Contrast adjustment in digital image processing method employing histogram normalization

• US 4,731,671 A
• Filed: 05/06/1985
• Issued: 03/15/1988
• Est. Priority Date: 05/06/1985
• Status: Expired due to Term

• Current Assignee: Eastman Kodak Company
• Original Assignee: Eastman Kodak Company
• Inventors: Alkofer, James S.
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): NOT, DEFINED
• Application Number:
  US06/730,629
• Time in Patent Office: 1,044 Days
• Field of Search: 358/75, 358/76, 358/78, 358/80, 358/280, 358/283, 358/284, 358/256, 382/18, 355/38, 355/35, 355/40, 355/41, 355/77, 355/88, 355/68
• US Class Current: 382/274
• CPC Class Codes: H04N 1/4074 using histograms

Abstract

A problem in digital image processing employing a tone reproduction function that maximizes the quality of the processed image is to automatically determine the overall contrast of the processed image. In an improved method for determining the contrast of the processed image, the contrast is automatically determined as a function of the standard deviation of a sample of tone values used to generate the tone reproduction function by normalizing a histogram of the sample of tone values. The sample of tone values is selected from one of a plurality of samples of tone values corresponding to a plurality of contrast intervals based upon the shape of the histogram of the selected sample of tone values. To facilitate the adjustment of contrast, the tone reproduction function is constructed to produce values in units of a standard normal variate Z. These Z values are then multiplied by a constant which is a function of the standard deviation of the sample of tone values to determine the contrast of the processed image. A constant representing the mean density of the output medium is added to the multiplied Z values to position the tone scale of the processed image with respect to the tonal range of the output medium.

Claims

1. A digital image processing method employing a tone reproduction function generated by normalizing a histogram of a sample of tone values selected from informational portions of a digital image to produce a processed digital image having improved contrast characterized by the steps of computing the standard deviation of the sample of tone values, and of adjusting the contrast of the processed digital image as a function of the standard deviation.

2. The digital image processing method claimed in claim 1, wherein the tone reproduction function relates tone values to Z values (i.e. values of a standard normal variate Z), the tone reproduction function being applied to tone values of the digital image to produce processed Z values, and said step of adjusting contrast comprises multiplying said processed Z values by a contrast adjusting constant which is a function of the standard deviation of the sample of tone values.

3. The digital image processing method claim ed in claim 2, wherein said digital image is to be displayed on an output medium with a desired mean density, said method further including the step of adding a constant representing the desiring mean density of the output medium to said processed Z values which have been multiplied by said contrast adjusting constant.

4. The digital image processing method claimed in claim 2, wherein said function of the standard deviation of the sample of tone values is of the form

5. The digital image processing method claimed in claim 2, wherein said sample of tone values is selected from a plurality of samples of tone values corresponding to a plurality of contrast intervals on the basis of statistics of the distributions of the tone values in the samples.

6. The digital image processing method claimed in claim 5, wherein the sample of tone values having the most nearly normal distribution of tone values is selected.

7. A digital image processing apparatus for processing tone values of a digital image to produce a processed digital image, comprising:

means for selecting a sample of tone values from informational portions of the digital image;

means for normalizing the sample of tone values to produce a tone reproduction function, the tone reproduction function relating tone values to values of a standard normal variate Z (Z values);

means for applying the tone reproduction function to tone values of the digital image to produce processed Z values;

means for computing the standard deviation of the sample of tone values; and

means for adjusting the contrast of the processed digital image comprising means for multiplying said processed Z values by a contrast adjusting constant which is a function of the standard deviation of the sample of tone values.

8. The digital image processing apparatus claimed in claim 7, including means for displaying the processed digital image on an output medium with a desired mean density, and means for adding a constant representing the desired mean density of the output medium to the processed Z values which have been multiplied by said contrast adjusting constant.

9. The digital image processing apparatus claimed in claim 8, wherein said function of the standard deviation of the sample of tone values is of the form

10. The digital image processing apparatus claimed in claim 8, wherein said means for selecting a sample of tone values includes means for forming a plurality of histograms of tone values from a corresponding plurality of samples of tone values corresponding to a plurality of contrast intervals, and means for selecting one of the samples of tone values corresponding to one of the contrast intervals on the basis of statistics of the histograms of the samples of tone values.

11. The digital image processing apparatus claimed in claim 10, wherein the sample of tone values having the most nearly normal histogram is selected.

12. A digital image processing method of the type employing a tone reproduction function generated by normalizing a sample of tone values from informational portions of a digital image to produce a processed digital image having improved contrast, said processed digital image to be displayed on an output medium with a desired mean density, characterized by:

the tone reproduction function being applied to tone values in the digital image to produce values of a standard normal variate Z (Z values) in the processed digital image, and the method including the steps of;

computing the standard deviation of the sample of tone values;

adjusting the contrast of the processed digital image by multiplying the Z values by a contrast adjusting constant which is a function of the standard deviation of the sample of tone values, and adding a constant representing the desired mean density of the output medium to the multiplied Z values.

13. A digital image processing method of the type employing a tone reproduction functional generated by normalizing a sample of tone values from informational portions of a digital image to produce a processed digital image having improved contrast, said processed digital image to be displayed on an output medium with a desired means density, characterized by:

the sample of tone values being selected from a plurality of samples of tone values corresponding to a plurality of contrast intervals on the basis of statistics of the distributions of the tone values in the samples, and the tone reproduction function being applied to tone values in the digital image to produce values of a standard normal variate Z (Z values) in the processed digital image, and the method including the steps of adjusting the contrast of the processed digital image by multiplying the Z values by a contrast adjusting constant which is a function of the standard deviation of the sample of tone values, the function having the form

space="preserve" listing-type="equation">σ

.sub.z =m·

σ

.sub.s +b

where;

σ

_z is the contrast adjusting constant,

σ

_s is the standard deviation of the sample of tone values, and

m and b are system-dependent constants; and

including the step of adding a constant representing the desired mean density of the output medium to the multiplied Z values.

14. A digital image processing method of the type employing a tone reproduction function generated by normalizing a sample of tone values from informational portions of a digital image, characterized by:

the tone reproduction function being applied to tone values in the digital image to produce values of a standard normal variate Z (Z values) in a processed digital image.

15. The digital image processing method claimed in claim 14, including the steps of:

computing the standard deviation of the sample of tone values;

multiplying the Z values by a constant which is a function of the standard deviation of the sample of tone values; and

adding a constant representing a desired mean density of an output medium to the multiplied Z values.