Image correction apparatus and method

Abstract

When the image of a face is detected from within the image of a subject, the brightness of the face image is calculated. If it is determined that the face image is dark are requires a correction, a revised gamma correction curve is calculated in such a manner that both the face image and the overall image of the subject take on the appropriate brightness. A gamma correction is applied using the revised gamma correction curve. Noise reduction processing is executed in order to suppress the amount of noise produced by the gamma correction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating part of the electrical structure of an image processing apparatus according to a first embodiment of the present invention;

FIG. 2 is a flowchart illustrating processing executed by the image processing apparatus according to the first embodiment;

FIG. 3 is a flowchart illustrating processing for calculating a revised gamma correction curve according to the first embodiment;

FIG. 4 is a brightness histogram of the image of a subject after application of a basic gamma correction according to the first embodiment;

FIG. 5 illustrates a basic gamma correction curve and a revised gamma correction curve according to the first embodiment;

FIG. 6 is a block diagram illustrating the electrical structure of a noise parameter calculation circuit according to the first embodiment;

FIG. 7 illustrates amount of noise in image data representing the image of a subject according to the first embodiment;

FIG. 8 illustrates noise data according to the first embodiment;

FIG. 9 illustrates amount of noise in image data that has undergone a gamma correction by a revised gamma correction curve according to the first embodiment;

FIG. 10 is a flowchart illustrating processing for calculating a noise parameter according to the first embodiment;

FIG. 11 illustrates the relationship between amounts of noise and filter coefficients according to the first embodiment;

FIG. 12 illustrates a basic gamma correction curve and revised gamma correction curves according to the first embodiment;

FIG. 13 illustrates amount of noise in image data that has undergone a gamma correction by a revised gamma correction curve according to the first embodiment;

FIG. 14 is a block diagram illustrating part of the electrical structure of an image processing apparatus according to a second embodiment;

FIG. 15 illustrates an example of a revised gamma correction curve according to the second embodiment;

FIG. 16A illustrates the relationship between slope angles and filter sizes, and FIG. 16B illustrates the relationship between slope angles and filter coefficients according to the second embodiment;

FIG. 17 illustrates the relationship between a basic gamma correction curve and a revised gamma correction curve according to a modification;

FIG. 18A illustrates the relationship between slope angles and filter sizes according to the modification, and FIG. 18B illustrates the relationship between slope angles and filter coefficients according to the modification;

FIG. 19 is a block diagram illustrating part of the electrical structure of an image processing apparatus according to a third embodiment;

FIG. 20 illustrates the relationship between slope angles and threshold values according to the third embodiment;

FIG. 21 illustrates the relationship between amounts of differentiation and threshold values according to the third embodiment;

FIG. 22 is a block diagram illustrating part of the electrical structure of an image processing apparatus according to a fourth embodiment;

FIG. 23 is a flowchart illustrating processing executed by the image processing apparatus according to the fourth embodiment;

FIG. 24 is a flowchart illustrating processing for calculating a revised gamma correction curve according to the fourth embodiment;

FIG. 25 illustrates amount of noise in image data corrected by revised gain and by basic gain according to the fourth embodiment;

FIG. 26 illustrates amount of noise in image data corrected by revised gain;

FIG. 27 is a block diagram illustrating part of the electrical structure of an image processing apparatus according to a fifth embodiment; and

FIG. 28 is a block diagram illustrating part of the electrical structure of an image processing apparatus according to a sixth embodiment.

Claims

1. An image correction apparatus comprising: a brightness correcting device for correcting subject image data, which represents the image of a subject, in such a manner that the brightness of a target image, which is one portion within the image of the subject, will take on a prescribed target value;a noise reduction parameter deciding device for deciding a noise reduction parameter based upon an amount of correction applied in said brightness correcting device; anda noise reducing device for applying noise reduction processing to the subject image data, which has been corrected by said brightness correcting device, based upon the noise reduction parameter decided by said noise reduction parameter deciding device.

2. The apparatus according to claim 1, wherein said noise reduction parameter deciding device decides the noise reduction parameter based upon amount of noise possessed by the subject image data and the amount of correction applied in said brightness correcting device.

3. The apparatus according to claim 1, further comprising a target image detecting device for detecting the target image from within the image of the subject; wherein said brightness correcting device corrects the subject image data in such a manner that the brightness of the target image detected by said target image detecting device will take on the appropriate target value.

4. The apparatus according to claim 1, wherein said brightness correcting device is a gamma correction circuit for applying a gamma correction to the subject image data based upon a revised gamma correction curve according to which the brightness of the target image takes on the prescribed target value.

