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.

Abstract: In a method for processing digital color images derived from photographic film, the method employing color reproduction functions generated by normalizing random samples of color values from different colors, sampling of color values from noisy areas of an image, or scanner noise can distort the randomness of sampling and result in unwanted contrast reductions and color shifts in the processed digital images. The problem is solved by correcting the shapes of the distributions of color value samples toward normal distributions having the same means and standard deviations as the sample distributions prior to forming the color reproduction functions.

Abstract: A problem in digital image processing is to automatically generate a tone reproduction function that maximizes the quality of the processed image. In an improved method for generating the tone reproduction function by normalizing the histogram of a sample of tone values, the sample of tone values is selected from an image dependent "floating" contrast interval using the statistical parameters of the distribution of tone values in the sample itself for the selection criteria. A plurality of histograms are formed from the tone values in a plurality of contrast intervals, and the most nearly normal histogram is selected for normalization.

Abstract: In automatic photographic printers, it is a problem to accurately detect and skip blank (unexposed or completely exposed) film frames automatically. The problem is solved by scanning a film frame in three colors to generate color value samples, compiling histograms of the color value samples, computing the standard deviations of the color histograms, comparing the standard deviations with predetermined constant values, and generating a signal indicating the presence of a blank frame when all the standard deviations are less than their respective predetermined constants. The method is particularly useful in a scanning-type color photographic printer employing histogram modification to effect tone-scale and color balance adjustments.

Abstract: In automatic color printing, there is a need to identify scenes that have been exposed with a very narrow-band illuminant and to apply no color correction to such scenes. Narrow-band illuminant scenes are identified according to the present invention by scanning the original image in three primary colors to generate three color value distributions. The statistics of the three color value distributions are computed, and if the ratio of the standard deviations for any two of the color value distributions is greater than a first predetermined constant (e.g. 4.0) or less than a second predetermined constant (e.g. 0.25), no color correction is employed. The method is particularly useful in a scanning color photographic printer employing histogram modification to effect tone-scale and color balance adjustments.

Abstract: In methods of digital image processing, it is a problem to automatically adjust the contrast of an image that contains a bimodal distribution of tone values, such as a back-lit scene, or some close-up flash pictures. The invention solves the problem in a digital image processing method employing a tone reproduction function generated by normalizing the histogram of a sample of tone values, by statistically testing the sample of tone values for bimodal distribution, and if the sample is determined to be bimodal, testing the statistics of the sample of tone values to determine whether the image contains useful information in the shadows, and if so, adjusting the contrast of the image by increasing the contrast of the shadow portions of the image, and decreasing the contrast of the highlight portions. A part of the sample of tone values representing the shadow portions of the image is tested for skewness, and the contrast is adjusted only if the skewness does not exceed a predetermined amount.

Abstract: In a method of digital image processing employing a tone reproduction function generated by normalizing the histogram of a sample of tone values selected from the image, it is an object to automatically detect images that were derived from over or under exposed photographic originals and to adjust the tone scale of such images to improve their appearance. The invention achieves this objective by testing the skewness of the sample of tone values of the image and if the amount of skewness exceeds a predetermined amount, adjusting the contrast of the processed image by an amount equal to the value of a standard variate computed for the sample at a mean tone value of the sample minus the value of a standard variate computed for the sample at a mode of the tone values of the sample.