specific gradation level is determined as equal to or larger than the second reference value.

10. The video signal processing method according to claim 9 , wherein the step of producing the gradation correction curve includes the steps of:

obtaining gain-correcting values that become smaller as variances become larger, the variances becoming larger as the corrected luminance histogram data converges on a particular gradation level of the gradation levels whereas smaller as the luminance histogram data diverges on a wide range of the gradation levels;

obtaining offset values based on the corrected luminance histogram data; and

applying the gain-correcting values and a predetermined constant gain to the offset values to obtain a plurality of values that constitute the gradation correction curve.

11. The video signal processing method according to claim 9 further comprising the steps of:

generating chrominance gains in accordance with ratios of the corrected luminance signal components to the luminance signal components of the input video signal; and

multiplying chrominance signal components of the input video signal by the chrominance gains to obtain corrected chrominance signal components.

12. A video signal processing apparatus comprising:

a low-frequency component extractor to extract low-frequency components from an input video signal at frequencies equal to or lower than a reference frequency, for pixels contained in a specific area of an image carried by the input video signal;

a luminance histogram data producer to allocate the low-frequency components, based on luminance signal components of the input video signal, to a plurality of gradation levels that range from maximum to minimum luminance levels exhibited by the luminance signal components, thus producing luminance histogram data that indicates the frequency of the gradation levels for the pixels;

a high-frequency component histogram data producer to allocate the gradation levels to the pixels based on the luminance signal components to produce high-frequency component histogram data that indicates the frequency of the gradation levels for specific pixels among the pixels, each specific pixel carrying a high-frequency component equal to or higher than a reference high frequency;

a processor to correct the luminance histogram data in accordance with the frequency of the gradation levels in the high-frequency component histogram data to produce corrected luminance histogram data and to produce a gradation correction curve for correcting the gradation exhibited by the input video signal using the corrected luminance histogram data; and

a gradation corrector to correct the luminance signal components of the input video signal based on the gradation correction curve,

wherein the processor corrects the luminance histogram data by:

determining whether a first value based on the luminance histogram data at each gradation level is equal to or larger than a first reference value;

determining whether a second value based on the luminance histogram data at a specific gradation level is equal to or larger than a second reference value, when the first value based on the specific gradation level is determined as equal to or larger than the first reference value; and

correcting luminance data at the specific gradation level in the luminance histogram data based on the high-frequency component histogram data, when the second value at the specific gradation level is determined as equal to or larger than the second reference value.

13. The video signal processing apparatus according to claim 12 , wherein the processor produces the gradation correction curve by:

obtaining gain-correcting values that become smaller as variances become larger, the variances becoming larger as the corrected luminance histogram data converges on a particular gradation level of the gradation levels whereas smaller as the luminance histogram data diverges on a wide range of the gradation levels;

obtaining offset values based on the corrected luminance histogram data; and

applying the gain-correcting values and a predetermined constant gain to the offset values to obtain a plurality of values that constitute the gradation correction curve.

14. The video signal processing apparatus according to claim 12 further comprising:

a chrominance gain generator to generate chrominance gains in accordance with ratios of the corrected luminance signal components to the luminance signal components of the input video signal; and

a multiplier to multiply chrominance signal components of the input video signal by the chrominance gains to obtain corrected chrominance signal components.