5. The apparatus according to claim 4, further comprising: a first histogram calculating device for calculating a histogram of image data obtained in a case where the target image data has been subjected to gamma correction using a basic gamma correction curve;a threshold value deciding device for deciding a threshold value based upon the histogram calculated by said first histogram calculating device;a revised gamma correction curve calculating device for calculating a revised gamma correction curve based upon an ideal value of brightness of the target image;a second histogram calculating device for calculating a histogram of image data obtained in a case where the target image data has been subjected to gamma correction using the revised gamma correction curve calculated by said revised gamma correction curve calculating device; anda loop control device for executing processing that lowers the level of the ideal value until a peak value of the histogram that is calculated by said second histogram calculating device falls below the threshold value decided by said threshold value deciding device, and controlling said revised gamma correction curve calculating device and said second histogram calculating device so as to repeat revised correction curve calculation processing that is based upon the ideal value whose level has been lowered and gamma correction processing that uses the revised correction curve;wherein the ideal value at which the peak value of the histogram falls below the threshold value calculated by said second histogram calculating device is adopted as the prescribed target value of brightness of the target image, and the subject image data is subjected to the gamma correction based upon the revised correction curve according to which the brightness of the target image takes on the prescribed target value.

6. The apparatus according to claim 4, wherein said noise reduction parameter deciding device includes: a differentiating circuit for calculating differential data by differentiating the revised correction curve; anda multiplying circuit for calculating the noise reduction parameter by multiplying the differential data, which has been calculated by said differentiating circuit, by data indicating amount of noise possessed by the subject image data prevailing immediately prior to the gamma correction.

7. The apparatus according to claim 1, wherein said brightness correcting device is a gamma correction circuit for subjecting the subject image data to a gamma correction based upon a revised gamma correction curve according to which the brightness of the target image takes on the prescribed target value; and said noise reduction parameter deciding device decides the noise reduction parameter based upon a slope angle of a gamma correction curve decided from a predetermined reference brightness and brightness of the gamma correction value vs. this reference brightness.

8. The apparatus according to claim 1, wherein said brightness correcting device is a gamma correction circuit for subjecting the subject image data to a gamma correction based upon a revised gamma correction curve according to which the brightness of the target image takes on the prescribed target value; and said noise reduction parameter deciding device decides the noise reduction parameter based upon a level difference between a level that results after a predetermined reference brightness has been subjected to a gamma correction using a reference gamma curve and a level that results after the reference brightness has been subjected to a gamma correction using the revised gamma correction curve.

9. The apparatus according to claim 1, further comprising: a contour extracting device for extracting contour component data from the subject image data that has been subjected to the brightness correction by said brightness correcting device;wherein said noise reducing device applies noise reduction processing to the contour component data extracted by said contour extracting device;said apparatus comprising an adding circuit for adding the contour component data that has been subjected to noise reduction by said noise reducing device and the subject image data that has been subjected to the brightness correction by said brightness correcting device.

10. The apparatus according to claim 1, wherein the subject image data is three-color subject image data; and said brightness correcting device is a white-balance adjusting circuit, which includes three gain-up circuits corresponding to the subject image data of respective ones of the three colors, for raising the level of the three-color subject image data in such a manner that the brightness of the subject image will take on the prescribed target value.

11. The apparatus according to claim 1, further comprising: a first histogram calculating device for calculating a histogram of image data obtained in a case where the three-color target image data has been subjected to the white-balance adjustment using a reference amount of gain;a threshold value deciding device for deciding a threshold value based upon the histogram calculated by said first histogram calculating device;a revised gain-amount calculating device for calculating a revised gain amount based upon an ideal value of brightness of the target image;a second histogram calculating device for calculating a histogram of image data obtained in a case where the target image data has been subjected to the white-balance adjustment by the revised gain amount; anda loop control device for executing processing that lowers the level of the ideal value until a peak value of the histogram that is calculated by said second histogram calculating device falls below the threshold value decided by said threshold value deciding device, and controlling said revised gain-amount calculating device and said second histogram calculating device so as to repeat gain-amount calculation processing that is based upon the ideal value whose level has been lowered and white-balance adjustment processing that uses the revised gain amount;wherein the ideal value at which the peak value of the histogram falls below the threshold value calculated by said second histogram calculating device is adopted as the prescribed target value of brightness of the target image, and the subject image data is subjected to the white-balance adjustment based upon the revised gain amount according to which the brightness of the target image takes on the prescribed target value.

12. An image correction method comprising the steps of: correcting subject image data, which represents the image of a subject, in such a manner that the brightness of a target image, which is one portion within the image of the subject, will take on a prescribed target value;deciding a noise reduction parameter based upon an amount of correction in the correction of subject image data; andapplying noise reduction processing to the corrected subject image data based upon the noise reduction parameter decided.