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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.

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First Claim