Contrast adjustment in digital image processing method employing histogram normalization

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.

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
• .sigma..sub.z =m.multidot..sigma..sub.s +b
• where:
• .sigma..sub.z is the contrast adjusting constant,
• .sigma..sub.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.